1. Field of the Invention
The present invention relates to a pigment-dispersed ink-jet ink having high ejection stability from an ink-jet nozzle, ink sets that use this pigment-dispersed ink and can provide high-quality color images, an ink tank, a recording unit, an ink-jet recording apparatus, an ink-jet recording process, and a production process of the pigment-dispersed ink-jet ink.
2. Description of the Background Art
In inks used in an ink-jet recording system, it is currently a subject of various investigations to use pigments excellent in weather-fastness as coloring materials for inks for the purpose of improving the weather-fastness of recorded articles formed on recording media using such inks. Since a pigment is not dissolved in an ink-jet ink, but is in a dispersed state in a liquid medium, however, the ejection velocity of a pigment-dispersed ink ejected from a nozzle is unstable, and the ejection stability of the ink has been extremely low, as demonstrated by the fact that the ink is sometimes not ejected from the nozzle. As a result, the problem has occurred that the ink does not exactly impact at an intended position on a recording medium, and so an image to be formed is disordered.
On the other hand, it is investigated to improve the ejection stability by adding a certain kind of nonionic surfactant into an ink (see, for example, Japanese Patent Application Laid-Open Nos. H05-140496 and H07-207202).
As an ink-jet ink, that having high penetrability into a recording medium as its physical property is generally used in view of the fixing speed thereof and bleeding between inks of different colors. Therefore, a coloring material penetrates into the interior of a recording medium when an image is formed on plain paper or the like, and so it is impossible to fix a sufficient amount of the coloring material on to the surface of the recording medium. As a result, it has been difficult to provide a recorded article having good coloring ability.
In order to achieve sufficient coloring, various proposals have been made about the fact that a reactive liquid reactive to a coloring material in an ink is used in combination with the ink upon formation of an image, and the reactive liquid is brought into contact with the coloring material upon the formation of the image, thereby depositing and aggregating the coloring material in the ink to conduct solid-liquid separation into a liquid medium and the coloring material so as to increase the amount of the coloring material fixed on to the surface of a recording medium (see, for example, Japanese Patent Application Laid-Open No. H05-202328). Although addition of that kind of nonionic surfactant into a pigment-dispersed ink has been made for the purpose of improving the ejection stability of the ink as described above, however, the effect thereof varies according to the kind of pigment and a dispersing agent, and there have been only a few surfactants capable of reliably retaining sufficient ejection stability. Further, the aggregating reaction of a pigment by virtue of the reactive liquid described above may have been impeded in some cases depending on the kind of the nonionic surfactant added. As described above, it has been extremely difficult to reconcile an improvement in the ejection stability from an ink-jet nozzle with improvement of the coloring properties of a recorded article when a pigment-dispersed ink is used for the purpose of providing a recorded article having good weather-fastness.
A proposal has been made that when a water-soluble resin is used as a resinous dispersing agent for an ink-jet ink using a pigment dispersion (hereinafter referred to merely as “pigment dispersion”) in which a pigment is dispersed in a liquid medium with the resinous dispersing agent, a water-soluble resin having higher solubility in water is preferably used for the purpose of reliably retaining the ejection stability (see, for example, Japanese Patent Application Laid-Open No. H05-263029). When a resin having high solubility in water is used as the resinous dispersing agent, however, the adsorptivity of the resinous dispersing agent to the pigment is weak, and so it has been difficult to reliably retain dispersion stability and storage stability.
In order to improve the water fastness and fixing ability of a recorded article formed on a recording medium, a proposal that a resin having a solubility in water of at most 2% by mass at 20° C. is added in addition to the resinous dispersing agent (Japanese Patent Application Laid-Open No. H05-331395), and a proposal that particles of an insoluble resin are added (Japanese Patent Application Laid-Open No. H05-25413) have been made. However, a resin having a high solubility in water is used as the resinous dispersing agent even in these proposals. Therefore, another problem that the ejection stability of the resulting ink is deteriorated when the resin having a low solubility in water is separately added in addition to the resinous dispersing agent, and so the impact accuracy of the ink on a recording medium is lowered, and the resulting recorded article is disordered has been involved.
Fine particles dispersed in a solution like pigments in an ink are held in a dispersed state by an action between van der Waals attraction (force) acting between the fine particles, and electrostatic repulsion generated by static charges on the surfaces of the fine particles and repulsion by steric hindrance of a resin and a surfactant adsorbed on the surfaces of the fine particles. With respect to the van der Waals attraction, a Hamaker constant is determined, and the van der Waals attraction may be calculated out from the constant for the fine particles and the particle diameter thereof. On the other hand, the degree of the effect of the electrostatic repulsion may be determined by measuring the zeta potential of the fine particles. A great number of proposals have heretofore been made on pigment-dispersed ink-jet inks improved in the dispersion stability of a pigment contained therein and the ejection stability of the inks by defining the electrostatic repulsion, i.e., the zeta potential. With respect to the effect of the repulsion by steric hindrance of the resin and surfactant adsorbed on the surfaces of the fine particles, however, a method for evaluating it has not been established, and a sufficient investigation has not been made about the influence of a repulsion factor by this steric hindrance on pigment-dispersed ink-jet inks.