1. Technical Field
The present invention relates to an ink composition, an ink set, and an image forming method.
2. Description of the Related Art
Recording methods utilizing inkjet methods allow recording of high-quality images on various recording media by ejecting droplets of inks from a number of nozzle holes provided at an inkjet head. Therefore, recording methods utilizing inkjet methods are widely used.
Pigments are widely used as a coloring material, which is one of the components contained in an ink.
Among the pigments used in inks, carbon black is widely used as a black pigment.
Further, a technique in which a pigment other than carbon black, such as a cyan pigment, is used together with carbon black (hereinafter, may be abbreviated to “CB”) is also known.
Specifically, for example, a pigment-dispersed aqueous recording liquid containing carbon black (CB) and a pigment other than CB, such as a cyan pigment, together with a urethane resin has been disclosed (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2004-285344). Further, an aqueous ink for inkjet recording which contains polymer particles containing carbon black has been disclosed (see, for example, JP-A No. 2009-144007).
However, pigment inks containing a pigment may thicken and gradually solidify, due to evaporation of solvent components in the inks. When pigment inks solidify, it is hard to dissolve the ink again afterwards. Thus, when such a thickened or solidified ink adheres to a nozzle tip portion of an inkjet head or the like and is gradually deposited thereon, the nozzle holes may become narrower or clogging may be caused, which leads to problems such as deviation in the ejection direction of the ink or non-ejection. When the adhered ink is readily deposited, it is difficult to maintain the performance by means of using a nozzle cap, a wiping unit, or the like, and as a result, image formability deteriorates over time.
In connection with the above, methods using a liquid for washing off adhered inks have been proposed and, for example, a maintenance liquid for inkjet recording containing a resin-dissolving solvent which is insoluble or slightly soluble in water and a moisturizing agent has been disclosed (see, for example, JP-A No. 2007-169314). It is said that when such a maintenance liquid is used, defects such as unstable ejection, clogging of nozzles, or the like are less likely to occur.
Further, in inks including a pigment, a dispersant for dispersing the pigment is generally used, and polyvinyl pyrrolidone is known as the dispersant. For example, an ink for aqueous recording which contains polyvinyl pyrrolidone as a polymer dispersant together with a pigment, a resin emulsion, and the like has been disclosed (see, for example, JP-A No. 4-356570 and Japanese Patent No. 3000853).