Compositions: coating or plastic – Materials or ingredients – Pigment – filler – or aggregate compositions – e.g. – stone,...
Reexamination Certificate
1999-12-23
2001-10-16
Green, Anthony (Department: 1755)
Compositions: coating or plastic
Materials or ingredients
Pigment, filler, or aggregate compositions, e.g., stone,...
C430S007000, C524S100000, C544S187000, C544S188000
Reexamination Certificate
active
06302953
ABSTRACT:
BACKGROUND OF THE INVENTION
a) Field of the Invention
This invention relates to pigment dispersions for the production of color filters (hereinafter called “pigment dispersions for color filters”), which are excellent in fluidity, storage stability, dispersion stability, and the like.
b) Description of the Related Art
A color filter, which is useful for the production of a liquid crystal color display, an image pickup device or the like, has conventionally been produced primarily by spin-coating a color filter substrate with a pigment dispersion for a color filter—said pigment dispersion containing pigments of three colors, that is, red (R), green (G) and blue (B) dispersed in a solution of a photosensitive resin—to form a color film, exposing the color film to light through a photomask, and then developing the exposed color film to form the color film into a pattern so that desired pixels are formed on the color filter substrate.
As primary pigments for use in the production of color filters, phthalocyanine green, for example, C.I. pigment green (hereinafter called “P.G.”) 36 is generally used as a green color, anthraquinone red, for example, C.I. pigment red (hereinafter called “P.R.”) 177 is generally employed as a red color, and phthalocyanine blue, for example, C.I. pigment blue (hereinafter called “P.B.”) 15:6 is generally used as a blue color. There are however differences between the hues of these pigments and color characteristics required for liquid crystal displays. For a green pigment and a red pigment, a yellow pigment is therefore used in combination in a small amount as a complementary color, and for a blue pigment, a purple pigment is also used in combination in a small amount as a complementary color.
A pigment dispersion for a color filter is generally composed of a pigment, a dispersant, a film-forming resin and a liquid medium. As the film-forming resin, an acrylic resin having an acid value high enough to permit development with an aqueous alkaline solution after the formation of a film is mainly adopted. However, a pigment dispersion formed of the above-described conventional pigment and the acrylic resin of the high acid value has poor storage stability in many instances. Flocculation of the pigment therefore takes place so that the viscosity of the pigment dispersion tends to become higher with time.
When a color filter is produced by using the pigment dispersion accompanied by such difficulties as described above, the pigment dispersion is coated on a color filter substrate by spin-coating. If the viscosity of the pigment dispersion is high or if pigment particles in the dispersion undergo flocculation and the pigment dispersion exhibits thixotropic viscosity, The pigment dispersion so coated becomes thicker around a central portion of the substrate. This leads to occurrence of unevenness in color hue and a difference in color density between the resulting color film at the central part of the substrate and that at a peripheral part of the substrate upon production of a large-screen color filter.
Accordingly, a pigment dispersion for a color filter must be in such a stable dispersion state of the pigment as being free from mutual flocculation of its pigment particles and must have a low viscosity of from 5 to 20 centipoises viscosity and excellent storage stability, despite its pigment content is generally in a range of from 5 to 10 wt. %. With a view to meeting performance requirements for such pigment dispersions for color filters, a variety of dispersants are disclosed in JP 60-237403 A and JP 60-247603 A. Pigment dispersions making use of these pigment dispersants (hereinafter simply called “dispersants”) are accompanied by a drawback in that they show neither sufficiently low viscosity nor sufficiently high dispersion stability and also by another drawback in that the maximum absorption wavelengths at RGB pixels in color filters produced from these pigment dispersions are caused to shift toward a lower wavelength side or a shorter wavelength side, thereby lowering their color quality as color filters for liquid crystal displays.
SUMMARY OF THE INVENTION
To develop a dispersant capable of solving the above-described drawbacks of the conventional dispersants and also of permitting formulation of a pigment dispersion for a color filter, said pigment dispersion being provided with improved color quality and lowered viscosity, the present inventors have proceeded with an extensive investigation. As a result, it has been found that particular derivatives of triazinylaminoanthraquinone dyestuff commonly act as excellent dispersants for a phthalocyanine blue as a primary pigment for a blue color and a purple pigment as a complementary pigment therefor, an anthraquinone red (P.R. 177) and pyrrolopyrrole pigments (P.R. 254, P.O. 71) and a yellow pigment as a complementary pigment therefor, and a phthalocyanine green (P.G. 36) as a primary pigment for a green color and a yellow color as a complementary pigment therefor and can achieve reductions in the viscosities of pigment dispersions. It has also been found that the resultant pigment dispersions can be prevented from thickening and gelatinization during storage and have been improved in clarity, a most important property for color filters. These findings have led to the completion of the present invention.
The present invention therefore provides a pigment dispersion for a color filter, said pigment dispersion being composed of a pigment, a dispersant, a film-forming resin and a liquid medium, wherein said dispersant comprises a compound represented by the following formula (I):
wherein X and X′ each independently represent a hydrogen atom, a hydroxyl group, an alkoxy group, a primary, secondary or tertiary amino group, or an acylamino group; Y represents an anthraquinonylamino, phenylamino or phenoxy group having a hydrogen atom, a hydroxyl group, an alkoxy group, a primary, secondary or tertiary amino group, or an acylamino group at the 4-position or 5-position thereof; A and B each independently represent an alkyl group, a cycloalkyl group or an aryl group, and at least one of A and B has at least one substituent group containing a basic nitrogen atom; and Z represents a hydrogen atom, a cyano group, a halogen atom, an alkyl group, an alkoxy group, a nitro group, a benzoylamino group or a 3-benzoyl group, and said 3-benzoyl group may be fused together with X to form an acridone ring.
The term “at least one substituent group containing a basic nitrogen atom” as used herein may mean a primary, secondary or tertiary amino group, a quaternary ammonium group or a pyridinium group, with a tertiary amino group being particularly preferred.
The pigment dispersion according to the present invention for the color filter is characterized in the use of the particular derivative of the triazinylamino-anthraquinone dyestuff as the dispersant for the pigment. Owing to the use of the dispersant, flocculation of particles of the pigment in the pigment dispersion for the color filter can be prevented. As a consequence, the pigment dispersion for the color filter has been lowered in structural viscosity, and has a low viscosity. The pigment dispersion is prevented from thickening and gelatinization, and is provided with increased storage stability. Further, the dispersant represented by the formula (I) has by itself a color tone of yellowish red-bluish red-blue. When this dispersant is used as a dispersant for a blue pigment and a purple pigment as a complementary color pigment for the blue pigment, a green pigment and a yellow pigment as a complementary color pigment for the green pigment, and a red pigment and a yellow pigment as a complementary color pigment for the red pigment, respectively, and the resulting pigment dispersions are used for the formation of pixels of color filters in a liquid crystal color display or the like, the pixels can be formed with desired color quality.
According to the present invention, the use of the specific dispersant makes it possible to prepare the pigment dispersion stably.
Abe Yoshio
Fukuda Tetsuo
Nakamura Michiei
Okamoto Hisao
Saikatsu Hiroaki
Dainichiseika Color & Chemicals Mfg. Co. Ltd.
Green Anthony
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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