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I n d . Eng. Chem. Res. 1989,28, 1712-1718
Design Criteria and Future Directions in Inkjet Ink Technology Melvin D. Croucher* and Michael L. Hair Xerox Research Centre of Canada, 2660 Speakman Drive, Mississauga, Ontario, Canada L5K 2Ll
Present inkjet inks consist of a dye dissolved in a water-based vehicle. Additives such as humectants, biocides, and chelating agents are also added to control the properties of the ink, which enable it to function effectively in a printing engine. Provided the ink is tailored to the properties of the paper that is used, prints of an acceptable copy quality are obtained. This ink-paper sensitivity limits the usefulness of inkjet printing devices. Consequently, considerable thought is being given to schemes that overcome this limitation. One such approach is to use a colloidal particle-based ink rather than a dyed fluid. In this paper, we describe the design criteria needed in order to formulate an inkjet ink. An approach based on a latex-based inkjet ink is described together with comments on the imaging characteristics that have been achieved. The problems associated with the use of particulate inkjet inks are also discussed, and suggestions are made for their solution. The past decade has seen the widespread introduction of word processing and personal computers in the workplace. These devices allow information to be created, manipulated, and stored in electronic form. Hard-copy output of these electronic images may be obtained with a variety of printing technologies. One of the most viable of these printing methods is inkjet printing, which has attracted considerable attention within the technical community. In inkjet printing, a uniform train of ink droplets is generated from a digital input. These ink drops are then deflected onto paper so as to produce a legible output. There are numerous ways in which a stream of droplets can be generated and directed to the correct position on the paper. However, only two modes of operation have been widely studied. These are shown schematically in Figure 1. The first method is known as the drop-on-demand system in which droplets of ink are generated as needed by a piezoelectric crystal and are ejected from 20-80-pm nozzles, producing an ink stream with a velocity of -3 m s-l. In order to produce characters on paper, the droplets are assigned the correct trajectory by the writing head. The second type of jet system is known as synchronous inkjet printing in which ink droplets are produced continuously. This is achieved by pressurizing the ultrasonically attenuated jet to -3 X lo5 Pa, which produces a stream of droplets (-106/s) with a velocity of -20 m s-l. The ink drops that are to be used to generate characters are inductively charged and deflected in a high-voltage electric field to a specific position on the paper. The uncharged ink droplets pass undeflected through the electric field to be caught in a gutter and recirculated through the fluidic circuit. In this paper, we will briefly review the physicochemical features demanded of inkjet inks. This will be followed by a description of the liquid inks presently used in this technology, together with a discussion of their imaging characteristics. Finally, we will discuss possible future material trends with special emphasis given to the role of colloidal dispersions as the marking material for this ink technology.
Functional Requirements of Inkjet Inks The physicochemical requirements for drop-on-demand systems and for synchronous inkjet inks are very similar as shown in Table I. It can be seen that the surface tension, the viscosity, and the conductivity of the ink are the parameters that have to be controlled and each is discussed separately.
Table I. Physicochemical Properties of Inkjet Inks drop-on- synchroproperty demand nous comments surface tension/ >35 >35 inkjet needs a stable mN m-' stream of droplets conductivity/ >10-3 (n cm)-* printing head performance viscosity/ 1-10 1-2 mN s m-2 is matched to the viscosity of the ink shelf 18 18 ink should not "age" life/months fast drying is essential by drying time/s
