A color filter for liquid-crystal display apparatuses include red pixel portions (R), green pixel portions (G), and blue pixel portions (B). These pixel portions each have a structure in which a thin film composed of a synthetic resin including an organic pigment dispersed therein is disposed on a substrate. These red, green, and blue pixel portions respectively include red, green, and blue organic pigments as the organic pigment.
There has been a demand for organic pigments used for producing color filters which have characteristics completely different to those required for conventional general-purpose applications. Specifically, for example, there has been a demand for a display of a liquid-crystal display apparatus that is clearer (increase in contrast) or brighter (increase in brightness) than existing displays. In order to meet these demands, organic pigments that are fine powders having an average primary particle size of 100 nm or less are often used.
However, on the other hand, the smaller the pigments, the larger the surface area of the pigments. This increases surface energy, which results in aggregation of the organic pigment due to a thermal history during production of the color filter. As a result, brightness and contrast are disadvantageously reduced.
Accordingly, a surface treatment using an organic pigment derivative, a surfactant, or a synthetic resin is commonly performed in order to impart heat resistance. This surface treatment improves the dispersibility, dispersion stability, and heat resistance of an organic pigment that has not yet been subjected to any surface treatment.
Examples of known methods for surface treatment include a kneading method, an acid/alkaline precipitation method, and a heating under pressure method. Specifically, a method in which an organic pigment is subjected to solvent-salt-milling in the presence of the nonvolatile component of an acrylic resin is known.
The surface treatment for an organic pigment using a synthetic resin has been studied for polymers belonging to each category, which are roughly divided into (meth)acrylic resins, epoxy resins, polyester resins, polyurethane resins, and the like. However, systematical studies on a category of polymer that has markedly improved dispersibility and dispersion stability and on the structure of a polymer that selectively has the best improved dispersibility and dispersion stability described above among polymers of a specific category have not been conducted and thus there are many unclear points.
However, because a fine organic pigment are more likely to cause aggregation than general-purpose organic pigments, the method of surface treatment for organic pigments used for general purposes does not always achieve the intended improvement effect regardless of the type of synthetic resin used. In reality, a synthetic resin that belongs to an optimal category and that has an optimal structure is selected by trial and error.
Specifically, examples of methods for surface treatment for an organic pigment used in preparation of color resists for color filters include a method in which a rosin ester and an organic pigment are subjected to solvent-salt-milling (PTL 1) and a method in which an organic pigment is subjected to heating under pressure in the presence of a polyurethane resin in a liquid medium (PTL 2). In addition, an organic pigment composition including copper phthalocyanine, bromo-chloro copper phthalocyanine, and a resin having a phosphate group is described (PTL 3).