Ink-jetting

Keta.Is, Trimethylpentanediol reportedly forms a cycHc ketal by heating it with benzophenone ia the presence of sulfonic acid catalysts at reflux temperatures ia toluene (64). These are said to be useful as aprotic solvents for ink-jet printing and as inflammation inhibitors for cosmetic preparations... 

There are two types of impulse printers (Eig. 19). A piezoelectric ink jet propels a drop by flexing one or more walls of the firing chamber to decrease rapidly the volume of the firing chamber. This causes a pressure pulse and forces out a drop of ink. The flexing wall is either a piezoelectric crystal or a diaphragm driven by a piezoelectric incorporated into the firing chamber (Eig. 19a). Thermal impulse ink jets also propel one drop at a time, but these use rapid bubble formation to force part of the ink in a firing chamber out the orifice (Eig. 19b). 

Fig. 19. Ink-jet system (a) piezoelectric ink-jet firing chamber and (b) bubble formation ia thermal ink-jet technology.

There was a logical progression of technology development from continuous to piezoelecttic ink jet. Designers of continuous ink-jet systems ensure that the ink stream breaks into drops of constant size and frequency by applying vibrational energy with piezoelecttic crystals at the natural frequency of drop formation. This overcomes the effects of any random forces from noise, vibrations, or air currents. 

The earliest significant technical work on piezoelecttic ink jet began in the 1930s and the first tme commercial activity was begun in the late 1960s. This early development effort, aimed at office printing appHcations, had limited commercial success. The first successful piezoelecttic ink-jet printer was introduced in 1977. It printed a relatively cmde character set using an array of 12 jets in its printhead. 

Inks. The main components of the inks ate typically water, colorants, and humectants. Additives ate used to control drying time, waterfastness, lightfastness, and consistency of drop formation. Water is an excellent vehicle for ink jet because of its high surface tension and safety in all environments. 

The principal physical properties influencing ink performance ate surface tension and viscosity. High surface tension is desired for good droplet formation and capillary refill in dtop-on-demand ink jet. Low viscosity is desired because less energy is required to pump and eject ink. Conductivity is also an important parameter. Continuous ink-jet inks must have some conductivity to allow for charging. Low conductivity is generally preferred for impulse, particularly thermal ink jet, because excess ions can cause corrosion of the printhead. 

Inks for continuous ink-jet printers typically comprise dyes dissolved in water or solvent having salts added to make the ink conductive for electrostatic charging. Whenever waterproof printing is required, low boiling solvent inks ate used. For printers that ate used in office environments, water is used as the ink solvent. Using water-based inks, humectants may be added to inhibit drying of ink in the sump and surfactants ate added to wet the printing surface. 

Piezoelecttic impulse ink-jet printers ate especially sensitive to bubbles in the ink. A bubble in the firing chamber absorbs some of the comptessional force from the flexing of the chamber wall and reduces drop volume and drop velocity, thereby affecting print quaHty. Because of the limited range of motion of the crystal, bubbles ate not readily ejected, and the loss of print quaHty owing to their presence is persistent. 

Humectants and low vapor pressure cosolvents are added to inhibit drying of ink in the no22les. Surfactants or cosolvents that lower surface tension are added to promote absorption of ink vehicle by the paper and to prevent bleed. For improvements in durabiUty, additional materials such as film-forming polymers have been added. Ink developments are providing ink-jet prints with improved lightfastness, waterfastness, and durabiUty. As a result, such prints are beginning to rival the quaUty of electrophotographic prints. 

Office. Various segments of the office market have the largest population of computers, and thus the greatest demand for computer printers. In 1994, however, more computers were purchased for home use than for the office. The fastest developing computer printer technology is thermal ink jet, whether measured by rates of performance increase, price decrease, or purchase. 

Fig. 20. Growth in image quaUty of ink-jet technology, where corresponds to quaUty in 1995 for (-) continuous flow, (—...

Sulfonic acid-based azo dyes (qv) and intermediates are characterized by the presence of one or more azo, R—N=N—R, groups. An example of a water-soluble polysulfonic acid-based azo dye (3), where M is a metal, typically Na, for use in ink-jet printers, is shown (75). 

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