JP2002258011A - Light diffusion sheet and method of producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light diffusion sheet excellent in light diffusion efficiency (haze degree) and light transmittance (light utilization efficiency) and having uniform characteristics and to provide a method of producing the light diffusion sheet. SOLUTION: The light diffusion sheet comprises a transparent resin body containing a dispersed light diffusing agent and this light diffusing agent comprises platy particles of silica having <=50 μm average particle diameter and an aspect ratio of 5-300, preferably secondary particles having a laminated structure formed by the face-to-face parallel orientation and overlapping of primary particles of foliate silica comprising layered polysilicic acid, e.g. silica X or silica Y, particularly a plurality of foliate silicas.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light diffusion efficiency (haze degree) and a light transmittance (light utilization efficiency) used for a display device such as a liquid crystal device, a projection television, and a surface light utilization device such as a lighting device. The present invention relates to a novel light-diffusing sheet having excellent characteristics, excellent in performance, and having no variation in performance among lots, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, in a surface light utilization device such as a liquid crystal display device, a projection television, and a lighting device, a light diffusion sheet is disposed adjacent to a tube-shaped light source or a light guide plate. The light is diffused to make the luminance uniform. This light diffusion sheet has a great need as the demand for a liquid crystal display device and the like increases rapidly, and is used in large quantities.

A light diffusion sheet is disclosed in, for example,
-289075, JP-A-6-347617, JP-A-11-2706 and JP-A-9-96
As shown in No. 705, a sheet in which the surface of a transparent substrate such as a sheet or a film of glass, polyester, polycarbonate or the like is embossed or coated with a transparent fine particle or the like, and There is known a sheet in which a light diffusing agent having a refractive index different from that of a substrate such as inorganic particles is dispersed in the substrate.

The characteristics of the light diffusion sheet mainly include light transmittance,
Haze, reflection characteristics, transmission color, degree of depolarization, is evaluated in terms of backscattering, etc., the latter light diffusion sheet in which a light diffusing agent such as inorganic particles is dispersed in the transparent substrate is generally
Compared with the former light-diffusion sheet subjected to unevenness, it is widely used because of its advantages such as high performance retention, easy production, and suitability for mass production. In the latter light diffusing sheet, the light diffusing agent such as inorganic particles contained and dispersed therein together with the transparent base material which is the base material is important for determining the properties of the light diffusing sheet.

[0005] As such light diffusing agents, conventionally, natural or synthetic materials such as mica (mica), kaolin, talc, surface-coated mica, crystalline silica, amorphous silica, glass, barium sulfate, calcium carbonate, titanium dioxide and the like have been used. Materials are used. Above all, mica is a material with relatively high transparency and low light absorption, and at the same time, the shape is scale-like, so that the light diffusion effect is large, the bulk specific gravity is small, and the specific gravity difference with the base resin is small. Therefore, it is widely used because a light diffusion sheet having good uniform dispersibility in a base material and excellent characteristics can be obtained.

However, mica, which has been widely used as a light diffusing agent, is required to satisfy the originally contradictory characteristics of increasing light diffusion efficiency (haze degree) and light transmittance (light use efficiency). Further, there is room for improvement, and there is a lot-to-lot variation due to natural materials, and the performance of the manufactured light diffusing sheet becomes uneven. For this reason,
In order to improve the characteristics, mica whose surface is coated with titanium dioxide, iron oxide or the like has been proposed. However, the effect of improving the characteristics is not always sufficient, and the manufacturing cost increases, which is not always advantageous in terms of cost.

[0007]

SUMMARY OF THE INVENTION In view of the above circumstances, the present invention is demanded as a light diffusing agent in a light diffusing sheet of a surface light utilizing device such as a display device such as a liquid crystal, a projection television, and a lighting device. Novel light diffusion sheet having excellent characteristics, excellent light diffusion efficiency (haze degree) and light transmittance (light utilization efficiency), and having no variation in performance due to a light diffusion agent, and a method for producing the same. The purpose is to provide.

[0008]

The present inventors have conducted various studies on the light diffusing agent in the above light diffusing sheet. As a result, the following plate-like particles of silica having an average particle diameter and an aspect ratio in a specific range are preferable. Is composed of a layered polysilicic acid such as silica X or silica Y and has a very small thickness of nano-sized primary particles of leaf-shaped silica, particularly a silica having a laminated structure in which a plurality of such leaf-shaped primary particles are overlapped in parallel. It has been found that when a material composed of secondary particles is used as a light diffusing agent, a light diffusing sheet having excellent properties can be obtained.

That is, the light diffusion sheet according to the present invention comprises:
The light diffusion efficiency (haze degree) is large, and the light transmittance (light utilization efficiency) is large, and the light transmitted through the light diffusion sheet according to the present invention does not show a coloring phenomenon or a rainbow pattern.
Furthermore, characteristics such as no variation in performance due to the light diffusing agent are exhibited.

The reason why the light diffusing sheet of the present invention exhibits excellent properties is not necessarily clear, but the light diffusing agent used in the present invention generally has high transparency and high chemical stability. A silica material, preferably silica X
Alternatively, since silica made of a layered polysilicic acid such as silica Y is used as a material, its light transmittance (light utilization) is large, and it has an average particle diameter and an aspect ratio in a specific range. Mixed. Further, when these plate-like particles are subjected to a stretching treatment or the like at the time of forming a sheet, they are uniformly oriented in a uniaxial direction in the diffusion sheet, so that the light diffusion property is extremely large.

In particular, when the primary particles of the plate-like particles represented by the leaf-like silica are composed of the secondary particles of the leaf-like silica having a laminated structure formed by being superposed and oriented in parallel with each other, the incident light is incident. Since the light is diffused and reflected multiple times in the gaps and the particle surfaces where the plurality of leaf-like particles are stacked, it is considered that a large diffusion effect can be obtained. Further, since the silica plate-like particles are synthetic compounds having stable properties, unlike natural products such as mica, a light diffusion sheet having stable uniform characteristics without variation between lots can be obtained.

Further, silica plate-like particles typified by leaf-like silica particles have silanol groups (—S
The iOH) concentration has a value several times to several tens times that of ordinary silica gel. When the silica particles having such a high silanol concentration are contained and dispersed in the resin sheet, they exhibit self-forming properties due to condensation of silanol groups and the like, and binding between the leaf-like particles occurs, so that the strength of the light diffusion sheet increases. This makes it possible to reduce the thickness of the film,
It is considered that the gas permeability of oxygen, carbon dioxide and the like also decreases, and the characteristics as a light diffusion sheet are further improved. Note that the description of these mechanisms is not always clear and is only an inference, and does not restrict the interpretation of the present invention.

Thus, the present invention achieves the above object sufficiently, and has the following constitution. (1) A transparent resin body containing a light diffusing agent dispersed therein, wherein the light diffusing agent is plate-like silica particles having an average particle diameter of 50 μm or less and an aspect ratio of 5 to 300. Light diffusion sheet. (2) The light diffusion sheet according to the above (1), wherein the plate-like silica particles are composed of layered polysilicic acid. (3) The light diffusion sheet according to the above (2), wherein the layered polysilicic acid comprises silica X or silica Y. (4) A plate-like particle of silica has a laminated structure in which a plurality of leaf-like primary particles are superposed and oriented in parallel with each other between their surfaces, and have a thickness of 0.01 to 0.5 μm. (1), (2) or (3) having a surface diameter of 2 to 5 μm ^2.
The light diffusion sheet according to 1. (5) The light diffusion sheet according to any one of the above (1) to (4), wherein the plate-like particles of silica are oriented in the length direction of the sheet in the light diffusion sheet. (6) The light diffusion sheet according to any one of (1) to (5), wherein the plate-like particles of silica are contained in an amount of 0.01 to 100 parts by mass per 100 parts by mass of the transparent resin body. (7) A method for producing a transparent resin body containing a light diffusing agent dispersed therein, wherein a plate-like particle of silica having an average particle diameter of 50 μm or less and an aspect ratio of 5 to 300 is mixed with a liquid transparent resin body. A method for producing a light-diffusing sheet, comprising forming the obtained mixture into a sheet. (8) During or after forming the mixture into a sheet,
(7) The method for producing a light diffusion sheet according to the above (7), wherein the molded article is stretched in a length direction of the sheet so that plate-like silica particles are uniaxially oriented in the light diffusion sheet.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION A resin is preferably used as the transparent base material forming the light diffusion sheet of the present invention, and either a thermoplastic resin or a thermosetting resin can be used.
Among them, the use of a thermoplastic resin is preferred. Preferred examples of the thermoplastic resin include olefin resins such as polyethylene, polypropylene or poly-4-methylpentene resin; polyvinyl chloride resins, vinyl chloride resins such as vinyl chloride-vinyl acetate copolymer; polyvinyl acetate;
Vinyl acetate resins such as ethylene-vinyl acetate copolymer;
Polyvinyl alcohol resins such as polyvinyl alcohol and ethylene-vinyl alcohol copolymer; acrylic resins such as polymethyl methacrylate and methyl methacrylate-acrylate copolymer; polystyrene;
Styrene-based resins such as styrene-methyl methacrylate copolymer, rubber-modified polystyrene and ABS resin; polyester-based resins such as polyalkylene terephthalate and modified polyester; nylon 6, nylon 66, nylon 610, nylon 612, modified amide and the like Polyamide resins; polycarbonate resins and the like. Among them, the use of an acrylic resin, a polycarbonate resin or a poly-4-methylpentene resin is preferred.

In the present invention, the transparent base material contains
The light diffusing agent to be dispersed is composed of plate-like silica particles having an average particle diameter of 50 μm or less and an aspect ratio of 5 to 300. In the present invention, silica may be non-crystalline or crystalline, but preferably has high white transparency and a refractive index of 1.30 to 1.52. In the present invention, the average particle size of the plate-like particles of silica refers to a laser scattering type particle size measuring device (for example,
Means the median diameter measured by HORIBA LA-920, manufactured by Horiba, Ltd. If the average particle diameter exceeds 50 μm, the number of bright spots increases, and the uniformity of the emitted light decreases, resulting in insufficient performance. The average particle diameter is preferably small, but if it is excessively small, secondary aggregation is likely to occur. Above all,
The average particle diameter is preferably from 0.01 to 3 μm.

The aspect ratio of the light diffusing agent indicates the ratio of the major axis to the thickness of the silica plate-like particles. One of the features of the present invention is that the aspect ratio is sufficiently larger than that of a conventional substrate. In the present invention, the aspect ratio needs to be 5 to 300. When the aspect ratio is less than 5, the thickness increases, so that the transparency is lost and sufficient characteristics are hardly obtained. If the aspect ratio exceeds 300, the thickness is excessively reduced, the strength is reduced,
Cracking occurs in the process of mixing into resin to form a sheet, making it difficult to obtain a light diffusion sheet having uniform characteristics. Above all, the aspect ratio is 10 to 200, especially 20 to 100.
Is more preferable.

The silica plate-like particles having the above-mentioned average particle diameter and aspect ratio, which form the light diffusing material of the present invention, can be obtained, for example, by forming extremely fine particles into a sheet. , Silica composed of layered polysilicic acid. The layered polysilicic acid is a polysilicic acid having a silicate structure whose basic structural unit is composed of SiO ₄ tetrahedron, and in the present invention, means the H type and / or a salt type such as an alkali metal. Examples of these layered polysilicic acids include silica X, silica Y, Kenyaite, macadite, macatite, islayite, kanemite, octosilicate, and the like. Among them, silica X or silica Y is preferred in the present invention.

As the light diffusing material of the present invention, foliar silica particles composed of silica X or silica Y are preferably used. What is characteristic of these leaf-like silica particles is that the silanol groups (-SiOH) per specific surface area are 30%.
１００100 μmol / m ² , especially 50-70 μmol / m ²
m ^{2, which} means that the same silica material has a value several times to several tens times larger than that of silica gel or the like. Due to the large content of such silanol groups, it is thought that particularly excellent properties are exhibited when used as a light diffusing agent in the present invention. Here, the content of the silanol group is a numerical value determined by the following equation and measured by a thermogravimetric analysis method. (W × 1111.1) / SA In the above formula, W is from heating at 120 ° C. to 120
The difference in weight (%) at equilibrium up to heating at 0 ° C., and SA is the specific surface area by the BET method.

The plate-like silica particles used in the present invention include:
Although primary particles of foliar silica may be used, secondary particles having a laminated structure in which a plurality of primary particles of foliar silica are substantially oriented in parallel with each other and overlap each other are preferable. The secondary particles of the flaky silica have a thickness of preferably 0.01 to 0.5 μm, particularly preferably 0.05 to 0.5 μm.
~ 0.5μm, preferably 0.5 ~ 10μm surface diameter,
Particularly preferably, the thickness is 1 to 5 μm. If the thickness and the surface diameter do not have the above ranges, the desired properties cannot be obtained, and physical properties such as strength cannot be obtained, which is not preferable.

The secondary particles of the above-mentioned foliated silica form pores in the gaps between the primary particles of the laminated ultra-thin pieces of foliated silica, and the pore volume is preferably 0.01 μm.
1.0 ml / g, especially 0.10 to 0.15 ml / g
Is preferred. The specific surface area is preferably 20 to
200 m ² / g, particularly preferably 50 to 80 m ² / g.

The method for producing plate-like silica particles having a specific average particle diameter and aspect ratio used in the present invention is as follows.
Manufactured in various ways. Among them, foliar silica composed of layered polysilicic acid such as silica X or silica Y is preferably produced by the following method. That is, the hydrothermal treatment of the silica hydrogel, active silicic acid or hydrous silicic acid at 150 to 220 ° C. in the presence of an alkali metal salt allows the primary particles and the secondary particles to be three-dimensionally overlapped with the foliar silica. Tertiary aggregated particles can be produced. The tertiary aggregated particles are pulverized and dispersed using a wet pulverizer or the like, whereby the above-mentioned secondary particles of leaf-like silica are produced. These foliar silicas are sold by Asahi Glass Co., Ltd. under the trade name “Sun Lovely (registered trademark)”.

The light-diffusing sheet according to the present invention containing a light-diffusing agent composed of silica plate-like particles is preferably produced as follows. Prepare a solution such as a resin that forms a transparent base material, an emulsion, a dispersion or a suspension, uniformly mix plate-like particles of silica or a slurry-like aqueous dispersion containing the same, and sheet the mixture. It is obtained by molding into. When an aqueous dispersion of plate-like silica particles is used, the concentration of the plate-like particles is preferably 1 to 30% by mass, particularly preferably 5 to 20% by mass. Extrusion molding, injection molding, compression molding, cast molding, continuous cast molding, cast molding and the like are used as a method for molding into a sheet. In molding, preferably 2 to 10 in the vertical or horizontal direction, respectively.
The light diffusing agent is stretched by a factor of 1 in the length direction of the sheet.
A light diffusion sheet oriented in the axial direction can be obtained. Further, upon forming into a sheet, a release agent, a heat stabilizer, an ultraviolet absorber, a coloring agent, and the like can be added to the mixture as needed.

The thickness of the light diffusion sheet in the present invention may be a thickness called a film or a plate, and is appropriately selected according to the purpose of use. However, in order to allow the light diffusing agent to be uniformly and sufficiently dispersed therein, preferably,
0.05 μm to 20 mm, especially 0.1 to 200 μm,
Especially, 1-50 micrometers is suitable. The content of the light diffusing agent dispersed in the light diffusion sheet is selected depending on the size of the particles and the purpose of use, but is preferably 0.01 to 100 parts by mass, particularly preferably 0 to 100 parts by mass per 100 parts by mass of the diffusion sheet. 0.05 to 50 parts by mass, particularly preferably 0.1 to 10 parts by mass.

The light diffusion sheet of the present invention has sufficiently excellent performance as it is, but preferably, its properties can be further improved by providing an antireflection tank on its surface. The anti-reflection layer on the surface prevents reflection of light incident on the light diffusion sheet, increases the amount of incident light, improves the light utilization rate, and is also effective in preventing visual interference due to reflected light. The antireflection layer preferably has a thickness of 10 to 50.
A known antireflection film such as an inorganic fluoride-containing layer having a thickness of nm and a fluorine-based resin layer can be used.

Since the light diffusing sheet of the present invention has the above-mentioned excellent characteristics, it can be used in a wide range of fields such as a liquid crystal display device, a projection television, a lighting device, and a surface light utilizing device including a window shutter. In particular, when the light diffusion sheet of the present invention is used for a display device such as a liquid crystal, excellent performance is exhibited. This display device exhibits excellent performance whether it is a transmissive (backlight) display device or a reflective display device.

In the transmission type display device, an image is visually recognized by light transmitted through the light diffusion sheet. In this case, among the light diffusion sheets of the present invention, the light diffusion agent is provided in the length direction of the sheet. When a uniaxially oriented light diffusion sheet is selected and used in parallel with the longitudinal direction of a tubular fluorescent tube as a light source, the light diffusion sheet can be uniformly applied to the display panel without lowering the overall luminance. This is preferable because surface light emission can be performed.

In the reflection type display device, external light is used without incorporating an internal light source. If the light diffusing sheet has excessively large diffusing performance from a specific direction, the sharpness of an image is deteriorated. However, in this case, among the light diffusion sheets of the present invention, when a light diffusion sheet in which a light diffusion agent is oriented in a random direction in the sheet is used, external light at all angles is diffused, and an image of an image is diffused. This is preferable because sharpness is ensured.

[0028]

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples of the present invention. However, it is needless to say that the present invention is not limited to these examples.

Example 1 A water-based acrylic resin emulsion was produced as follows. 60 g of deionized water was charged into the polymerization reactor, and the temperature was raised to 60 ° C. On the other hand, methyl methacrylate / styrene / 2-ethylhexyl acrylate (molar ratio: 40
/ 25/35), 64 g of surfactant Sannol NP-2030 (trade name of Lion Corporation) and 40 g of deionized water were added and mixed well to obtain a uniform emulsion.

5 g of this emulsion was placed in a polymerization vessel,
When the internal temperature was stabilized, a solution prepared by dissolving 0.1 g of sodium persulfate in 5 g of deionized water was added, and the remaining emulsion was added while maintaining the internal temperature of the polymerization vessel at 80 ° C.
It was dropped over time. After completion of the dropwise addition, the reaction was completed at an internal temperature of 80 ° C. for 2 hours. After completion of the reaction, the reaction mixture was cooled, and ammonia water was added to obtain an emulsion having a pH of 8.0.

On the other hand, a slurry of "Sun Lovely LFS" (trade name of Asahi Glass Co., Ltd.) having a solid concentration of 5% by mass as secondary particles of foliar silica was prepared. The silica particles in the slurry are silica X particles, and are secondary particles of silica having a layered structure in which a plurality of leaf-like primary particles are formed by overlapping and oriented in parallel, and have a thickness of 0.1 μm.
m, leaf-shaped particles having a surface diameter of 4 μm were the largest, the average particle diameter was 1.2 μm, and the aspect ratio was 40. The pore volume is 0.13 ml / g and the specific surface area is 5
It was 5 m ² / g. Further, the content of the silanol group per specific surface area was 65 μmol / m ² .

The above-mentioned aqueous acrylic resin emulsion 10
The above leaf-like silica slurry was added so as to be 1 g (in terms of solid content) with respect to 0 g (in terms of solid content), and was uniformly mixed and dispersed. The dispersion was cast on a glass plate and cured at 120 ° C. for 30 minutes to obtain a light diffusion sheet having a thickness of 50 μm. The obtained light diffusion sheet had a total light transmittance of 90% and a haze value of 90%.

The above characteristic values were measured using an optical system measuring device. The measuring device is such that a light from a light source passes through a light diffusion sheet, an integrating sphere, a shutter, and a detector in this order, and a haze meter (HGX-3P) is used.
D, manufactured by Suga Test Instruments Co., Ltd.).

Example 2 (Comparative Example) In Example 1, mica (average particle diameter 10 μm, aspect ratio 10) was used in place of the leaf-like silica slurry so as to become 1 g in terms of solid content, and an aqueous system containing the same was used. Acrylic resin emulsion dispersion is cast on a glass plate,
By curing at 20 ° C for 30 minutes, a thickness of 50μ
m light diffusion sheet was obtained. The obtained light diffusion sheet had a total light transmittance of 70% and a haze value of 40%.

[0035]

According to the present invention, plate-like particles of silica having a specific range of an average particle diameter and an aspect ratio are used as a diffusing agent contained in a light diffusing sheet. A novel light diffusion sheet having excellent characteristics such as light transmittance (light use efficiency) and having no variation in characteristics is provided.

The light diffusion sheet of the present invention is used for a display device such as a liquid crystal display, a projection television, and a surface light utilization device such as various lighting devices. It is suitable for use in equipment.

Further, according to the present invention, there are provided both a light diffusing sheet in which the light diffusing agent in the sheet is substantially oriented in one axis direction and a light diffusing sheet in which the light diffusing agent is randomly oriented, The former exhibits excellent characteristics as a light diffusion sheet in a transmissive display device and the latter in a reflective display device. Further, according to the present invention, there is also provided a method for producing a light diffusion sheet having the above-described excellent characteristics.

Claims (8)

[The claims] 1. A transparent resin body containing a light diffusing agent dispersed therein, wherein said light diffusing agent is plate-like silica particles having an average particle diameter of 50 μm or less and an aspect ratio of 5 to 300. Light diffusion sheet. 2. The light diffusing sheet according to claim 1, wherein the silica plate-like particles are composed of layered polysilicic acid. 3. The layered polysilicic acid is silica X or silica Y.
The light diffusion sheet according to claim 2, comprising: 4. A laminated structure in which a plurality of plate-like silica particles are formed by overlapping a plurality of leaf-like primary particles oriented parallel to each other between their surfaces and having a thickness of 0.01 to 0. .5μ
The light diffusion sheet according to claim 1, 2 or 3, wherein m and the surface diameter are 0.5 to 10 µm ² . 5. The light diffusing sheet according to claim 1, wherein the plate-like particles of silica are substantially oriented in one axial direction of the sheet in the light diffusing sheet. 6. The light diffusion sheet according to claim 1, wherein the plate-like particles of silica are contained in an amount of 0.01 to 100 parts by mass per 100 parts by mass of the transparent resin body. 7. A method for producing a transparent resin body containing a light diffusing agent dispersed therein, wherein plate-like particles of silica having an average particle diameter of 50 μm or less and an aspect ratio of 5 to 300 and a liquid transparent resin body are uniformly formed. A method for producing a light diffusion sheet, comprising forming a mixture obtained by mixing into a sheet into a sheet. 8. The molded product is stretched in the longitudinal direction of the sheet during or after the mixture is formed into a sheet, so that the silica plate-like particles are uniaxially oriented in the light diffusion sheet. 7. The method for manufacturing a light diffusion sheet according to 7.