Ink Chemistry. The most critical component of ink-jet printing is probably the ink. Ink chemistry and formulations not only dictate the quality of the printed image, but they also determine the drop ejection characteristics and the reliability of the printing system. Many different types of inks have been developed and used in ink-jet applications. Figure 26 illustrates a technology map of different types of ink-jet inks.

Aqueous- or water-based inks are commonly used in home and small-office ink-jet printers such as in the Hewlett-Packard DeskJet series, Canon BJC series, and Epson Color Stylus series ink-jet printers. In the case of thermal ink-jet, due to the basic vapor bubble formation process, water seems the material of choice for the method. Typical composition of a water-based ink for ink-jet printing is presented in Table II. Viscosity of water-based ink-jet inks range from 2 to 8 cps.

Figure 27 illustrates the behavior of a water-based ink droplet when it lands on the surface of an uncoated media such as bond, copy, or plain papers. The ink tends to spread along the paper fibers and penetrate into the bulk of the paper. The water-based ink actually depends on penetration and absorption for its drying mechanism. Some evaporation of water is taking place, but this drying mechanism is often very slow. Such ink behavior lowers color density and spot resolution on paper. It has been known for some time that paper or other media with a coated water-receiving layer can greatly improve both color density and resolution by controlling the ink spreading and penetration at the coated layer. However, just within the past few years, the market for the specialty-coated ink-jet media has exploded, especially in the home photo quality and large-format ink-jet printing areas.32 Recent availability of printhead technologies with high resolution (such as 1440 dpi Epson Color Stylus 800, 1200 x 1200 dpi Lexmark 7000, 10 pL drop Hewlett-Packard DeskJet 890C), multilevel dye load gray-scale (such as in photo pens from Epson, Canon, and Hewlett-Packard), and multilevel dot volume gray-scale capability (such as in Hewlett-Packard DeskJet 720C and 890C) certainly have made a positive impact on this trend.

A cross section light microscope of pigment-based (on the left) and dye-based (on the right) inks on paper.

One significant advantage of pigment-based as compared to dye-based ink is its color durability when exposed to light or outdoor weather conditions. This feature is definitely critical to applications such as billboards or other large-format displays. However, as compared to dye base, pigment-based ink-jet ink has the inherent disadvantage of particle dispersion instability that may lead to nozzle clogging. Even though the pigment dispersion chemistry and process of making ink-jet ink has improved significantly in the past few years,37­40 the trend in the ink-jet industry is toward smaller nozzle diameter for high resolution and a higher number of jets for print speed. The reliability issue should be a part of the decision process when it comes to the pigment or dye-base question.

Another recent development in the ink-jet ink area is the introduction of the Canon BJC-7000 ink-jet printer. This new printer implemented a new process called Plain Paper Optimized Printing (P-POP). The black printhead contains black ink and a precoat fluid applied to the paper surface a few microseconds before dye-based ink drops hit the paper. The precoat fluid is clear and believed to contain a compound that will aggregate with the dye, thus fixing it instantly on the surface of paper.41 This unique approach is proven to provide excellent waterfastness. The P-POP process not only improved waterfastness of the printed image but fixed the ink on the top surface of the paper, making a plain paper perform like a coated media. If this approach does not impact the overall reliability of the system in any way, the P-POP process will become one of Canon's significant technological breakthroughs.

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