The present invention relates to a water-color ink which provides high print quality on ordinary paper, recycled paper and coated paper, and which has excellent storage stability.
Inkjet recording is a method of recording letters and graphics on the surface of a recording medium by discharging ink as droplets from a fine nozzle. Inkjet recording systems which are in practical use include a method of converting an electrical signal into a mechanical signal using an electrostrictive element to intermittently discharge ink stored in a nozzle head part and record letters and symbols on the surface of a recording medium, and a method of rapidly heating that part of the ink stored in a nozzle head part which is closest to the ejection part to produce bubbles, and intermittently discharging the ink by means of the volume expansion caused by the bubbles to record letters and symbols on the surface of a recording medium.
Properties required of the inks used in such inkjet recording include absence of bleeding and good drying of the print on the paper which is the recording medium, the ability to print uniformly on a variety of recording medium surfaces, and absence of mixing of adjacent colors in multicolor printing including color printing.
In conventional inks and most inks which use pigments in particular, means of have been studied and put into practice of controlling wetting of the paper surface by the ink primarily by controlling permeability, and of ensuring print quality by confining the ink drops to near the paper surface.
However, with inks which are designed to control wetting of the paper there is a large difference in bleeding depending on the kind of paper, and in the case of recycled paper consisting of a mixture of various paper components in particular bleeding occurs due to differences in the wetting properties of the ink with respect to those different components. Moreover, with such inks it takes time for the print to dry, raising the issue of mixing of adjacent colors in the case of color printing and other multicolor printing, and in the case of inks which use pigments as coloring materials the pigments remain on the surface of the paper or the like, detracting from abrasion resistance.
To resolve such issues, efforts are being made to improve permeability of the ink into the paper, and research is being done into adding diethylene glycol monobutyl ether (U.S. Pat. No. 5,156,675, Specifications), adding Surfynol 465 (Nisshin Chemical), adding an acetylene glycol surfactant (U.S. Pat. No. 5,183,502, Specifications), or adding both diethylene glycol monobutyl ether and Surfynol 465 (U.S. Pat. No. 5,196,056, Specifications). The use of diethylene glycol ethers in inks is also being studied (U.S. Pat. No. 2,083,372, Specifications).
In the case of inks which use pigments, because it is generally difficult to improve the permeability of the ink while maintaining the dispersion stability of the pigment the selection of penetrating agents is small, and examples of a glycol ether combined with a pigment have heretofore included triethylene glycol monomethyl ether (Japanese Patent Application Laid-open No. S56-147861) and ethers of ethylene glycol, diethylene glycol or triethylene glycol (Japanese Patent Application Laid-open No. H9-111165) used with pigments.
However, conventional water-color inks have unsatisfactory print quality, with frequent bleeding when printed on ordinary paper such as PPC paper, and color density and color development have also been inadequate. Conventional dispersions have also been unstable, and adsorption-desorption is likely to occur in the presence of surfactants, glycol ethers and other substances with hydrophilic and hydrophobic parts, detracting from the storage stability of such water-color inks. In order to reduce bleeding in the paper ordinary water-color inks have required substances with hydrophilic parts and hydrophobic parts, such as surfactants and glycol ethers. Inks which lack these substances have inadequate permeability in paper, so for purposes of uniform printing the type of paper is limited, and the printed image is often adversely affected.
Moreover, when the sort of additives (acetylene glycol, acetylene alcohol or silicon surfactants, di(tri)ethylene glycol monobutyl ether, (di)propylene glycol monobutyl ether or 1,2-alkylene glycol or mixtures of these) used in the present invention are used in conventional dispersions long-term storage stability is not obtained, and because the ink has poor re-solubility the ink has been likely to dry and clog the nozzle of the ink jet head or the pen tip or the like of the writing instrument.
Moreover, in the case of pigments dispersed by such dispersants a dispersant residue remains in the ink, with the problem that the dispersant is released from the pigment without contributing sufficiently to dispersion and viscosity is increased. This high viscosity means that there are limits on the amount added to pigments and other coloring materials, and adequate image quality is not obtained particularly on ordinary paper.