Liquid ink has been used in many types of ink-jet printers of which the major categories are "Drop-On-Demand" ink-jet and "Continuous" ink-jet. For Drop-On-Demand ink-jet, ink is normally stored in a reservoir and delivered by capillary action to a nozzle in the print head of the printer. A means exists to force a single drop of ink out of the nozzle whenever it is needed to print a single spot on the printed medium (for example, paper). For Continuous ink-jet, ink is forced out of the nozzle in the form of a jet of ink which is unstable and subsequently breaks up into a steady stream of droplets. The trajectory of each drop is controlled typically by either electrostatic or magnetic forces. The drop can be directed either to a catcher for recycling through the main ink system or can be directed to the printed medium to form a printed spot.
The major problems with liquid ink-jet ink are (i) media dependent print quality, (ii) poor reliability, (iii) poor waterfastness, and (iv) a long drying (set) time for the printed ink. The print quality usually depends on the type of paper used, which also has an effect on the drying time and on waterfastness. Although water-based ink has been widely used, it exhibits poor waterfastness. Also, in order to prevent the ink from drying in the jet, high concentrations of humectant such as diethylene glycol have been used. This leads to a long drying (set) time for the print on the medium and poor print quality. The above mentioned factors are interrelated and trade-offs must be made. It is very difficult to satisfy all the conditions outlined above that make for an excellent ink-jet ink.
One method of solving several of the above mentioned problems is to use a phase change or hot melt ink. The ink is normally solid at room temperature. When the ink is heated the ink melts to form a low viscosity fluid which can be ejected as droplets. Most of the hot melt inks reported so far above have involved one phase change, that is, from solid at room temperature to liquid at operating temperature.
Hot melt ink was originally used by Berry et al. (U.S. Pat. No. 3,653,932, April 1972) in electrostatically controlled continuous ink-jet printing. The ink was comprised of a waxy component which is solid at room temperature, and the term used by Berry et al., "hot melt" ink, " . . . defines ink in a solid phase at room temperature and in a fluid phase at the operating temperature, which is above the melting temperature of the waxy material." The hot-melt ink was used in the conventional Drop-On-Demand ink-jet printing ejected by a pressure pulse from a piezoelectric crystal (U.S. Pat. No. 4,484,948). In both Continuous ink-jet and Drop-On-Demand ink-jet printing only one phase change is required.
Vaught et al. U.S. Pat. No. 4,490,731 discloses a process for driving ink-jet ink through a jet using a thermally induced vapor bubble. The pressure pulse from vaporization drives liquid ink drops through a small orifice of the print head toward the printed medium. The Vaught process requires transforming most of the ink two phases, that is, from solid to liquid and then from liquid to vapor.