WO2005052677A1

WO2005052677A1 - Light diffusion film for lcd back-light unit

Info

Publication number: WO2005052677A1
Application number: PCT/KR2004/002169
Authority: WO
Inventors: Seung-Hwan Jung; Hyun-Jin Kim
Original assignee: Kolon Industries Inc
Current assignee: Kolon Industries Inc
Priority date: 2003-11-26
Filing date: 2004-08-30
Publication date: 2005-06-09
Anticipated expiration: 2006-05-26
Also published as: TWI261710B; TW200517746A; JP2007512573A

Abstract

Disclosed is a light diffusion film (1) for LCD (Liquid Crystal Display) back-light unit, which comprises a light diffusion layer (2, 4) prepared by coating a composition comprising a binder resin and light diffusion beads on at least one side of a high-transparency plastic support (3), the light diffusion layer (2, 4) comprising, based on 100 parts by weight of the binder resin solid, 50 to 350 parts by weight of the light diffusion beads having an arithmetic average particle diameter of 1 to 50 um and a polydispersity index (PDI) of more than 1.00 and less than 1.50 (that is, excluding 1.00 but including 1.50) as calculated by the following Equation 1: Equation 1 Formula (1) average diameter; N is the number of beads analyzed; and di is the diameter o i beads. The light diffusion film (1) of the present invention, when installed in a back-light unit, guarantees an enhanced luminance relative to the existing products.

Description

LIGHT DIFFUSION FILM FOR LCD BACK-LIGHT UNIT

Technical Field The present invention relates to a high-luminance light diffusion film for an LCD (Liquid Crystal Display) back-light unit. More particularly, the present invention relates to a light diffusion film prepared by dispersing transparent spherical organic beads in a transparent binder resin such as an acrylic resin for surface hardness and adhesion to a base film formed from a high-transparency plastic and then coating the either side or both sides of the substrate film with the dispersed beads in the binder resin.

Background Art With the innovative development of display technologies, enlarged screen, lower power consumption and higher luminance of the LCD are the future core technology of TFT-LCD. To cope with these demands, many studies have been made on the parts of the TFT-LCD, i.e., liquid crystal module and back-light unit. Enlarged screen, lower power consumption and higher luminance of the backlight unit are also in progress with an attempt to eliminate the back-light unit itself. It is, however, impossible in the current techniques to form an LCD display without a separate light source as an LED unit because of the basic characteristics of the LCD. In the back-light unit, a light diffusion film has a function of diffusing a light beam from a lamp used as a lateral light source installed on the one side or the rear side of the LCD on the whole face of the screen and refracting the light beam into a uniform beam in the front direction. In addition, a reflection film, a light guiding plate, and prism films are used for the respective operations in the LCD. The intensity of the light emitted from the light source attenuates gradually through the above-mentioned different media, so the luminance on the screen actually viewed by the users is only one of hundreds of the light intensity from the original light source. To solve this problem, many approaches have been taken that involves, for example, increasing the brightness of the light source, or reducing the thickness of each medium of the light guiding plate, the reflection film, the prism, and the diffusion plate.

However, the method of reducing the thickness of each medium encounters a limitation because of the manufacturer's productivity and operating efficiency. Many approaches of changing the patterning of the light guiding plate have recently been tried. For example, a light diffusion film, which is prepared by coating a light diffusion layer on a substrate film, is used to uniformly diffuse the light beam with beads excellent in light diffusion efficiency. In regard to this, many approaches for high luminance have been taken, including the selection of beads, the control of coating thickness, and the antireflection for the backside film coating. In the light diffusion film, the light diffusion layer has a composition including light diffusion beads in a binder resin. The light diffusion beads used for this purpose include organic polymer beads excellent in light diffusion efficiency. But, there is a limitation in the type of the organic polymer beads, which are mostly prepared by emulsion polymerization. On the other hand, inorganic beads are incompatible with the binder resin and currently poor in light diffusion efficiency. According to the usage, the light diffusion film has been developed differently for notebook computer and for LCD monitor, because the light diffusion film for notebook computer has a different structure of the back-light unit and hence a different optical properties from the light diffusion film for LCD monitor. Although the same configuration is not applied to all the products, the back-light unit for notebook computer includes a brightness enhancement film on a single light diffusion film and a cover film on the brightness enhancement film, while the back-light unit for LCD monitor includes two light diffusion films and a dual brightness enhancement film or a diffuse reflective polarizer film on the light diffusion films. The light diffusion film in the back-light unit for LCD monitor includes, based on 100 parts by weight of the binder resin, 150 to 350 parts by weight of beads having a diameter of greater than 20 μm in the light diffusion layer composition, so the light diffusion layer has a large coating thickness with a great difference in total transmittance between front and rear sides. On the other hand, the light diffusion film in the back-light unit for notebook computer includes, based on 100 parts by weight of the binder resin, 50 to 250 parts by weight of beads having a diameter of around 10 μm in the light diffusion layer composition, so the coating of the light diffusion layer is thin with a relatively small difference in total transmittance between front and rear sides. . The recent tendency is providing high luminance for all the parts with a demand for high-luminance display devices. The luminance after the back-light unit attenuates to about one of ten of the initial luminance from a single lamp and, with a liquid crystal panel, further diminishes to one of one or two hundreds of the initial luminance. The enhancement of luminance is a matter of grave concern for image implementation of the LCD device. For this purpose, LCD manufacturers are required to make an attempt to design an LCD with a high opening rate, which is already at the uppermost limit, while back-light manufacturers attempt to reduce the thickness of the back-light unit. An example of the conventional method related to the light diffusion film is Korean Patent Laid-Open No. 2001-054274 that discloses a light diffusion film having a light diffusion layer comprising an inorganic or organic light diffuser and a transparent binder resin, the light diffusion layer being formed on either side of a base film comprising a transparent polymer resin film. In the light diffusion film, the light diffusion layer contains beads having a coagulated particle size of smaller than 50 μm and a stacking ratio of less than 10 %.

Disclosure of Invention Accordingly, the inventors of the present invention found out that the use of monodispersed beads having a polydispersity index (PDI) of more than 1.00 and less than 1.50(that is, excluding 1.00 but including 1.50) measured as a degree of dispersion for a light diffusion layer composition in a high-luminance light diffusion film for LCD backlight unit can guarantee a high luminance from a light source using a smaller amount of beads, thereby completing the present invention. It is therefore an object of the present invention to provide a light diffusion film having a light diffusion layer that provides a high luminance from a light source with a smaller amount of beads. To achieve the object of the present invention, there is provided a light diffusion film that comprises a light diffusion layer prepared by coating a composition comprising a binder resin and light diffusion beads on at least one side of a high-transparency plastic support. The light diffusion layer includes, based on 100 parts by weight of the binder resin solid, 50 to 350 parts by weight of the light diffusion beads having an arithmetic average particle diameter of 1 to 50 μm and a polydispersity index (PDI) of more than 1.00 and less than 1.50(that is, excluding 1.00 but including 1.50) as calculated by the following Equation 1 : Equation 1

Dw Polydispersity Index (PDI) = Dn where Dn = ; Dn is a number average diameter; Dw is a weight

average diameter; N is the number of beads analyzed; and di is the diameter of i beads. Hereinafter, the present invention will be described in further detail as follows. A cross-section of a light diffusion film 1 according to the present invention is presented in FIG. 1. The light diffusion film 1 has light diffusion layers 2 and 4 comprising a binder resin and light diffusion beads on either side or both sides of a high-transparency plastic support 3. The individual layers will be described in further detail as follows. (1) Light diffusion layer The light diffusion layer formed on either side or both sides of a high- transparency plastic support includes a binder resin, and light diffusion beads. The light diffusion layer not only involves scattering, reflection and refraction of incident light to diffuse the light but also induces more transmittance of the light to diffuse the light through a light guiding plate uniformly on the whole surface of the LCD screen. Hence, the light diffusion layer uses a binder resin that is excellent in both adhesion to the plastic support and compatibility with the beads serving as a light diffuser. Specific examples of the resin as used herein may include unsaturated polyester; acrylic resins, such as polymer or copolymer or terpolymer of methylmethacrylate, ethylmethacrylate, isobutylmethacrylate, n-butylmethacrylate, n-butylmethylmethacrylate, acrylic acid, methacrylic acid, hydroxyethylmethacrylate, hydroxypropylmethacrylate, hydroxyethylacrylate, acrylamide, methyrolacrylamide, glycidylmethacrylate, ethylacrylate, isobutylacrylate, n-butylacrylate, or 2-ethylhexylacrylate; urethanes; epoxys; or melamines. If necessary, a hardening agent may be used to harden the coating film of the resin for the purpose of enhancement of heat resistance, wear resistance and adherence. The preferred resin has a high light transmittance and, particularly, a high adherence to the base film because it forms the light diffusion layer. The light diffusion beads include various organic or inorganic beads that enhance light transmittance and diffusion rate with a difference in refractive index with normal resins. The specific examples of the light diffusion beads as used herein may include organic beads, including acrylic beads (e.g., polymer or copolymer or terpolymer of methylmethacrylate, ethylmethacrylate, isobutylmethacrylate, n-butylmethacrylate, n- butylmethylmethacrylate, acrylic acid, methacrylic acid, hydroxyethylmethacrylate, hydroxypropylmethacrylate, hydroxyethylacrylate, acrylamide, methylolacrylamide, glycidylmethacrylate, ethylacrylate, isobutylacrylate, n-butylacrylate, or 2- ethylhexylacrylate), olefin beads (e.g., polyethylene, polystyrene, or polypropylene), acryl-olefin copolymer or multilayer multicomponent beads prepared by forming beads of monopolymer and coating a different monomer on the bead layer; and inorganic beads, such as silicon oxide, aluminum oxide, titanium oxide, zirconium oxide, or magnesium fluoride. The present invention preferably uses organic beads, which are superior in light diffusion efficiency to inorganic beads. The content of the light diffusion beads is preferably in the range of 50 to 350 parts by weight based on 100 parts by weight of the binder resin solid content. The content of the light diffusion beads less than 50 parts by weight deteriorates the light diffusion efficiency, while the content of the light diffusion beads exceeding 350 parts by weight results in white turbidity caused by the stacked beads, with the difficulty in using the light diffusion beads. The above range can be represented by the number of beads per a unit area of the light diffusion layer, i.e., approximately 10 to 200 beads in an area of 100 μm x

100 μm. With less than 10 beads, light diffusion efficiency deteriorates. With more than 200 beads, the number of fine beads is relatively increased to raise the polydispersity index and cause the stacking of beads with a deterioration of light diffusion efficiency. The average size of the beads is measured with a particle size analyzer, ANALYSETTE22 manufactured by Fritsch Company. The arithmetic average particle diameter measured with the particle size analyzer is preferably in the range of 1 to 50 μm. The arithmetic average particle diameter smaller than 1 μm results in a deterioration of light diffusion efficiency, while the arithmetic average particle diameter greater than 50 μm increases the thickness of the light diffusion layer, with a difficulty in installation of the light diffusion layer into a thickness-reduced product. More preferably, the arithmetic average particle diameter is in the range of 5 to 30 μm. The present invention develops a light diffusion film using monodispersed beads having a polydispersity index of more than 1.00 and less than 1.50(that is, excluding 1.00 but including 1.50) as light diffusion beads to exhibit a high luminance with a smaller amount of beads than the existing products. The luminance of the light diffusion film installed in a back-light unit is detemiined by the particle proportion and the particle distribution of the light diffusion layer. The polydispersity index is calculated according to the Equation 1 and, specifically, measured from the analysis of particle diameter distribution using an FE-SEM (Field Emission Scanning Electronic Microscope; manufactured by Hitachi Company) with 500-fold magnification. More specifically, N beads observed with the FE-SEM were taken as a sample and measured in regard to diameter to determine a number average diameter Dn and a weight average diameter Dw and calculate a polydispersity index according to the Equation 1. The polydispersity index is 1.00 for a completely monodispersed bead, so the luminance of a light source is greater with a lower bead proportion when using a monodispersed bead having a polydispersity index of more than 1.00 and less than 1.50(that is, excluding 1.00 but including 1.50) rather than beads having a polydispersity index of greater than 1.50. More preferably, the bead has a polydispersity index of more than 1.00 and less than 1.40(that is, excluding 1.00 but including 1.40). The polydispersity index given by the Equation 1 is specified as a measure for evaluating the degree of dispersion of monodispersed beads in Colloid Polym. Sci. 1999, ρρ210 to 216. The bead composition having such a particle distribution of high-luminance light diffusion layer must be dispersed on the whole surface of the plastic support. The light diffusion solution thus prepared is coated on the plastic support by means of a comma knife or a gravure. The film thickness is desirably in the range of 5 to 50 μm. With the film thickness of less than 5 μm, the beads cannot be so distributed as to provide a light diffusion characteristic. With the film thickness exceeding 50 μm, the light diffusion layer cannot be installed in a thickness-reduced product. (2) Plastic support Any transparent support can be used for the plastic support on which the light diffusion layer is coated. The specific examples of the plastic support may include polycarbonate, polypropylene, polyethyleneterephthalate, polyethylene, epoxy, etc. The preferred plastic support is polyethyleneterephthalate. Recently, polyethylenenaphthalate having a UV-screening effect is used for the light diffusion layer without addition of a UV stabilizer. However, it is too expensive to be applied for commercial use. The plastic support is required to provide adhesion of the light diffusion layer to the binder resin without an effect on the light diffusion layer caused by its high light transmittance and to have a surface smoothness without a deviation of luminance. The thickness of the plastic support is desirably in the range of 50 to 250 μm, more preferably 75 to 200 μm. The thickness of less than 50 μm results in poor mechanical properties and heat resistance of the light diffusion film, while the thickness exceeding 250 μm thickens the product to cause a problem in the installation of the thickness-reduced product.

Brief Description of the Drawings FIG. 1 is a cross-section of a light diffusion film according to the present invention.

Best Mode for Carrying out the Invention Hereinafter, the present invention will be described in detail by way of the following examples, which are not intended to limit the scope of the present invention. In the following examples, the arithmetic average particle diameter and the polydispersity index of the light diffusion beads were measured with a particle size analyzer, ANALYSETTE22 manufactured by Fritsch Company. Example 1 100 parts by weight of an acrylic resin, 52-666 (supplied by Aekyung Chemical. Company) was diluted with 100 parts by weight of methylethylketone and 100 parts by weight of toluene. With respect to the binder resin solid content, 120 parts by weight of polymethylmethacrylate beads, PX010 (supplied by Kolon Company) having an arithmetic average particle diameter of 12.7 μm and a polydispersity index of 1.046 (as measured with a particle size analyzer, ANALYSETTE22 manufactured by Fritsch Company) was added to the binder resin and dispersed with a milling machine (Dynomill) to obtain light diffusion solution. Then, the light diffusion solution was coated on either side or both sides of a lOOμm-thickness ultrahigh-transparency polyethyleneterephthalate film (T600, supplied by Mitsubishi Company) in a thickness of 10 to 30 μm with a gravure to complete a light diffusion film. Example 2 100 parts by weight of an acrylic resin, 52-666 (supplied by Aekyung Chemical. Company) was diluted with 100 parts by weight of methylethylketone and 100 parts by weight of toluene. With respect to the binder resin solid content, 130 parts by weight of polymethylmethacrylate beads, PX010 (supplied by Kolon Company) having an arithmetic average particle diameter of 14.2 μm and a polydispersity index of 1.136 (as measured with a particle size analyzer, ANALYSETTE22 manufactured by Fritsch Company) was added to the binder resin and dispersed with a milling machine (Dynomill) to obtain light diffusion solution. Then, the light diffusion solution was coated on either side or both sides of a lOOμm-thickness ultrahigh-transparency polyethyleneterephthalate film (T600, supplied by Mitsubishi Company) in a thickness of 10 to 30 μm with a gravure to complete a light diffusion film. Example 3 100 parts by weight of an acrylic resin, Surcol836 (supplied by Allied Colloids Company) was diluted with 100 parts by weight of methylethylketone and 100 parts by weight of toluene. With respect to the binder resin solid content, 100 parts by weight of polymethylmethacrylate beads, PX010 (supplied by Kolon Company) having an arithmetic average particle diameter of 11.5 μm and a polydispersity index of 1.310 (as measured with a particle size analyzer, ANALYSETTE22 manufactured by Fritsch Company) was added to the binder resin and dispersed with a milling machine (Dynomill) to obtain light diffusion solution. Then, the light diffusion solution was coated on either side or both sides of a lOOμm-thickness ultrahigh-transparency polyethyleneterephthalate film (A4300, supplied by Toyobo Company) in a thickness of 10 to 30 μm with a gravure to complete a light diffusion film. Example 4 100 parts by weight of an acrylic polyol, BR-113 (supplied by Mitsubishi, Rayon Company) was diluted with 100 parts by weight of methylethylketone and 10 parts by weight of toluene. With respect to the binder resin solid content, 110 parts by weight of polymethylmethacrylate beads, PX010 (supplied by Kolon Company) having an arithmetic average particle diameter of 9.8 μm and a polydispersity index of 1.272 (as measured with a particle size analyzer, ANALYSETTE22 manufactured by Fritsch Company) was added to the binder resin and dispersed with a milling machine (Dynomill) to obtain light diffusion solution. Then, the light diffusion solution was coated on either side or both sides of a lOOμm-thickness ultrahigh-transparency polyethyleneterephthalate film (A4300, supplied by Toyobo Company) in, a thickness of 10 to 30 μm with a gravure to complete a light diffusion film. Comparative Example 1 100 parts by weight of an acrylic polyol, BR-113 (supplied by Mitsubishi Rayon

Company) was diluted with 100 parts by weight of methylethylketone and 100 parts by weight of toluene. With respect to the binder resin solid content, 150 parts by weight of polymethylmethacrylate beads, PX010 (supplied by Kolon Company) having an arithmetic average particle diameter of 13.5 μm and a polydispersity index of 1.698 (as measured with a particle size analyzer, ANALYSETTE22 manufactured by Fritsch Company) was added to the binder resin and dispersed with a milling machine (Dynomill) to obtain light diffusion solution. Then, the light diffusion solution was coated on either side or both sides of a lOOμm-thickness ultrahigh-transparency polyethyleneterephthalate film (A4300, supplied by Toyobo Company) in a thickness of 10 to 30 μm with a gravure to complete a light diffusion film. Comparative Example 2 100 parts by weight of an acrylic polyol, Surcol836 (supplied by Allied Colloids

Company) was diluted with 100 parts by weight of methylethylketone and 100 parts by weight of toluene. With respect to the binder resin solid content, 170 parts by weight of polymethylmethacrylate beads, PX010 (supplied by Kolon Company) having an arithmetic average particle diameter of 12.2 μm and a polydispersity index of 1.716 (as measured with a particle size analyzer, ANALYSETTE22 manufactured by Fritsch

Company) was added to the binder resin and dispersed with a milling machine (Dynomill) to obtain light diffusion solution. Then, the light diffusion solution was coated on either side or both sides of a lOOμm-thickness ultrahigh-transparency polyethyleneterephthalate film (A4300, supplied by Toyobo Company) in a thickness of 10 to 30 μm with a gravure to complete a light diffusion film. Comparative Example 3 100 parts by weight of an acrylic polyol, 52-666 (supplied by Aekyung Chemicals Company) was diluted with 100 parts by weight of methylethylketone and 100 parts by weight of toluene. With respect to the binder resin solid content, 150 parts by weight of polymethylmethacrylate beads, PX010 (supplied by Kolon Company) having an arithmetic average particle diameter of 10.6 μm and a polydispersity index of 1.724 (as measured with a particle size analyzer, ANALYSETTE22 manufactured by Fritsch Company) was added to the binder resin and dispersed with a milling machine (Dynomill) to obtain light diffusion solution. Then, the light diffusion solution was coated on either side or both sides of a lOOμm-thickness ultrahigh-transparency polyethyleneterephthalate film (A4300, supplied by Toyobo Company) in a thickness of 10 to 30 μm with a gravure to complete a light diffusion film. Comparative Example 4 100 parts by weight of an acrylic polyol, 52-666 (supplied by Aekyung Chemicals Company) was diluted with 100 parts by weight of methylethylketone and 100 parts by weight of toluene. With respect to the binder resin solid content, 150 parts by weight of polymethylmethacrylate beads, PX010 (supplied by Kolon Company) having an arithmetic average particle diameter of 11.7 μm and a polydispersity index of 1.952 (as measured with a particle size analyzer, ANALYSETTE22 manufactured by Fritsch Company) was added to the binder resin and dispersed with a milling machine (Dynomill) to obtain light diffusion solution. Then, the light diffusion solution was coated on either side or both sides of a lOOμm-thickness ultrahigh-transparency polyethyleneterephthalate film (A4300, supplied by Toyobo Company) in a thickness of 10 to 30 μm with a gravure to complete a light diffusion film. The light diffusion films prepared in the Examples and Comparative Examples were analyzed in regard to luminance and total transmittance according to the following measurement methods . (1) Particle size distribution on light diffusion layer of light diffusion film The light diffusion layer of the light diffusion film was observed with 500-fold magnification using an S-4300 FESEM (Field Emission Scanning Electronic Microscope) manufactured by Hitachi Company to measure the diameter di of all beads distributed in an area of 100 μm x 100 μm and the number N of the beads. Then, the number average diameter and the weight average diameter of the beads were calculated according to the Equation 1 to determine the polydispersity index. (2) Particle size distribution of beads The number average molecular weight and the weight average molecular weight of the beads were measured with a particle size analyzer, LS-13320 manufactured by Beclαnan Coulter Company using a laser having a wavelength of 750 nm and an output of 5 W for 90 seconds to calculate the polydispersity index of the bead. (3) Luminance The light diffusion film was cut to be installed in a back-light unit, and the luminance with the light diffusion film on a light guiding plate of the back- light unit was measured. Then, the luminance after installation of the light diffusion layer and a dual brightness enhancement film (Vikuiti™ DBEF-D440 supplied by 3M Company) or a diffuse reflective polarizer film (Vikuiti™ DRPF supplied by 3M Company) was measured at nine points with a luminance measurer (BM7 supplied by TOPCON Company). (4) Total transmittance (TT) and haze With a 40mmx40mm film sample installed in the back-light unit, the total transmittance (TT) and the haze were measured with two haze measurers, NDH2000 and COH300A manufactured by Nippon Denshoku Company when the light beam emitted from the light source passes from the front side to the backside of the film sample and when the light beam passes from the backside to the front side of the film sample. The properties of the light diffusion films according to the Examples and the Comparative Examples are presented in Table 1. Table 1

As can be seen from Table 1, compared with the Comparative Examples 1 to 4, the Examples 1, 2 and 3 of the present invention provide an excellent luminance. Namely, the measurements of luminance with [(A) back-light + diffusion film], [(A) + dual brightness enhancement film] and [(A) + dual brightness enhancement film + cover film] show that the Examples of the present inventio.n guarantee a higher luminance than the Comparative Examples.

Industrial Application As described above, when installed in a back-light unit, the light diffusion film with a light diffusion layer containing monodispersed beads having a polydispersity index of more than 1.00 and less than 1.50(that is, excluding 1.00 but including 1.50) provides an enhanced luminance, compared with the existing products.

Claims

What is claimed is:

1. A light diffusion film for LCD (Liquid Crystal Display) back-light unit, which comprises a light diffusion layer prepared by coating a composition comprising a binder resin and light diffusion beads on at least one side of a high-transparency plastic support, the light diffusion layer comprising, based on 100 parts by weight of the binder resin solid, 50 to 350 parts by weight of the light diffusion beads having an arithmetic average particle diameter of 1 to 50 μm and a polydispersity index (PDI) of more than 1.00 and less than 1.50(that is, excluding 1.00 but including 1.50) as calculated by the following Equation 1 : Equation 1

Dw Polydispersity Index (PDI) Dn

wherein Dn = ; Dn is a number average diameter; Dw is a weight

average diameter; N is the number of beads analyzed; and di is the diameter of i beads.

2. The light diffusion film for LCD back-light unit as claimed in claim 1, wherein the polydispersity index (PDI) is in the range of more than 1.00 and less than 1.40(that is, excluding 1.00 but including 1.40).

3. The light diffusion film for LCD back-light unit as claimed in claim 1, wherein the light diffusion beads has an arithmetic average particle diameter of 5 to 30 μm.

4. The light diffusion film for LCD back-light unit as claimed in claim 1, wherein the light diffusion layer has a thickness of 5 to 50 μm.

5. The light diffusion film for LCD back-light unit as claimed in claim 1, wherein the light diffusion layer includes 10 to 200 beads per area of 100 μm * 100 μm.