Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Reexamination Certificate
1998-02-24
2003-06-17
Thibodeau, Paul (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Including a second component containing structurally defined...
C428S500000, C428S220000, C428S328000
Reexamination Certificate
active
06579606
ABSTRACT:
TECHNICAL FIELD OF INVENTION
The present invention relates to a reflection sheet, particularly a reflection sheet disposed so as to surround a straight tube type fluorescent lamp provided along the edge of a light guide plate in an edge type back light device among back light devices of liquid crystal display panels, and a reflection sheet disposed on the reverse side of the light guide plate.
PRIOR ART
In an edge type back light device among back light devices of liquid crystal display panels, a straight tube type fluorescent lamp is disposed along the edge of a light guide plate. Part of light emitted from the fluorescent lamp on the light guide plate is directly guided to the light guide plate. The remainder of the light is reflected by a reflection sheet disposed so as to surround the fluorescent lamp and then enters the light guide plate. The light guided to the light guide plate is reflected by a light diffusing layer, which has been printed in a dot form on the reverse side of the light guide plate, and then leaves the surface of the light guide plate (illuminating surface). On the other hand, the light directed to the reverse side of the light guide plate through between the light diffusing layers is reflected by the reflection sheet disposed on the reverse side of the light guide plate. The light diffusing layer is generally formed of a white pigment ink with glass beads optionally dispersed therein.
Many proposals on the light diffusing layer have hitherto been made as means for increasing the brightness of the illuminating surface. Japanese Examined Patent Publication (Kokoku) No. 4-66519 discloses a light diffusing layer having fine cells formed by foaming or a light diffusing layer formed by screen printing of a plastic balloon powder. Japanese Unexamined Patent Publication (Kokai) No. 4-76593 discloses a light diffusing layer formed by screen printing of a balloon polymer, having a diameter of 0.1 to 20 &mgr;m, of an acrylic resin. Japanese Unexamined Patent Publication (Kokai) No. 5-273552 discloses a light diffusing layer formed by screen printing of fine transparent beads or reflective beads including a white pigment. Japanese Unexamined Patent Publication (Kokai) No. 5-303017 discloses a light diffusing layer formed by mixing an acrylic binder with ultrafine particulate titanium oxide coated with an organic fluorescent material to prepare an ink and forming a dot gradation pattern using the ink by screen printing. Japanese Unexamined Patent Publication (Kokai) No. 6-94923 discloses a light diffusing layer formed by printing a foamable coating and forming fine cells.
All of the above means for improving the brightness utilize a light diffusing layer provided on the reverse side of the light guide plate by screen printing or the like, and attempts have been hardly made to improve the brightness by improving components constituting the back light device. In order to enhance the brightness, a polyester film with silver deposited thereon is, in some cases, used as the reflection sheet disposed so as to surround the fluorescent lamp. This type of reflection sheet, however, involves problems including that a high frequency current leaks from the fluorescent lamp, resulting in increased current consumption. In recent years, a 75 &mgr;m-thick white polyester film or the like has become used as the reflection sheet. Further, a 188 &mgr;m-thick white foamed polyester film has been used on the reverse side of the light guide plate. The inventions described in the above patent documents too use the white foamed polyester film. The white foamed polyester film has a lower reflectance than the polyester film with silver deposited thereon, resulting in about 10% or more lowering of the brightness when the white foamed polyester film is built in the back light device.
For this reason, as shown in
FIG. 2
, a reflection sheet comprising a 75 &mgr;m-thick white foamed polyester film
5
of the type described above, a white ink coating layer
6
provided on the reverse side of the polyester film, and a reflecting layer
7
, formed of barium sulfate and an acrylic binder, provided on the reflecting surface side has been developed and used as an improved reflection sheet having an enhanced brightness (RW75C manufactured by Kimoto Co., Ltd.). For this reflection sheet, however, the improvement in brightness over the brightness for the conventional reflection sheet is unsatisfactory, i.e., only 3 to 5%.
In recent years, a reduction in thickness of liquid crystal panels has been desired in the art, leading to a reduction in diameter of the fluorescent lamp. The 75 &mgr;m-thick foamed polyester film which has been used up to now has a problem that, when the film is built in a back light device so as to surround the fluorescent lamp, due to excessively high stiffness, the work efficiency is so low that the productivity cannot be increased. Further, the foamed polyester film often poses a problem that it pushes out seams in the body of the liquid crystal panel, creating a gap through which light leaks. This results in lowered brightness. On the other hand, reducing the thickness of the white foamed polyester film for the purpose of lowering the stiffness causes a problem that the brightness lowers with a reduction in the film thickness.
PROBLEMS TO BE SOLVED BY THE INVENTION
For the above reason, in order to provide a screen having higher brightness, the development of a high-reflectance, low-stiffness reflection sheet has been desired in the art. An object of the present invention is to provide such a reflection sheet.
MEANS FOR SOLVING THE PROBLEMS
The present invention provides a reflection sheet comprising: a film support, such as polyester, and, applied on one side of the support, a reflective paint coat comprised of a mixture of a resin binder of a (meth)acrylate (the term “methacrylate” referring to acrylate or methacrylate; the same shall apply hereinafter) copolymer with a small particle balloon.
The particle diameter of the small balloon may be in the range of from about 0.05 to 10 &mgr;m. When the particle diameter is less than about 0.05 &mgr;m, it is difficult to prepare such a small balloon by emulsion polymerization. Even though the small balloon could be successfully prepared, the capability of scattering light is low, resulting in lowered brightness. When the particle diameter is more than about 10 &mgr;m, the capability of scattering light is lowered except for the case where a number of small empty spaces are present within the particle, i.e., where the empty space is not of a single spherical type. The inside empty diameter to particle diameter ratio is preferably in the range of from 0.2 to 0.9. When this ratio is less than 0.2, the proportion of the empty space within the particle balloon after the formation of a reflective coating layer (hereinafter often referred to as a “reflective layer”) becomes very small, resulting in lowered brightness. On the other hand, when the ratio exceeds 0.90, the strength of the small particle balloon becomes so low that the particle balloon, when dried, cannot maintain its spherical shape and is crushed or collapsed, here again resulting in lowered brightness.
The small particle balloon may be formed of an organic material or an inorganic material. Regarding the small particle balloon formed of an inorganic material, there is a glass balloon. However, the small particle balloon is preferably formed of an organic material because it is difficult to provide particle balloons having the above particle diameter from the inorganic material. The organic small balloon may be prepared by emulsion polymerization or suspension polymerization of an acrylic monomer or a styrene monomer. Details relating to the production of the organic particle balloon are known and described, for example, in Japanese Unexamined Patent Publication (Kokai) No. 62-127336 and Japanese Examined Patent Publication (Kokoku) No. 3-9124.
The mixing ratio of the small particle balloon to the binder comprised of a (meth)acrylate copolymer also influences the brightness.
Nagamatsu Hideki
Nanba Takaaki
Okada Mitsuhiko
Shiota Koh
Uchiya Tomoaki
Ahmed Sheeba
Buckingham Stephen W.
Thibodeau Paul
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