subject: Ink-jet Printing Inks [print this page] An ink-jet printing ink composition which has high printing stability after long periods of time and at high temperatures is provided. The ink-jet printing ink composition is an aqueous ink having a nitrite ion concentration between about 0.2 to 10 parts per million (ppm) in order to lower the concentration of dissolved gas in the ink.
Ink-jet printers are known in the art and have advantages over other printing techniques, such as freedom from noise and attainment of high-speed printing, high-quality printing, and color printing. In the ink-jet printing process, an ink is compressed by an electromechanical transducing-vibration or electrothermal transducing-bubble and a high-pressure ink particle is then ejected from a nozzle for printing.
A disadvantage of ordinary inks used in this process is that ordinary ink contains dissolved gases which upon compression from bubbles or bubble nuclei. This results in blank spots in the printing known as the "missing dot" condition.
For this reason, ink-jet printing inks have been subjected to treatment, such as deaeration or fine filtration, to remove bubble nuclei, and then stored in a bag or vessel having low air permeability.
The disadvantage of these treatment and storage methods is that after a period of time, and particularly when the ink is exposed to high temperatures, bubbles still form in the ink notwithstanding the initial deaeration. Thus, after a period of time or exposure to high temperatures, the ink develops the same problems as untreated ink, i.e. an inability to be uniformly ejected from the nozzle. Thus, the prior art inks for ink-jet printers are subjected to limitations of time and temperature which affect the transportation, shelf-life, storage, and use of the ink.
Accordingly, an ink-jet printing ink which can perform in an ink jet printer after long periods of time and at high temperatures is desired
The amount of dissolved nitrogen was measured by gas chromatography (with a column packed with Molecular Sieve 5A manufactured by Gasukuro Kogyo, Inc.). As can be seen from FIG. 2, the concentration of nitrogen in the ink rises dramatically over time whereas the concentration of nitrogen in the water increases very little during the same period of time. This increase of nitrogen concentration in the ink occurs even though the bag used to store the ink has excellent gas shielding properties, and the ink has been deaerated or an oxygen reducer has been added to the ink. As shown in FIG. 2, after 120 hours at 70 C., the amount of dissolved nitrogen has increased to 14 ppm. The presence of bubbles in the ink has been confirmed when the amounts of dissolved nitrogen exceeded this value.
Based on the assumption that the generation of nitrogen is caused by impurities in the ink, the present inventors determined that the presence of a nitrite ion and a dye, particularly one having at least one azo linkage, causes the nitrogen generation. When the ink contains a dye having at least one azo linkage, if the nitrite ion concentration in the ink is adjusted to 10 ppm or less, the formation of bubbles is completely eliminated even when the ink is left to stand at a temperature of 70 C. for 15 days. Thus, this ink prepared in accordance with the invention can satisfy the severest possible operating conditions.
When the nitrite ion concentration is reduced below 0.2 ppm, the resulting print tends to undergo the "curved flight" phenomenon, after the printer has been out of use for a long period of time. "Curved flight" means that the printing is either misaligned or missing. Accordingly, the nitrite ion concentration of the ink should be between about 0.2 to about 10 ppm.
