1. Field of the Invention
The present invention provides an aqueous metallic ink composition. More particularly, the aqueous metallic ink composition of the invention has utility in the ink for writing instruments, for printing, for coating, and the like.
2. Description of Related Arts
It has been a conventional practice to prepare an aqueous metallic ink composition by adding to an aqueous solution with a metallic powder pigment and a colorant, a water-soluble organic solvent, if required, and a water-soluble resin as a bonding agent (a binder resin), a viscosity controlling agent, or a dispersant, and the like. For example, Japanese Unexamined Patent Publication No.8-199108 (1996) discloses an aqueous ink composition for ball-point pens comprising a metallic powder pigment, a resin, a specific compound capable of providing lubricity, a water-soluble organic solvent, and water. In this report, the resin is used as the bonding agent (the binder resin) and the viscosity controlling agent. The binder resin includes, for example, a water-soluble acrylic resin, an acrylic emulsion and the like. Examples of thickening agents include a microbial polysaccharide or derivatives thereof such as xanthan gum or dextran, a water-soluble vegetable polysaccharide or derivatives thereof, and a water-soluble seaweed polysaccharide or derivatives thereof, water-soluble cellulose derivatives, and a water-soluble synthetic polymer. Frequently, an anionic polymer such as the acrylic resin is used as a pigment dispersant.
However, when the aqueous metallic ink composition containing the conventional water-soluble resin is used for coating (writing or printing) on an absorbent surface (the surface capable of absorbing aqueous ink or being penetrated by aqueous ink) such as a paper (for example, a paper for copy), the pigment (the colorant) penetrates into the paper and the density of the color development at the ink film such as prints or images decreases with the passage of time (for example, from the moment of writing to a few minutes after writing). Also the problem arises that the ink film such as prints or images blurs easily. At the same time, the ink film fixability to a nonabsorbent surface (the surface which neither absorbs the aqueous ink nor be penetrated by the aqueous ink) is low. Moreover, using only the cellulose derivative as a water-soluble resin, the dispersibility falls off because of gelation. These ink compositions have low viscosity stability with time, so the viscosity increases or decreases after storage. Therefore, it is difficult to keep the adequate viscosity for a long time. Accordingly, it is difficult to control the viscosity of the ink composition for each purpose.
The anionic polymer as the pigment dispersant also may cause a viscosity change of the ink (such as an increase of the viscosity) with aggregation or precipitation. The metallic powder pigment (particularly, an aluminum powder pigment) tends to ionize in the ink and the pH of the ink changes because of the ion element which is produced by the metallic powder pigment. Specifically, when the pH of the ink decreases, the aggregation or the precipitation of the anionic polymer changes the viscosity of the ink. Furthermore, the change of the viscosity, which is caused by the aggregation or the precipitation of the anionic polymer in the ink, is often the increase of the viscosity. Accordingly, the writing characteristic deteriorates in some cases.
Thus, the conventional aqueous metallic ink composition has problems in that the density of color development at the ink film decreases while the viscosity stability with time is low.