WO2006083101A1

WO2006083101A1 - Light-diffusion plate

Info

Publication number: WO2006083101A1
Application number: PCT/KR2006/000346
Authority: WO
Inventors: Byeong Heui Han; Hee Cheong Lee; Dong Hyun Kim; Jun Kil Doo; Yoon Hee Cho; Tae Yong Ryu; Chung Seock Kang
Original assignee: Kolon Ind. Inc./Kr
Priority date: 2005-02-01
Filing date: 2006-02-01
Publication date: 2006-08-10
Also published as: TW200706576A

Abstract

Disclosed is a light-diffusion plate for use in a backlight unit of a liquid crystal display or an illumination apparatus. Specifically, this invention provides a light-diffusion plate including a substrate layer composed mainly of a first base resin; and a surface layer composed mainly of a second base resin the same as or different from the first base resin of the substrate layer and including fluorine resin particles formed on either or both surfaces of the substrate layer. Thus, such a light-diffusion plate exhibits high dimensional stability thanks to high heat resistance and moisture resistance, and seldom causes a bending phenomenon even under conditions of high temperature and high humidity. Further, the light-diffusion plate has excellent light properties including total luminous transmittance, and also has a low yellowing index even upon exposure to a light source for a long period of time.

Description

LIGHT-DIFFUSION PLATE

Technical Field

The present invention relates to a light-diffusion plate for use in a backlight unit of a liquid crystal display (LCD) or an illumination apparatus .

Background Art

As conventional light-diffusion plates , there are disclosed © a light-diffusion plate having surface protrusions through a physical process (Japanese Patent Laid- open Publication No . Hei . 4-275501) , (2) a light-diffusion plate obtained by coating a transparent substrate of a polyester resin with a light-diffusion layer containing transparent resin particles (Japanese Patent Laid-open Publication No . Hei . 6-59108 ) , © a light-diffusion plate obtained by melt-mixing beads with a transparent resin and then extruding a mixture of beads and resin, (Japanese Patent

Laid-open Publication No . Hei . 6-123802 ) , and ® a light- diffusion plate having a sea-island structure formed by melt- kneading at least two kinds of transparent thermoplastic resins (Japanese Patent Laid-open Publication No . Hei . 9- 311205) .

The light-diffusion plates of © and (2) are a surface light-diffusion plate exhibiting a light-diffusing effect by the surface protrusions or coated light-diffusion layer . In addition, the light-diffusion plates of (3) and ® are a light- diffusion plate having the light-diffusing component even in the substrate . Typically, a light-diffusion plate prepared mainly using a methylmethacrylate resin is advantageous because it has excellent light properties, such as total luminous transmittance . However, since such a light-diffusion plate has low dimensional stability, when it is used along with a light source such as a cold cathode fluorescent lamp or an LED and a change in temperature is caused by turning on and off the light source, the water absorption rate of the light- diffusion plate may easily vary, therefore causing problems including deformation, leading to bending, corrugations, or cracking . That is, under conditions of high temperature and high humidity, deformation such as a bending phenomenon may be caused (Japanese Patent Laid-open Publication Nos . Hei . 07-100985 and 08-198976) .

In order to solve such problems, a multi-layer sheet including methylmethacrylate-styrene capable of decreasing absorption ability is proposed (Japanese Patent Laid-open Publication No. 2004-37483 and Korean Patent Laid-open Publication No . 2003-95262 ) . However, the resin used essentially has insufficient light resistance, and may entail a problem of deterioration such as discoloration. As a known process to improve light resistance of the resin, a UV absorbent may be added. However, in some of resins having low light resistance, such improvement effects are not sufficient.

Disclosure of the Invention

Leading to the present invention, intensive and thorough research on light-diffusion plates, carried out by the present inventors aiming to avoid the problems encountered in the related art, led to the development of a light-diffusion plate having high dimensional stability thanks to high heat resistance and low absorption performance even under conditions of high temperature and high humidity, without a yellowing phenomenon even upon exposure to a light source for a long period of time by virtue of light resistance of a surface layer .

Therefore, it is an object of the present invention to provide a light-diffusion plate having high dimensional stability, thus generating less of a bending phenomenon even under conditions of high temperature and high humidity.

Another object of the present invention is to provide a light-diffusion plate which generates less of a yellowing phenomenon.

In order to achieve the above objects, the present invention provides a light-diffusion plate comprising a substrate layer composed mainly of a first base resin; and a surface layer composed mainly of a second base resin the same as or different from the first base resin of the substrate layer and including fluorine resin particles formed on either or both surfaces of the substrate layer.

The fluorine resin particles may comprise particles of at least one fluorine resin selected from the group consisting of a vinylidenefluoride (VDF) homopolymer, a copolymer of vinylidenefluoride (VDF) and methylmethacrylate, a copolymer of vinylidenefluoride (VDF) and hexafluoropropylene (HFP) , a copolymer of vinylidenefluoride (VDF) and tetrafluoroethylene (TFE) , and a terpolymer of vinylidenefluoride (VDF) , hexafluoropropylene (HFP) and tetrafluoroethylene (TFE) .

The fluorine resin particles of the surface layer may has a particle size of 0.2~50 μm. The fluorine resin particles of the surface layer may be used in an amount of 0.5~35 wt%, based on the total weight of the composition of the surface layer.

The first base resin or second base resin may comprise at least one selected from the group consisting of acrylic resins, styrene resins, styrene-acrylic copolymer resins, and polycarbonate resins .

As such, the acrylic resin preferably comprises a homopolymer, a copolymer, or mixtures thereof obtained using at least one monomer selected from the group consisting of methacrylic acid alkylester, acrylic acid alkylester, methacrylic acid cycloalkylester, • acrylic acid cycloalkylester, methacrylic acid arylester, and acrylic acid arylester .

The styrene resin preferably comprises a homopolymer, a copolymer, or mixtures thereof obtained using at least one monomer selected from the group consisting of styrene, α- methylstyrene, m-methylstyrene, p-methylstyrene and p~ methoxystyrene .

The styrene-acrylic copolymer resin is preferably a copolymer obtained using at least one acrylic monomer selected from the group consisting of methacrylic acid alkylester, acrylic acid alkylester, methacrylic acid cycloalkylester, acrylic acid cycloalkylester, methacrylic acid arylester, and acrylic acid arylester and at least one styrene monomer selected from the group consisting of styrene, α-methylstyrene, m-methylstyrene, p-methylstyrene and p-methoxystyrene .

The styrene-acrylic copolymer resin may be obtained by reacting the acrylic monomer with the styrene monomer at a weight ratio of 9 : 1-1 : 9. Further, the styrene-acrylic copolymer resin may be obtained by reacting the acrylic monomer with the styrene monomer at a weight ratio of 6 : 4~2 : 8.

In addition, at least one layer selected from the substrate layer and the surface layer may further comprise a light-diffusing agent . The surface layer may have an embossed shape caused by protrusion of the fluorine resin particles or light-diffusing agent from a surface thereof .

Such a surface layer preferably has a surface roughness of 0.1-50 μm.

The light-diffusing agent may be used in an amount of 0.1-35 wt%, based on the total weight of the composition of the substrate layer or surface layer .

In addition, at least one layer selected from the substrate layer and the surface layer may further comprise a light stabilizer .

Such a light stabilizer may be used in an amount of 0.01-5 wt%, based on the total weight of the composition of the substrate layer or surface layer .

Best Mode for Carrying Out the Invention

Hereinafter, a detailed description will be given of the present invention .

A light-diffusion plate of the present invention comprises a substrate layer and a surface layer formed on either or both surfaces of the substrate layer, in which the surface layer is composed mainly of a base resin the same as or different from the base resin of the substrate layer, and further includes fluorine resin particles .

In the light-diffusion plate of the present invention, as a first base resin of the substrate layer or a second base resin of the surface layer, at least one selected from among acrylic resins , styrene resins, styrene-acrylic copolymer resins and polycarbonate resins , having excellent light transmittance and light resistance, may be used . The acrylic resin preferably comprises a homopolymer, a copolymer, or mixtures thereof obtained using at least one monomer selected from the group consisting of methacrylic acid alkylester, such as methylmethacrylate, ethylmethacrylate, butylmethacrylate, 2- ethylhexylmethacrylate, etc . ; acrylic acid alkylester, such as methylacrylate, ethylacrylate, butylacrylate, etc . ; methacrylic acid cycloalkylester, such as cyclohexylmethacrylate, 2-methylcyclohexylmethacrylate, dicyclopentanylmethacrylate, etc . ; acrylic acid cycloalkylester, such as cyclohexylacrylate, 2- methylcyclohexylacrylate, etc . ; methacrylic acid arylester, such as phenylmethacrylate, benzylmethacrylate, etc . ; acrylic acid arylester, such as phenylacrylate, benzylacrylate, etc .

The styrene resin preferably comprises a homopolymer, a copolymer, or mixtures thereof obtained using at least one monomer selected from the group consisting of styrene, α- methylstyrene, m-methylstyrene, p-methylstyrene, and p- methoxystyrene .

The styrene-acrylic copolymer resin is prepared using the above-mentioned resin as the acrylic monomer and styrene monomer . In particular, in the styrene-acrylic copolymer resin used in the present invention, the acrylic monomer and styrene monomer are preferably copolymerized at a weight ratio of 9 : 1~1 : 9, and more preferably at a ratio of 6 : 4~2 : 8 in consideration of adhesion to the substrate layer.

As the polycarbonate resin, the polycarbonate resin may be used alone, or the mixture comprising polycarbonate resin and polystyrene resin may be used.

The polycarbonate resin has excellent impact resistance, light transmittance, cold resistance and electrical properties, and in particular has high heat resistance and absorption resistance, and thus is outstanding in dimensional stability and has a wide temperature range . Hence, this resin is mainly used for optical lenses, optical disk materials, helmets, guards, covers, etc .

The polycarbonate resin used in the present invention is a general aromatic polycarbonate resin, which includes linear and branched carbonate homopolymers, polyester copolymers, or mixtures thereof resulting from the reaction of dihydroxyphenol and phosgene or of dihydroxyphenol and a carbonate precursor. . As such, dihydroxyphenol is exemplified by 2, 2-bis (4-hydroxyphenyl) propane (bisphenol A) , bis (4-hydroxyphenyl)methane, 2, 2-bis (4-hydroxy-3, 5- dimethylphenyl) propane and 1, 1-bis (4-hydroxyphenyl) cyclohexane, and the carbonate precursor is exemplified by diphenylcarbonate, carbonyl halide, and diarylcarbonate. The polycarbonate resin preferably has a melt index (MI) of 7-30 g/10min at 300⁰C under a load of 1.2 kg according to ASTM D1238.

Although the polycarbonate resin has excellent impact resistance, absorption resistance and light transmittance, it is disadvantageously expensive . Thus, the polycarbonate resin may be mixed with the polystyrene resin having a similar refractive index and being relatively inexpensive, in order to reinforce stiffness of the polycarbonate resin, decrease material expense in preparation cost, and maintain and improve optical and mechanical properties .

Since the polystyrene resin used to prepare the base resin is hard, colorless, transparent, inexpensive (thanks to mass production) , and has excellent electrical properties, it has been widely used for daily necessities such as kitchen utensils , writing materials, furniture, etc . , and electrochemical articles, such as large automobile molds, television cabinets, etc .

The polystyrene resin used in the present invention has a melt index (MI) of 0.5-3 g/10min at 200°C under a load of 5 kg according to ASTM D1238.

With the intention of mixing the polycarbonate resin with the polystyrene resin, these resins may be melt-kneaded at 200-300⁰C, preferably 25O⁰C, and at a motor speed of 250 rpm using a twin-screw extruder having a screw diameter of 30 mm. As such, the polycarbonate resin and polystyrene resin may be mixed at a ratio of 1 : 9~9 : 1. Each of these resins should be added in an amount of at least 10% to exhibit advantages of polycarbonate, for example, flexibility and dimensional stability, and advantages of polystyrene, for example, high absorption resistance and strength .

The fluorine resin particles have excellent thermal stability and electrical properties, and are superior in chemical durability, weatherability, light resistance and oxygen resistance, and in particular, have a very low water absorption rate and high heat resistance, and thus have a high service temperature of 250~300°C . Also, since such resin particles have outstanding surface friction resistance, they are presently applied to various valves, pumps, tanks, filters, pipes, cables, and computers and are widely used in the aerospace industry.

Particularly, the fluorine resin particles used in the present invention preferably comprise particles of at least one fluorine resin selected from the group consisting of a vinylidenefluoride (VDF) homopolymer, a copolymer of vinylidenefluoride (VDF) and methylmethacrylate, a copolymer of vinylidenefluoride (VDF) and hexafluoropropylene (HFP) , a copolymer of vinylidenefluoride (VDF) and tetrafluoroethylene (TFE) , and a terpolymer of vinylidenefluoride (VDF) , hexafluoropropylene (HFP) and tetrafluoroethylene (TFE) .

The application of the fluorine resin particles to the light-diffusion plate results in superior water repellent properties with a light-diffusing function. That is, the fluorine resin particles protrude from the surface of the surface layer to form an embossed shape, thereby exhibiting a non-glossy surface and low reflectivity. As such, the surface layer preferably has a surface roughness of about 0.1-50 μm. If the roughness exceeds 50 μm, surface impact strength becomes insufficient . On the other hand, if the roughness is less than 0.1 μm, non-glossy effects are deteriorated. Hence, it is preferred that the fluorine resin particles used in the present invention have a particle size of 0.2-50 μm.

Further, in order to exhibit effects depending on the use of fluorine resin particles, the fluorine resin is used in an amount of 0.5-35 wt%, based on the total weight of the composition of the surface layer .

In addition, the substrate layer or surface layer may further include a light-diffusing agent, which has a refractive index different from the base resin and is used to increase the diffusion rate of light . To this end, various organic and inorganic particles may be used. As such, if the light-diffusing agent has a large difference in refractive index from the base resin, even though it is used in a small amount, desired light-diffusing effects may be exhibited. On the other hand, if a difference in refractive index is small, this material should be used in a relatively large amount .

Typically, examples of the organic particles include acrylic polymer particles, such as methylmethacrylate, ethylmethacrylate, isobutylmethacrylate, n-butylmethacrylate, n-butylmethylmethacrylate, acrylic acid, methacrylic acid, hydroxyethylmethacrylate, hydroxypropylmethacrylate, hydroxyethylacrylate, acrylamide, methylolacrylamide, glycidylmethacrylate, ethylacrylate, isobutylacrylate, n- butylacrylate, 2-ethylhexylacrylate polymers, or copolymers or terpolymers thereof; olefin polymer particles, such as polyethylene and polypropylene; acryl-olefin copolymer particles, multi-layered multicomponent particles obtained by forming a layer of the homopolymer, copolymer or terpolymer particles and then applying another monomer layer onto the above polymer particle layer, siloxane polymer particles, tetrafluoroethylene particles, etc.

Examples of the inorganic light-diffusion particles include calcium carbonate, barium sulfate, silicon oxide, aluminum hydroxide, titanium oxide, zirconium oxide, magnesium fluoride, talc, glass, mica, etc. Typically, the organic particles have superior light diffusibility to inorganic particles . Also, at least two kinds of light- diffusion particles may be mixed, if necessary.

When the light-diffusing agent is further included in the surface layer, light-diffusing effects may be more increased. As well, the light-diffusion particles protrude from the surface of the surface layer to form an embossed shape, thereby exhibiting a non-glossy surface and low reflectivity. Also, handling and management is easy upon manual working, thus increasing workability. As such, the surface layer preferably has a surface roughness of about 0.1-50 i-iin. If the roughness exceeds 50 μm, surface impact strength becomes insufficient . On the other hand, if the roughness is less than 0.1 μm, non-glossy effects are deteriorated.

Accordingly, it is preferred that the light-diffusing agent has a particle size of 0.2-50 μm.

The light-diffusing agent is contained in the substrate layer or surface layer of the present invention in an amount of 0.01-35 wt% , preferably 0.5-15 wt% , based on the total weight of the composition of the base resin of the substrate layer or surface layer . When the amount of light- diffusing agent is less than 0.01 wt% , it is difficult to expect sufficient light-diffusing effects and masking effects . On the other hand, when the above amount exceeds 35 wt%, light transmittance becomes bad. As such, the amount of light-diffusing agent may be determined depending on the difference in refractive index from the base resin .

In addition, the substrate layer or surface layer may further include a light stabilizer. Examples of the light stabilizer include a UV absorbent having a maximum absorption wavelength of 250-380 nm, or a radical scavenger such as a hindered amine UV stabilizer capable of maximizing a light stabilizing effect . This component should exhibit such an effect for a long period of time, and should not be separated or removed from the sheet due to evaporation or extraction. An absorbent having high compatibility with the substrate should be selected.

The UV absorbent may be selected from the group consisting of cyanoacryl, salicylate, malonic acid ester, oxalic anilide, diketone, hydroxyl benzophenone, hydroxy benzotriazole, organic metals, and mixtures thereof .

The UV stabilizer may be selected from the group consisting of piperidinyl ester, oxazolidine and piperidino- oxazolidine, piperidispiroacetal, diazacycloalkanone, and mixtures thereof .

The light stabilizer may be used in an amount of 0.01-5 wt%, and preferably 0.1~2 wt%, based on the total weight of the resin composition of the substrate layer or surface layer . In particular, this component is contained in the surface layer, thereby obtaining a desired light stabilizing effect, without a decrease in total luminous transmittance or physical properties of the substrate layer . In addition, the preparation cost may be reduced.

The surface layer is preferably 20-200 μm thick, and more preferably 30-100 μm thick. If the thickness of surface layer is less than 20 μm, the surface layer is difficult to process and the thickness of surface layer becomes nonuniform. As well, the water absorption rate is not sufficiently decreased, and thus improvement effects in dimensional stability and light resistance are decreased. On the other hand, if the above thickness exceeds 200 μm,- dimensional stability and light resistance are improved because of the low water absorption rate, but material cost is increased, negating economic benefits . Hence, the surface layer of the present invention is formed on either or both surfaces of the substrate layer . Thus, although the light- diffusion plate of the present invention is formed to be much thinner than a conventional light-diffusion plate, it can exhibit effects equal or superior to such a conventional light-diffusion plate .

The light-diffusion plate of the present invention may be used for various indoor or outdoor purposes . That is, the light-diffusion plate may be applied to signboards, illuminated signboards, illuminated covers, glass display cases, etc . , and may be preferably used as light-diffusion plates for displays . The light-diffusion plate for a display is typically exemplified by light-diffusion plates for backlight units of LCDs or edge-light-type backlight units .

A better understanding of the present invention may be obtained in light of the following examples which are set forth to illustrate, but are not to be construed to limit the present invention .

In the examples of the present invention, fluorine resin particles were contained in a surface layer . In Examples 1 to 12 and 46 to 57 , PMMA was applied to a substrate layer; the compositions and composition ratios of the substrate layer and surface layer are shown in Table 1 below. In Examples 13 to 24 and 58 to 69, MS was applied to a substrate layer; the compositions and composition ratios of the substrate layer and surface layer are shown in Table 2 below. In Examples 25 to 32 and 70 to 77, PS was applied to a substrate layer; the compositions and composition ratios of the substrate layer and surface layer are shown in Table 3 below. In addition, in Examples 33 to 37 and 78 to 82, PC was applied to a substrate layer; the compositions and composition ratios of the substrate layer and surface layer are shown in Table 4 below. In Examples 38 to 45 and 83 to 90, a resin mixture of PC and PS was applied to a substrate layer; the compositions and composition ratios of the substrate layer and surface layer are shown in Table 5 below. On the other hand, the cases where fluorine resin particles were not contained in a surface layer are shown in Comparative Examples, and the compositions and composition ratios of the substrate layer and surface layer are given in Table 6 below.

In the examples and comparative examples, polycarbonate and polystyrene were used as the base resin of the substrate layer, and they were uniformly added according to the composition ratios of Table 1 and then melt-kneaded at

25O⁰C using a twin-screw extruder .

As such, the polycarbonate resin had a melt index of

22 g/10min at 300⁰C under a load of 1.2 kg according to ASTM

D1238 , while the polystyrene resin had a melt index of 1.5 g/10min at 200⁰C under a load of 5 kg according to ASTM

D1238. In addition, as the base resin of the surface layer, polymethylmethacrylate, polystyrene or a styrene-acrylic copolymer resin copolymerized using an acrylate monomer and a styrene monomer according to a typical process was used.

The light-diffusion plate was formed at 250°C through a co-extruding process using an extruder having screws with diameters of 135 mm and 60 mm. In the light-diffusion plate of each of Examples 1 to 45 and Comparative Example 1 and 2, the surface layer was formed on one surface of the substrate layer, in which the substrate layer was 1.9 mm thick and the surface layer was 0.1 mm. Additionally, the light-diffusion plate of Examples 46 to 90 and Comparative Example 3 and 4 , the surface layer was formed such that surface layers were provided on both surfaces of the substrate layer, in which the substrate layer was 1.8 mm thick and each surface layer was 0.1 mm thick.

TABLE 1

PMMA, PMMA, TFE Particles, MM Beads,

8, 53

100 wt parts 100 wt parts 60 wt parts 5 wt parts

PMMA, MS, TFE Particles, MM Beads,

9, 54

100 wt parts 100 wt parts 1 wt part 5 wt parts

PMMA, MS, TFE Particles, MM Beads, 0 , 55

100 wt parts 100 wt parts 25 wt parts 5 wt parts

PMMA, MS, TFE Particles, MM Beads, 1 , 56

100 wt parts 100 wt parts 45 wt parts 5 wt parts

PMMA, MS, TFE Particles, MM Beads, 2 , 57

100 wt parts 100 wt parts 60 wt parts 5 wt parts

TABLE 2

Substrate Layer Surface Layer

No. Fluorine Resin Light-Diffusing

Base Resin Base Resin Particles Agent

MS, TFE Particles, 3, 58 MS, 100 Wt parts -

100 wt parts 1 wt part

MS, TFE Particles, 4 , 59 MS, 100 Wt parts -

100 wt parts 25 wt parts

MS, TFE Particles, 5, 60 MS, 100 Wt parts -

100 wt parts 45 wt parts

MS, TFE Particles , 6, 61 MS, 100 Wt parts -

100 wt parts 60 wt parts

MS, TFE Particles, MM Beads, 7, 62 MS, 100 wt parts

100 wt parts 1 wt part 5 wt parts

MS, TFE Particles, MM Beads, 8 , 63 MS, 100 Wt parts

100 wt parts 25 wt parts 5 wt parts

MS, TFE Particles, MM Beads, 9, 64 MS, 100 wt parts

100 wt parts 45 wt parts 5 wt parts

MS, TFE Particles, MM Beads, 0, 65 MS, 100 wt parts

100 wt parts 60 wt parts 5 wt parts

PMMA, TFE Particles, MM Beads, 1, 66 MS, 100 wt parts

100 wt parts 1 wt part 5 wt parts

PMMA, wt TFE Particles, MM Beads, 2, 67 MS, 100 parts

100 wt parts 25 wt parts 5 wt parts

PMMA, TFE Particles, MM Beads, 3, 68 MS, 100 Wt parts

100 wt parts 45 wt parts 5 wt parts

PMMA, TFE Particles,

MS, MM Beads, 4 , 69 100 wt parts

100 wt parts 60 wt parts 5 wt parts

TABLE 3

100 wt parts 25 wt parts

PMMA, TFE Particles, 7, 72 PS, 100 Wt parts -

100 wt parts 45 wt parts

PMMA, TFE Particles, 8, 73 PS, 100 wt parts -

100 wt parts 60 wt parts

PMMA, TFE Particles, MM Beads, 9, 74 PS, 100 wt parts

100 wt parts 1 wt part 5 wt parts

PMMA, TFE Particles, MM Beads, 0, 75 PS, 100 wt parts

100 wt parts 25 wt parts 5 wt parts

PMMA, TFE Particles, MM Beads, 1, 76 PS, 100 wt parts

100 wt parts 45 wt parts 5 wt parts

PMMA, TFE Particles, MM Beads, 2, 77 PS, 100 wt parts

100 wt parts 60 wt parts 5 wt parts

TABLE 4

TABLE 5

PC, 100 wt parts MS, TFE Particles, Silicone Beads,

45, 90 +PS, 100 wt parts 100 wt parts 60 wt parts 4 wt parts

TABLE 6

The materials used in the examples and comparative examples were as follows :

PMMA: polymethylmethacrylate

MS : methylmethacrylate-styrene copolymer resin PS : polystyrene PC : polycarbonate (polycarbonate resulting from the reaction of 2 , 2-bis ( 4-hydroxyphenyl) propane and phosgene)

TFE particles : tetrafluoroethylene resin particles, having a particle size of 8 μm

Silicone beads : silicone resin beads , having a particle size of 2 μm

MM beads : methylmethacrylate resin beads, having a particle size of 20 μm

In the compositions of the light-diffusion plates of the examples and comparative examples, the substrate layer was formed by mixing the base resin with 4 parts by weight of silicone resin beads (average particle size : 2 μm) as a light-diffusing agent and 0.05 parts by weight of B- cap (tetra-ethyl-2, 2' - (1, 4-phenylene-dimethylidene) - bismalonate) as a UV absorbent in a light stabilizer, and the surface layer was formed by mixing the base resin with 0.5 parts by weight of the above light stabilizer. The light-diffusion plates manufactured in the examples and comparative examples were measured for water absorption rate, bending, total luminous transmittance, haze, yellowing index and thermal deformation temperature . The results are given in Tables 7 to 12. The water absorption rate was measured from a difference in weight after cutting the light-diffusion plate to 10 x 10 cm and then dipping it into water at 23⁰C for 24 hours . The bending was determined in such a manner that the light-diffusion plate was mounted onto a 20" sized backlight unit and then allowed to stand at 60⁰C with a relative humidity of 75% for 96 hours, and a distance between four corners of the light-diffusion plate which were warped upwards and the surface of the backlight unit was measured. The thermal deformation temperature was measured according to ASTM D648.

The yellowing index was measured according to ASTM

D1003 after exposure was conducted at 50⁰C for 240 hours using a Q-UV tester equipped with an FS-40 313/280 lamp under conditions of ASTM D1925. The total luminous transmittance and haze were measured according to ASTM D1003. TABLE 7

From the results of the above properties, the light- diffusion plate of the present invention having the surface layer including the fluorine resin particles can be confirmed to have relatively high dimensional stability even under conditions of high temperature and high humidity, regardless of the type of base resin of the surface layer.

In the light-diffusion plate of Example 9, in which the MS resin is applied to the surface layer and the fluorine resin particles are used in relatively small amounts, the yellowing index is measured to be slightly high. In addition, as the amount of fluorine resin particles is increased, dimensional stability is increased, but total luminous transmittance is slightly decreased.

TABLE 8

From the results of the above properties, the light- diffusion plate of the present invention having the surface layer including the fluorine resin particles can be confirmed to have relatively high dimensional stability under conditions of high temperature and high humidity, regardless of the type of base resin of the surface layer, even in the case where the MS resin is applied to the substrate layer.

In the cases of Examples 13, 17 and 21 in which the fluorine resin particles are used in relatively small amounts, the yellowing index is measured to be slightly high.

TABLE 9

From the results of the above properties, the light- diffusion plate of the present invention having the surface layer including the fluorine resin particles can be confirmed to have relatively high dimensional stability under conditions of high temperature and high humidity, even in the case where the PS resin is applied as the base resin of ' the substrate layer. Compared to the light-diffusion plates prepared using PMMA or MS resin as the base resin of the substrate layer in Tables 7 and 8 , it is apparent that the water absorption rate is lower but the yellowing index is slightly higher .

TABLE 10

From the results of the above properties, the light- diffusion plate of the present invention having the surface layer including the fluorine resin particles can be confirmed to have relatively high dimensional stability and anti- yellowing properties under conditions of high temperature and high humidity, even in the case where the PC resin is applied as the base resin of the substrate layer .

TABLE 11

From the results of the above properties, the light- diffusion plate of the present invention having the surface layer including the fluorine resin particles can be confirmed to have relatively high dimensional stability and anti- yellowing properties under conditions of high temperature and high humidity, even in the case where the mixture of PC and PS is applied as the base resin of the substrate layer . Moreover, the water absorption rate is measured to further decrease in proportion to an increase in the amount of fluorine resin particles .

Accordingly, in the case where the polycarbonate resin is mixed with the relatively inexpensive polystyrene resin to be used as the base resin of the substrate layer, dimensional stability is equal or superior to the case where only the polycarbonate resin is used as the base resin of the substrate layer, thus decreasing preparation cost .

TABLE 12

From the results of the above properties, the light- diffusion plate in which the fluorine resin particles are not contained can be found to increase the water absorption rate and drastically increase the bending phenomenon, resulting in decreased dimensional stability.

Therefore, since the light-diffusion plate of the present invention has the surface layer including the fluorine resin particles, it has excellent heat resistance and absorption resistance, and thus has superior dimensional stability, with high total luminous transmittance and light diffusibility.

In addition, in the light-diffusion plate of the present invention, the base resin of the substrate layer or surface layer is appropriately selected, such that the light- diffusion plate is improved in anti-yellowing properties upon exposure to a light source for a long period of time .

Industrial Applicability

As previously described herein, the present invention provides a light-diffusion plate . According to the present invention, the light-diffusion plate has a surface layer including fluorine resin particles , whereby it has high dimensional stability thanks to high absorption resistance and heat resistance, thus generating less of a bending phenomenon even under conditions of high temperature and high humidity. In addition, when the light-diffusion plate is exposed to a light source for a long period of time, less yellowing is caused.

Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims .

Claims

1. A light-diffusion plate, comprising : a substrate layer composed mainly of a first base resin; and a surface layer composed mainly of a second base resin same as or different from the first base resin of the substrate layer and including fluorine resin particles formed on either or both surfaces of the substrate layer .

2. The light-diffusion plate according to claim 1, wherein the fluorine resin particles comprise particles of at least one fluorine resin selected from the group consisting of a vinylidenefluoride (VDF) homopolymer, a copolymer of vinylidenefluoride (VDF) and methylmethacrylate, a copolymer of vinylidenefluoride (VDF) and hexafluoropropylene (HFP) , a copolymer of vinylidenefluoride (VDF) and tetrafluoroethylene (TFE) , and a terpolymer of vinylidenefluoride (VDF) , hexafluoropropylene (HFP) and tetrafluoroethylene (TFE) .

3. The light-diffusion plate according to claim 1, wherein the fluorine resin particles of the surface layer has a particle size of 0.2~50 μm.

4. The light-diffusion plate according to claim 1, wherein the fluorine resin particles of the surface layer are used in an amount of 0.5~35 wt%, based on a total weight of a composition of the surface layer.

5. The light-diffusion plate according to claim 1, wherein the first base resin or second base resin comprises at least one selected from the group consisting of acrylic resins, styrene resins, styrene-acrylic copolymer resins, and polycarbonate resins .

6. The light-diffusion plate according to claim 5, wherein the acrylic resin comprises a homopolymer, a copolymer, or mixtures thereof obtained using at least one monomer selected from the group consisting of methacrylic acid alkylester, acrylic acid alkylester, methacrylic acid cycloalkylester, acrylic acid cycloalkylester, methacrylic acid arylester, and acrylic acid arylester .

7. The light-diffusion plate according to claim 5, wherein the styrene resin comprises a homopolymer, a copolymer, or mixtures thereof obtained using at least one monomer selected from the group consisting of styrene, α- methylstyrene, m-methylstyrene, p-methylstyrene and p- methoxystyrene .

8. The light-diffusion plate according to claim 5, wherein the styrene-acrylic copolymer resin is a copolymer obtained using at least one acrylic monomer selected from the group consisting of methacrylic acid alkylester, acrylic acid alkylester, methacrylic acid cycloalkylester, acrylic acid cycloalkylester, methacrylic acid arylester, and acrylic acid arylester and at least one styrene monomer selected from the group consisting of styrene, α-methylstyrene, m- methylstyrene, p-methylstyrene and p-methoxystyrene .

9. The light-diffusion plate according to claim 8 , wherein the styrene-acrylic copolymer resin is obtained by reacting the acrylic monomer with the styrene monomer at a weight ratio of 9 : 1~1 : 9.

10. The light-diffusion plate according to claim 8 , wherein the styrene-acrylic copolymer resin is obtained by reacting the acrylic monomer with the styrene monomer at a weight ratio of 6 : 4-2 : 8.

11. The light-diffusion plate according to claim 1, wherein at least one layer selected from the substrate layer and the surface layer further comprises a light-diffusing agent .

12. The light-diffusion plate according to claim 1 or 11, wherein the surface layer has an embossed shape caused by protrusion of the fluorine resin particles or light-diffusing agent from a surface thereof .

13. The light-diffusion plate according to claim 12, wherein the surface layer has a surface roughness of 0.1~50 μm.

14. The light-diffusion plate according to claim 11, wherein the light-diffusing agent is used in an amount of 0.01-35 wt%, based on a total weight of a composition of the substrate layer or surface layer .

15. The light-diffusion plate according to claim 1, wherein at least one layer selected from the substrate layer and the surface layer further comprises a light stabilizer .

16. The light-diffusion plate according to claim 15, wherein the light stabilizer is used in an amount of 0.01~5 wt%, based on a total weight of a composition of the substrate layer or surface layer .