JP3478973B2 - Light diffusion sheet - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light diffusing sheet, and more particularly to a light diffusing sheet suitable for a liquid crystal display backlight.

[0002]

2. Description of the Related Art In a liquid crystal display, a light guide plate system in which light emitted from a cold cathode tube is guided to the front by a light guide plate is mainly used. Then, the light diffusion sheet has an effect of diffusing the light emitted from the light source and making the image of the light source invisible. As such a light diffusing sheet, a sheet in which inorganic fine particles such as calcium carbonate and silica particles are dispersed as a light diffusing agent in a polymethylmethacrylate resin, a polycarbonate resin or the like has been proposed. It has also been proposed to use organic polymer fine particles as a light diffusing agent. For example, one using polystyrene fine particles (JP-A-56-33677).
No.) or using silicone resin particles (JP-A-1-
17280) has been proposed.

Further, in order to obtain a sufficient brightness in the front direction of the display screen, a method (JP-A-6-59107) in which a binder resin contains particles such as zirconium oxide having a higher refractive index than the polymer particles is proposed. Has been done. However, in this method, when particles having a high specific gravity such as zirconium oxide are used, the particles tend to settle in the mixture, and therefore it is necessary to use ultrafine particles. However, there is a problem that ultrafine particles are generally easily aggregated and it is difficult to disperse them.

As described above, in the light guide plate method, the light diffusing sheet is required to have a uniform and high diffusing property. Although the light diffusing sheet used so far diffuses light uniformly, the amount of transmitted light in the front direction is reduced. However, there is a problem that the brightness of the display is lowered. In recent years, there has been a strong demand for higher brightness and thinner liquid crystal displays. Conventional light diffusing sheets have good light diffusivity, but they are not satisfactory in terms of high brightness.

An object of the present invention is to provide a light diffusing sheet having excellent light transmittance in the front direction (light condensing property) and having sufficient haze.

[0006]

Means for Solving the Problems As a result of intensive studies for achieving the above object, the present inventor has found that flat polymer particles are used as a light diffusing agent to form a transparent substrate through a binder resin. The present invention has been completed by finding that the above problems can be solved by coating.

That is, according to the present invention, in a light diffusing sheet having a light diffusing layer made of a binder resin and polymer fine particles on at least one surface of a transparent substrate, the polymer fine particles are flat irregular shaped fine particles made of a vinyl polymer. A light diffusing sheet is provided.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The polymer fine particles as a light diffusing agent used in the present invention are flat irregular-shaped fine particles made of a vinyl-based polymer, having a substantially circular planar shape, a convex surface and a back surface. The irregular shaped fine particles (bottom) are concave.
Such flat irregular-shaped fine particles have a bottom diameter (D) of 0.3 to 5 μm and a ratio of (D) to thickness (d) (D / d) of 1.5 to 5. It is known (Japanese Patent Publication No. 2-14222). Further, a muticle manufactured by Mitsui Chemicals, Inc., Microsphere M-311 manufactured by Matsumoto Yushi-Seiyaku Co., Ltd., and the like can also be used.

Further, one or more kinds of vinyl monomers such as aromatic vinyl monomers such as styrene and vinyltoluene and methacrylic acid esters such as methyl methacrylate and butyl methacrylate are polymerized in combination. It is also possible to use those obtained. If a crosslinkable monomer is used in the production of these polymer fine particles, solvent resistance can be imparted. When a crosslinkable monomer is used, its use ratio is preferably 1 to 30%, more preferably 3 to
20%.

The size of the polymer fine particles of the present invention is not particularly limited and may be appropriately selected depending on the required characteristics of the light diffusing sheet, but is usually preferably about 0.5 to 20 μm, more preferably 0.8 to It is about 10 μm. These polymer particles are coated on a transparent substrate together with a binder resin.

The binder resin is not particularly limited as long as it is usually used in this field. For example, polyester resin, acrylic resin, polystyrene resin, polyvinyl chloride resin, polyurethane resin, polyvinyl alcohol resin. Examples thereof include resins and polyvinyl acetate resins. As the binder resin, a resin previously dissolved in an appropriate organic solvent or a resin dispersed in an organic solvent or water may be used. For example, as a polyester-based binder resin, Byron 200 (trade name) manufactured by Toyobo Co., Ltd. can be used. Byron 200SS manufactured by Toyobo Co., Ltd. is prepared by dissolving Byron 200 in a mixed solvent of toluene and methyl ethyl ketone in a resin solid content of 30% by weight in advance.
(Brand name resin solid content 30% by weight) may be used,
You may use Vylonal MD1200 (brand name: solid content 34%) manufactured by Toyobo Co., Ltd. in which a polyester resin is dispersed in a mixed solvent of water and an organic solvent so that the solid content is 34% by weight.

The transparent base material is preferably a film-like or sheet-like material such as polymethylmethacrylate, polycarbonate, polyvinyl chloride, polyester, acetate resin, etc. In terms of light resistance and processability, a polyester film-like material is preferable. Those are particularly preferable. The coating of the light diffusion layer on the transparent substrate is carried out by dissolving the binder resin in a solvent, dispersing the polymer fine particles therein, and coating the substrate on the substrate. The ratio of the binder resin to the polymer fine particles depends on the use of the light diffusion sheet, the thickness of the coating layer, etc., but is usually the binder resin 1
30 to 300 parts by weight of polymer fine particles are preferable with respect to 00 parts by weight. As a coating method, a spray method,
Dipping method, roll coater method, knife coating method,
A known method such as a curtain flow method can be adopted. The thickness of the coating layer is usually about 5 to 30 μm.

The light diffusion layer may be coated on both sides of the transparent substrate. The light diffusion sheet of this invention is
By containing flat irregular shaped fine particles in the light diffusion layer,
It increases the light emitted in the front direction and has a well-balanced light diffusion characteristic.

[0014]

EXAMPLES Synthesis Example 1 600 g of water and 100 g of methyl methacrylate were placed in a separable flask equipped with a stirrer, a thermometer, and a reflux condenser.
Then, 0.5 g of n-octyl mercaptan (n-OM) as a chain transfer agent is charged and heated to 70 ° C. with nitrogen substitution while stirring. The internal temperature is kept at 70 ° C., 0.5 g of potassium persulfate as a polymerization initiator is added, and then a polymerization reaction is carried out for 8 hours. The obtained emulsion has a particle size of 0.
It contained 14% of solid content consisting of spherical particles having a particle size of 4 μm and a weight average molecular weight of 45,000.

71.5 g of the emulsion obtained above
0.15 g of sodium lauryl sulfate, 500 g of ion-exchanged water, 95 g of methyl methacrylate and 5 g of ethylene glycol dimethacrylate were mixed with (10 g of solid content as seed particles), and the mixture was stirred at 30 ° C. for 2 hours to give single seed particles. Absorbed the quantity. Then, this mixture was heated to 70 ° C. under a nitrogen stream, 50 g of ion-exchanged water in which 0.5 g of potassium persulfate was dissolved was added as a polymerization initiator, and a polymerization reaction was carried out for 5 hours. Good polymer particles were obtained. When the obtained polymer microparticles were observed with a scanning electron microscope, the microparticles had a flat diameter of 1.2 μm, a thickness of 0.6 μm, and a flat irregular shape with a recess having a diameter of 0.5 μm in the center of the back surface of the particle. It was fine particles. An electron micrograph of the fine particles is shown in FIG.

Example 1 Polyester film [Lumirror T-60 Toray Co., Ltd.
The thickness of 100 μm] was coated on one side with the following composition using an applicator having a gap height of 75 μm. After coating, hot air drying was performed in a drying oven to obtain a light diffusion sheet. Binder resin: Polyester resin [Vylonal MD12000 manufactured by Toyobo Co., Ltd. 34% binder resin content] 100 parts by weight Polymer fine particles: Flat irregular shaped fine particles having an average particle diameter of 1.2 μm obtained in Synthesis Example 1 Parts by weight

Example 2 Polyester film [Lumirror T-60 thickness 100
[μm] on one side with the following composition and a gap height of 75 μm
Was coated using the applicator of. After coating, hot air drying was performed in a drying oven to obtain a light diffusion sheet. Binder resin: Polyester resin [Vylonal MD12000 manufactured by Toyobo Co., Ltd., binder content of 34%] ...... 100 parts by weight Polymer fine particles: Flat polymer particles having an average particle diameter of 1.2 µm obtained in Synthesis Example 1 ...... 12 parts by weight

Example 3 Polyester film [Lumirror T-60 thickness 100
[μm] on one side with the following composition and a gap height of 75 μm
Was coated using the applicator of. After coating, hot air drying was performed in a drying oven to obtain a light diffusion sheet. Binder resin: Polyester resin [Vylonal MD12000 manufactured by Toyobo Co., Ltd., 34% binder resin content] 100 parts by weight Polymer fine particles: Flat irregular shaped fine particles having an average particle diameter of 1.2 μm obtained in Synthesis Example 1 85 Parts by weight Solvent: Methyl ethyl ketone ...... 75 parts by weight

Comparative Example 1 As polymer fine particles, Tospearl 120 [average particle diameter 2 μm
m Organic Silicone Fine Particles manufactured by Toshiba Silicone Co., Ltd.]
A light diffusing sheet was obtained in the same manner as in Example 1 except that 34 parts by weight of was used.

Comparative Example 2 The same procedure as in Example 1 was carried out except that 34 parts by weight of MBX-2 [average particle size 2 μm polymethylmethacrylate particles manufactured by Sekisui Chemical Co., Ltd.] was used as the polymer particles. A diffusion sheet was obtained.

Comparative Example 3 Polyester film [Lumirror T-60 thickness 100
[μm] on one side with the following composition and a gap height of 75 μm
Was coated using the applicator of. After coating, hot air drying was performed in a drying oven to obtain a light diffusion sheet. Binder resin: polyester resin [Vylon 200 manufactured by Toyobo Co., Ltd.] 50 parts by weight Polymer fine particles: cross-linked polystyrene [average particle size 12 μm SBX-12 manufactured by Sekisui Plastics Co., Ltd.] 50 parts by weight solvent: toluene 75 parts by weight Methyl ethyl ketone ...... 75 parts by weight The optical characteristics of the light diffusion sheet obtained in each of the examples and comparative examples were evaluated. The evaluation was performed by the following method.

[Measurement of Light Transmittance and Haze] The total light transmittance and haze were measured by a haze meter [NDH-2000 manufactured by Nippon Denshoku Co., Ltd.]. A product having a total light transmittance of 80% or more and a haze of 70% or more was judged as a good product.

[Measurement of angle characteristic of transmitted light intensity] The angle characteristic of transmitted light intensity was measured by an automatic gonio-type photometer [GP-200 manufactured by Murakami Color Research Laboratory Co., Ltd.]. The results are shown in Table 1 and FIGS. 2-7. The light condensing property is measured by setting the transmitted light intensity in the front direction to 100 and the angle γ when the transmitted light intensity becomes 33.
Was measured. As apparent from FIGS. 2 to 7, when the light diffusing sheet of each example is used, the transmitted light intensity in the vicinity of the front surface (angle of 0 degree) is extremely larger than that of the light diffusing sheet of each comparative example. The light-condensing property was significantly higher than that of the light-type light diffusing sheet. Further, the light diffusion sheet of each example has a sufficient haze, and the pattern on the light guide plate is diffused and is not visually recognized.

[0024]

[Table 1]

[0025]

The light diffusing sheet of the present invention contains flat irregularly shaped fine particles in the light diffusing layer, whereby the amount of light emitted in the front direction can be extremely increased as compared with the conventional light diffusing sheet. It has excellent light properties. Therefore, when this is used for a liquid crystal display, its brightness can be dramatically improved.

[Brief description of drawings]

1 is an electron micrograph of flat irregular-shaped fine particles obtained in Synthesis Example 1. FIG.

FIG. 2 is a graph showing angular characteristics of transmitted light intensity of the light diffusion sheet obtained in Example 1.

FIG. 3 is a graph showing the angle characteristic of transmitted light intensity of the light diffusion sheet obtained in Example 2.

FIG. 4 is a graph showing angular characteristics of transmitted light intensity of the light diffusion sheet obtained in Example 3.

5 is a graph showing angular characteristics of transmitted light intensity of the light diffusion sheet obtained in Comparative Example 1. FIG.

FIG. 6 is a graph showing the angular characteristic of transmitted light intensity of the light diffusion sheet obtained in Comparative Example 2.

7 is a graph showing the angle characteristic of transmitted light intensity of the light diffusion sheet obtained in Comparative Example 3. FIG.

─────────────────────────────────────────────────── ─── Continuation of front page (56) Reference JP-A-9-176366 (JP, A) JP-A-9-281307 (JP, A) JP-A-9-171709 (JP, A) JP-A-9- 96705 (JP, A) JP 9-281339 (JP, A) JP 10-3079 (JP, A) JP 9-211208 (JP, A) JP 11-109113 (JP, A) JP-A-10-265580 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G02B 5/02

Claims (2)

(57) [Claims] 1. A light diffusion sheet having at least a light diffusing layer made of a binder resin and polymer microparticles on one surface of the transparent substrate, the polymer particles Ri is Na <br/> from vinyl polymer, the shape of the plane Is almost circular, the front surface is convex, the back surface is
Light diffusion sheet, which is a concave der Ru flat profiled microparticles. 2. The light diffusion sheet according to claim 1, wherein the light diffusion layer contains 30 to 300 parts by weight of polymer fine particles with respect to 100 parts by weight of the binder resin.