KR100754412B1 - Light-diffusion plate - Google Patents

Abstract

The present invention relates to a light diffusion plate used for a backlight unit, an illumination device, or the like of a liquid crystal display, comprising: a base layer comprising a first base resin mixed with a polycarbonate resin and a polystyrene resin; And a second base resin made of styrene-acrylic copolymer resin and fluorine resin particles, the light diffusing plate comprising a surface layer formed on one side or both sides of the base layer, thereby providing high heat resistance and water absorption and high dimensional stability. Therefore, the warpage does not occur even in an environment of high temperature and high humidity, and excellent optical properties such as total light transmittance and low manufacturing cost can provide an economical light diffusion plate.

Liquid Crystal Display, Polycarbonate Resin, Dimensional Stability, Light Diffusion Plate

Description

Light-diffusion plate

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light diffusion plate used for a backlight unit, an illumination device, or the like of a liquid crystal display.

Conventionally used light diffusing plates include (1) a light diffusing plate (Japanese Patent Application Laid-Open No. 4-275501) obtained by performing a process of imparting physical concavities and convexities to a surface, and (2) a transparent material containing fine particles on a transparent substrate such as a polyester resin. Optical diffuser plate obtained by coating a light diffusing layer made of resin (Japanese Patent Application Laid-Open No. 6-59108), and a light diffusing plate obtained by melt-mixing beads in a transparent resin and extruding them (Japan, Patent Application Laid-Open Patent No. 6-59108). 123802) and (4) disclose a light diffusion plate (Japanese Patent Application Laid-open No. 9-311205) having a sea island structure formed by melt kneading two or more kinds of transparent thermoplastic resins.

The light diffusing plates 1 and 2 are surface diffusing plates, that is, light diffusing effects are obtained by the uneven or coated light diffusing layer formed on the surface. On the other hand, the light diffusing plates 3 and 4 are light diffusing plates having a light diffusing component at least inside the substrate.

The light diffusion plate made of the existing methyl methacrylate resin, which is mainly used as the light diffusion plate, has excellent optical properties such as total light transmittance, but has low dimensional stability, so it is used together with cold cathode fluorescent lamps and light sources such as LEDs to light and When the temperature is changed by turning off, the water absorption rate of the light diffusing plate can be easily changed, so there are problems such as warpage, wrinkles, deformations such as cracking, and the like. That is, there is a problem that deformation such as bending phenomenon occurs in a high temperature and high humidity environment (Japanese Patent Application Laid-Open No. 07-100985, Japanese Patent Application Laid-open No. 08-198976).

In order to solve this problem, a light diffusion plate using polycarbonate resin having excellent dimensional stability due to relatively high light transmittance, high heat resistance and high water absorption is proposed, but in the case of a light diffusion plate to which polycarbonate resin is applied, Although high dimensional stability is exhibited by heat resistance and water absorption, there is a problem that yellowing phenomenon occurs when exposed to a light source for a long time due to low light resistance.

The present inventors have applied for a light diffusion plate using a polycarbonate resin and a polystyrene resin as a substrate layer and using an acrylic resin as a surface layer in order to solve the above problems. However, in the case of manufacturing the surface layer using only acrylic resin, the manufacturing cost is rather high, thereby providing a light diffusion plate at a lower cost, and trying to further increase the dimensional stability.

Thus, styrene-acrylic copolymer resin was used for the surface layer to improve the dimensional stability while lowering the manufacturing cost. Conventionally, a light diffusion plate could not be manufactured by using a polycarbonate resin having excellent dimensional stability as a base layer and a styrene-acrylic copolymer resin as a surface layer. Because there is a problem.

In order to solve this problem, the light diffusing plate of the present invention mixed the polycarbonate resin and the polystyrene resin with the base layer in consideration of the adhesive force with the surface layer, and applied the same to improve the adhesion between the surface layer and the base layer.

Accordingly, the inventors of the present invention have confirmed that the present invention can provide an economically advantageous light diffusing plate by applying a resin having better dimensional stability and lower manufacturing cost as a surface layer, thereby completing the present invention.

An object of the present invention is to provide a light diffusing plate having excellent optical characteristics without causing warpage even in a high temperature and high humidity environment with high dimensional stability.

In addition, an object of the present invention is to provide an economical light diffusion plate at a low manufacturing cost.

The present invention for achieving the above object is a base layer comprising a first base resin mixed with a polycarbonate resin and a polystyrene resin; And a second base resin and a fluorine resin particle made of styrene-acrylic copolymer resin, and including a surface layer formed on one or both surfaces of the substrate layer.

The first base resin has a mixing ratio of polycarbonate resin and polystyrene resin in a weight ratio of 9: 1 to 1: 9, and the second base resin has a ratio of 6: 4 to 1: 9 weight ratio of acrylic monomer and styrene monomer to be copolymerized. It is characterized by that.

The polycarbonate resin is characterized in that the melt index (MI) of 7 ~ 30 g / 10min at 300 ℃, 1.2kg load under the conditions of ASTM D1238.

The polystyrene resin is characterized by a melt index (MI) of 0.5 ~ 3 g / 10min at 200 ℃, 5kg load under the conditions of ASTM D1238.

The first base resin is manufactured by melt kneading a polycarbonate resin and a polystyrene resin at 200 ~ 300 ℃ using an extruder.

The fluorine-based resin particles of the surface layer is characterized in that it comprises 0.5 to 50 parts by weight based on 100 parts by weight of the second base resin.

The fluororesin particles are a tetrafluoroethylene (TFE) homopolymer, a copolymer of tetrafluoroethylene and perfluoropropyl ether (PFA), tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) Characterized in that the particles are made of at least one fluorine resin selected from a copolymer, a copolymer of tetrafluoroethylene and ethylene (ETFE), and a copolymer of vinylidene fluoride (VDF) and tetrafluoroethylene (TFE). do.

The surface layer is characterized in that the surface roughness is 0.1 ~ 50㎛.

Hereinafter, the present invention will be described in more detail.

It manufactured by mixing polycarbonate resin and polystyrene resin as 1st base resin of the base material layer of this invention. This is because the polycarbonate resin has a disadvantage in that it is rather expensive in spite of the impact resistance, absorption and light transmittance of the polycarbonate resin, and thus, a polystyrene resin having a similar refractive index and a relatively inexpensive price is mixed to reinforce the rigidity of the polycarbonate resin. This is to maintain and improve optical and mechanical properties while reducing raw material costs in manufacturing costs.

The polycarbonate resin used to prepare the first base resin has excellent impact resistance and light transmittance, good cold resistance and electrical properties, and particularly has high heat resistance and water absorption resistance, and thus has excellent dimensional stability, and thus has a wide temperature range. It is used in optical lenses, optical disk materials, helmets, protective equipment, covers, and the like.

The polycarbonate resins used in the present invention are aromatic polycarbonate resins which are generally used, and linear and branched carbonate homopolymers and polypolymers prepared by reacting dihydroxy phenol with phosgene or by reaction of dihydroxy phenol with a carbonate precursor. Ester copolymers or mixtures of one or more thereof. The dihydroxy phenol is 2,2-bis (4-hydroxyphenyl) propane (ie bisphenol A), bis (4-hydroxy phenyl) methane, 2,2-bis (4-hydroxy-3,5 -Dimethylphenyl) propane and 1,1-bis (4-hydroxyphenyl) cyclohexane and the like, and the carbonate precursors include diphenyl carbonate, carbonyl halide and diaryl carbonate.

The polycarbonate resin preferably has a melt index (MI) in the range of 7 to 30 g / 10 min at a load of 1.2 kg at 300 ° C. under ASTM D1238 standard conditions.

The polystyrene resin used to manufacture the first base resin is hard, colorless, transparent, has good electrical properties, and is cheap in mass production, so kitchen products, stationery, furniture, daily necessities such as furniture, TV cabinets for automobiles, etc. It is used in many places such as telecommunications products.

As for the polystyrene resin used in the present invention, it is preferable to use a melt index (MI) of 0.5 to 3 g / 10min at a load of 200 ° C and 5 kg under the conditions of ASTM D1238.

For the mixing of the polycarbonate resin and the polystyrene resin, a twin screw extruder having a screw diameter of 30 mm was used, and a molding temperature was prepared by melt kneading at 200 rpm to 300 ° C., preferably 250 ° C., at 250 rpm.

In this case, the mixing ratio of the polycarbonate resin and the polystyrene resin is 1: 9 to 9: 1 weight ratio, which is used to take advantage of the flexibility and dimensional stability of the polycarbonate, and the advantages of high absorption and strength of polystyrene In order to save, it is better to mix each content to at least 10%.

On the other hand, the light diffusing plate of the present invention to add a surface layer to one or both surfaces of the base layer to produce a light diffusing plate.

As the second base resin of the surface layer, a styrene-acrylic copolymer resin in which an acrylic monomer and a styrene monomer are copolymerized is used.

The acrylic monomer that can be used as the monomer is one or more selected from methacrylic acid alkyl ester, acrylic acid alkyl ester, methacrylic acid cycloalkyl ester, acrylic acid cycloalkyl ester, methacrylic acid aryl ester, acrylic acid aryl ester, and the styrene Examples of the monomers include styrene or substituted styrene. Substituted styrenes include alkyl styrenes such as α-methylstyrene, halogenated styrenes such as chlorostyrene, vinyl styrene, and the like, and two or more styrene monomers may be used in combination if necessary.

In particular, the styrene-acrylic copolymer resin used in the present invention preferably has a ratio of 6: 4 to 1: 9 by weight of the acrylic monomer and styrene monomer to be copolymerized. It is preferable to use in said ratio in consideration of the bonding force with a base material layer.

On the other hand, the surface layer of this invention contains fluororesin particle | grains in styrene-acrylic-type resin.

Fluorine-based resin particles have excellent thermal stability and electrical properties, and are excellent in chemical durability, weather resistance, light resistance and oxygen resistance, and in particular, have almost no water absorption, high heat resistance, high operating temperature of 250-300 ° C., and high surface friction. It is excellent and is currently applied to various valves, pumps, tanks, filters, pipes, cables, computers, and aerospace industries.

In particular, the fluororesin particles applied in the present invention are tetrafluoroethylene (TFE) homopolymers, copolymers of tetrafluoroethylene and perfluoropropyl ether (PFA), tetrafluoroethylene (TFE) and hexafluoro At least one fluorine resin selected from copolymers of propylene (HFP), copolymers of tetrafluoroethylene and ethylene (ETFE), and copolymers of vinylidene fluoride (VDF) and tetrafluoroethylene (TFE) It is preferable that the particles are formed, and the fluorine-based resin particles are applied to have a light diffusing function and excellent water repellency. That is, the fluorine-based resin particles protrude to the surface to have an embossed shape, thereby achieving the effect of having a matte surface and low reflectance. At this time, the surface roughness of the surface layer is appropriately about 0.1 ~ 50㎛, if the roughness is more than 50㎛ has insufficient surface impact strength, if the roughness is less than 0.1㎛ the matt effect.

It is preferable that the fluorine-type resin particle contained in the said surface layer is 0.2-50 micrometers in particle size.

In addition, the content of the fluorine-based resin particles included in the surface layer is a range of 0.5 to 50 parts by weight relative to 100 parts by weight of the second base resin in consideration of dimensional stability, permeability and thickness of the surface layer to be prepared, preferably 5 ~ 20 parts by weight is good.

The light diffusing agent included in the base layer is usually the same refractive index as the base resin, is used to increase the light diffusion rate, various organic and inorganic particles are used. At this time, the light diffusing agent exhibits a light diffusing effect even in a small amount when the refractive index with the base resin is large, and a relatively large amount should be included when the refractive index difference is small.

Representative organic particles include methyl methacrylate, ethyl methacrylate, isobutyl methacrylate, normal butyl methacrylate, normal butyl methyl methacrylate, acrylic acid, methacrylic acid and hydroxyethyl methacrylate. , Hydroxypropyl methacrylate, hydroxy ethyl acrylate, acrylamide, metyrol acrylamide, glycidyl methacrylate, ethyl acrylate, isobutyl acrylate, normal butyl acrylate, 2-ethylhexyl acrylate polymer Or acrylic polymer particles such as copolymers or terpolymers thereof; Olefin polymer particles such as polyethylene and polypropylene; Multi-layered multicomponent particles, siloxane-based polymer particles, tetrafluoroethylene-based particles formed by forming copolymer particles of acryl and olefin, particles of the homopolymer, copolymer or terpolymer, and then covering the layers with other monomers. Particles and the like.

On the other hand, examples of the inorganic light diffusing particles include calcium carbonate, barium sulfate, silicon oxide, aluminum hydroxide, titanium oxide, zirconium oxide, magnesium fluoride, talc, glass, mica and the like. Normally, the organic particles have excellent light diffusivity compared to the inorganic particles, and if necessary, two or more kinds of light diffusing particles can be used in combination.

In addition, the light diffusing agent included in the base layer may be further included in the surface layer. The light diffusing agent is further included in the surface layer to further increase the light diffusing effect, and the light diffusing particles protrude to the surface of the surface layer to have an embossed shape, thereby achieving an effect of having a matte surface and a low reflectance. At this time, the surface roughness of the surface layer is suitably about 0.1 ~ 50㎛, if the roughness is more than 50㎛ surface impact strength is insufficient, if the roughness is less than 0.1㎛ the matt effect.

The content of the light diffusing agent used in the substrate layer or the surface layer of the present invention is 0.01 to 50 parts by weight, preferably 0.5 to 20 parts by weight, based on 100 parts by weight of the first base resin or the second base resin of the surface layer. Parts by weight. If the amount of the light diffusing agent is less than 0.01 part by weight, it is difficult to expect sufficient light diffusing effect and concealability, and if it is more than 50 parts by weight, light transmittance is not good. At this time, the content of the light diffusing agent may be determined according to the difference in refractive index with the base resin.

On the other hand, in the present invention, a light stabilizer may be added to the base layer or the surface layer, the light stabilizer that can be added as a UV absorber and a radical scavenger having a maximum absorption wavelength in the range 250nm ~ 380nm can maximize the light stabilizer effect Ultraviolet stabilizers of hindered amines are preferred. They must be effective for a long time, must not be evaporated or extracted to be freed or removed from the sheet, and an absorbent with good compatibility with the substrate should be selected.

Examples of the ultraviolet absorber include cyanoacrylic, salicylate, malonic ester, oxalanilide, diketone, hydroxy benzophenone, hydroxy benzotriazole, organometallic, and the like. The above can be used in combination.

Examples of ultraviolet stabilizers include piperidinyl esters, oxazolidines and piperidino oxazolidines, piperidisspiroacetals, diazacycloalkanones, and the like, and two or more selected from these may be used in combination.

The content of the light stabilizer is 0.01 parts by weight to 5 parts by weight, and preferably 0.1 parts by weight to 2 parts by weight, based on 100 parts by weight of the base resin of the base layer or the surface layer. In particular, by including them in the surface layer, it is possible to obtain the expected light stability effect without lowering the total light transmittance and physical properties of the substrate layer, and also bring about the effect of cost reduction.

When the light diffusion plate of the present invention forms a surface layer on one surface or both surfaces of the substrate layer, it can be produced by a method such as coextrusion molding, lamination, thermal bonding, surface coating, etc., which are well-known techniques.

The light diffusion plate of the present invention can be used for various applications indoors and outdoors. That is, it can be used for signage, lighting signage, lighting cover, glass showcase, and preferably used as a light diffusion plate for display. Representative examples of display light diffusers include light diffusers for liquid crystal display backlights or edge light type backlights.

Hereinafter, examples of the present invention will be described in more detail, but the scope of the present invention is not limited to these examples.

Compositions and composition ratios of Examples and Comparative Examples are shown in the following [Table 1] and [Table 2], [Table 1] is a composition and composition ratio constituting the first base resin of the base layer of the light diffusion plate, [Table 2 ] Relates to the composition and composition ratio constituting the second base resin of the surface layer of the light diffusion plate.

In the first base resins of Examples 1 to 10, polycarbonate and polystyrene were uniformly added at a composition ratio of the following [Table 1], and melt kneaded at a temperature of 250 ° C. using a twin screw extruder.

In this case, the polycarbonate has a melt index (MI) of 22 g / 10min at 300 ° C. and 1.2 kg load under ASTM D1238 standard conditions, and the polystyrene resin has a melt index (200 ° C. and 5 kg load under ASTM D1238 standard conditions). MI) of 1.5 g / 10 min was used.

On the other hand, the second base resin of the surface layer is a monomer according to the composition of the above [Table 2] copolymerized by a conventional method to prepare a styrene-acrylic copolymer resin.

Examples 1 to 5 and Comparative Examples 1 to 3 form a surface layer on one surface of the base layer, Examples 6 to 10 and Comparative Examples 4 to 6 have the same composition as Examples 1 to 5 and Comparative Examples 1 to 3. The substrate layer and the surface layer are formed, and the surface layer is formed on both surfaces of the substrate layer.

At this time, molding was coextruded at a screw diameter of 135 mm and 60 mm, respectively, and molding temperatures were performed at 250 ° C. and 220 ° C., respectively. Examples 1 to 5 and Comparative Examples 1 to 3 were used to determine the thickness of the base layer. The thickness of the substrate layer was 1.9 mm and the surface layer was 0.1 mm. In Examples 6 to 10 and Comparative Examples 4 to 6, the thickness of the substrate layer was 1.8 mm and the thickness of the surface layer was 0.1 mm on both surfaces.

PC PS PMMA Example 1 Example 6 100 parts by weight 100 parts by weight - Example 2 Example 7 100 parts by weight 100 parts by weight - Example 3 Example 8 100 parts by weight 100 parts by weight - Example 4 Example 9 100 parts by weight 100 parts by weight - Example 5 Example 10 100 parts by weight 100 parts by weight - Comparative Example 1 Comparative Example 4 - - 100 parts by weight Comparative Example 2 Comparative Example 5 - 100 parts by weight - Comparative Example 3 Comparative Example 6 100 parts by weight - - * PC: Polycarbonate (Polycarbonate reacted with 2,2-bis (4-hydroxyphenyl) propane and phosgene), LG Dow, Caliber 300-22 * PS: Polystyrene, Toyo, HRM40 * PMMA: Polymethyl methacrylate, Mitsubishi-Rayon VH-001

division 2nd base resin Fluorine resin particles (content with respect to 100 parts by weight of the second base resin) MMA-Styrene Copolymer PMMA MMA Styrene Example 1 Example 6 600 parts by weight 400 parts by weight - 0.3 parts by weight of TFE resin particles Example 2 Example 7 600 parts by weight 400 parts by weight - 1 part by weight of TFE resin particles Example 3 Example 8 600 parts by weight 400 parts by weight - 25 parts by weight of TFE resin particles Example 4 Example 9 600 parts by weight 400 parts by weight - 50 parts by weight of TFE resin particles Example 5 Example 10 600 parts by weight 400 parts by weight - 60 parts by weight of TFE resin particles Comparative Example 1 Comparative Example 4 600 parts by weight 400 parts by weight - - Comparative Example 2 Comparative Example 5 600 parts by weight 400 parts by weight - 25 parts by weight of TFE resin particles Comparative Example 3 Comparative Example 6 600 parts by weight 400 parts by weight - - * MMA-Styrene copolymer resin: Styrene-acrylate copolymer resin, Shin-Il Chem Chem, MS600 * PMMA: Polymethyl methacrylate, Mitsubishi-Rayon VH-001 * TFE resin particle: Tetrafluoroethylene resin particle Particle size 8㎛

As the composition of the light diffusing plate according to the Examples and Comparative Examples, the base layer is 0.2 parts by weight of a silicone resin bead (average particle diameter 2㎛) as a light diffusing agent to the first base resin according to the composition of Table 1 and an ultraviolet absorber as a light stabilizer. 0.05 parts by weight of B-Cap (Tetra-ethyl-2,2 ′-(1,4-phenylene-dimethylidene) -bismalonate) was mixed, and the surface layer was prepared in Table 2 on the copolymerized second base resin according to the composition of Table 2. 2 parts by weight of silicone resin particles (average particle diameter: 2 μm) as a composition and a fluorine-based resin particle as a light diffusing agent, and B-Cap (Tetra-ethyl-2,2 ′-(1,4-phenylene-dimethylidene) as an ultraviolet absorber as a light stabilizer. 0.5 parts by weight of the bismalonate) is mixed.

The results of the measurement of the warpage, moisture absorption rate, heat deformation temperature, total light transmittance, and haze of the light diffusing plates manufactured in the above Examples and Comparative Examples are shown in Table 3 below.

Here, the warpage was measured after the sheet was mounted on a 20˝ backlight unit and left for 96 hours at 60 ° C and 75% relative humidity, and then the degree of floating of the four corners from the bottom was measured. The water absorption rate was 10 × 10 cm After cutting in water, the water absorption was measured from the change in weight after standing for 25 hours in water at 25 ℃, the heat deformation temperature was measured by the ASTM D648 method.

Total light transmittance and haze were measured by ASTM D1003 method.

Deflection (mm) Water absorption rate (%) Heat Deflection Temperature (℃) Total light transmittance (%) Haze (%) Example 1 0.20 0.18 121.6 64.0 99.3 Example 2 0.20 0.18 122.0 63.7 99.3 Example 3 0.17 0.17 124.3 62.9 99.5 Example 4 0.15 0.16 130.0 61.7 99.5 Example 5 0.15 0.16 130.8 60.8 99.5 Example 6 0.19 0.17 118.1 63.8 99.3 Example 7 0.19 0.17 118.5 63.5 99.3 Example 8 0.16 0.16 120.8 62.8 99.5 Example 9 0.14 0.14 121.6 61.6 99.5 Example 10 0.14 0.14 122.4 60.6 99.5 Comparative Example 1 0.26 0.46 103.2 64.0 99.3 Comparative Example 2 0.22 0.12 98.7 63.0 99.5 Comparative Example 3 0.22 0.20 143.7 64.0 99.3 Comparative Example 4 0.25 0.45 101.4 64.0 99.3 Comparative Example 5 0.22 0.13 97.4 63.0 99.5 Comparative Example 6 0.21 0.19 137.4 64.0 99.3

As a result of the physical property evaluation, it can be seen from Examples 1 to 10 that the dimensional stability due to the reduction of the warpage and the water absorption is improved as the content of the tetrafluoroethylene resin particles in the styrene-acrylic resin of the surface layer is increased. In Examples 4, 5, 9, and 10 having a high content of the resin particles, the total light transmittance is lower than those of Examples 1 to 3 and 6 to 8.

On the other hand, in Comparative Examples 1, 3 and 4, 6 it was confirmed that the dimensional stability is relatively low, and in Comparative Examples 1, 2 and 4, 5 it was confirmed that the heat resistance is relatively low heat deformation temperature.

As described above, the light diffusion plate of the present invention has high dimensional stability with high heat resistance and water absorption by adding fluorine resin particles to the surface layer, and does not cause warpage even in a high temperature and high humidity environment, and optical characteristics such as total light transmittance It is excellent and the manufacturing cost is low, thereby providing an economical light diffusion plate.

Claims (8)

A base layer comprising a first base resin having a polycarbonate resin and a polystyrene resin mixed at a weight ratio of 9: 1 to 1: 9; And The second base resin includes a styrene-acrylic copolymer resin in which an acrylic monomer and a styrene monomer are copolymerized in a weight ratio of 6: 4 to 1: 9, and contains fluorine resin particles, and includes a surface layer formed on one or both surfaces of the substrate layer. Light diffusion plate. delete The method of claim 1, The polycarbonate resin is light diffusion plate, characterized in that the melt index (MI) of 7 ~ 30 g / 10min at 300 ℃, 1.2kg load under the conditions of ASTM D1238. The method of claim 1, The polystyrene resin is a light diffusion plate, characterized in that the melt index (MI) is 0.5 ~ 3 g / 10min at 200 ℃, 5kg load under the conditions of ASTM D1238. The method of claim 1, The first base resin is a light diffusion plate, characterized in that the polycarbonate resin and polystyrene resin by melt kneading at 200 ~ 300 ℃ using an extruder. The method of claim 1, The fluorine-based resin particles of the surface layer comprises 0.5 to 50 parts by weight based on 100 parts by weight of the second base resin. The method of claim 1, The fluororesin particles are a tetrafluoroethylene (TFE) homopolymer, a copolymer of tetrafluoroethylene and perfluoropropyl ether (PFA), tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) Characterized in that the particles are made of at least one fluorine resin selected from a copolymer, a copolymer of tetrafluoroethylene and ethylene (ETFE), and a copolymer of vinylidene fluoride (VDF) and tetrafluoroethylene (TFE). Light diffusion plate. The method of claim 1, The surface layer is a light diffusion plate, characterized in that the surface roughness is 0.1 ~ 50㎛.