Journal of Imaging Science and Technology · Volume 42, Number 1, January/February 1998

Progress and Trends in Ink-jet Printing Technology

Part 4

Hue P. Le*

Le Technologies, Inc., Beaverton, Oregon

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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.

Figure 26. An ink-jet ink technologies map.

Table II. Water-Based Ink-Jet Ink Composition

Component	Function	Concentration, %
Deionized water	Aqueous carrier medium	60 - 90
Water soluble solvent	Humectant, viscosity control	5 - 30
Dye or pigment	Provides color	1 - 10
Surfactant	Wetting, penetrating	0.1 - 10
Biocide	Prevents biological growth	0.05 - 1
Buffer	Controls the pH of ink	0.1 - 0.5
Other additives	Chelating agent, defoamer, solublizer etc.	> 1

 

Figure 27. Drying mechanisms of a water-based ink-jet drop on a plain paper.

Figure 28. A SEM photograph of phase-change ink drops on the surface of a bond paper.

Figure 29. A SEM photograph of phase-change ink drops after fuse by cold pressure rollers.

Figure 30. The basic configuration of the Tektronix's Phaser 350 offset drum transfer ink-jet printer.

Figure 31. A SEM photograph of a phase-change ink drop on the surface of aluminum substrate.

 

Phase-change ink is also called hot melt or solid ink which is solid at room temperature. The ink is jetted out from the printhead as a molten liquid. Upon hitting a recording surface, the molten ink drop solidifies immediately, thus preventing the ink from spreading or penetrating the printed media. The quick solidification feature ensures that image quality is good on a wide variety of recording media. Figure 28 shows a SEM photograph of phase-change ink drops printed on the surface of a Xerox 4024 bond paper. Notice that the ink drops maintain their hemisphere shape with little or no evidence of ink spreading, even along rough paper fiber structures. Tektronix currently implements phase-change ink-jet technology in the Phaser 300 ink-jet printer. However, in practice, the solidified ink drops need to be fused on top of paper to increase ink adhesion and prevent light scattering owing to the lens effect of the hemisphere shaped ink dot.³³ Figure 29 shows a SEM photograph of several Tektronix Phaser 300 phase-change ink drops after being fused by a cold pressure fusing roller.

Another successful implementation of phase-change ink-jet technology is in the Tektronix Phaser 350 offset trans fer printing architecture. Figure 30 describes major components of a Tektronix Phaser 350 color ink-jet printer. Basically, the printing process starts with coating a thin silicon oil film onto a warm rotating aluminum drum. Ink is then jetted onto this intermediate drum. Once an entire image is printed, it is then transferred from the drum onto a preheated media via a pressure nip. It is very critical for the drum to be heated at a temperature above the glass transition temperature and below the ink melting point so that the phase-change-ink material is soft enough to fuse into paper with moderate pressure, but still strong enough to prevent the ink from cohesive failure when exiting the pressure nip.³⁵

A SEM photograph of a phase-change ink drop on an aluminum surface is shown in Fig. 31. A SEM photograph of an offset-transferred phase-change-ink dot on an aluminum surface onto a bond paper is showed in Fig. 32. Because of its ability to print good color quality images on a variety of bond and plain papers at speeds of up to 6 ppm, Tektronix's Phaser 350 phase-change ink-jet printer has been very successful in the office network printer market. Other companies that actively develop and commercialize the phase-change ink-jet-printing method are Spectra³⁶ and Dataproducts.¹³

Table III describes a typical composition for a phase-change ink formulation. Operating temperatures for phase-change inks range from 120 to 140C. Viscosity at the operating temperatures are from 8 to 15 cps.

Solvent-based inks are commonly used in industrial marking or coating applications where the printing is done on a nonporous substrate such as plastic, metal, or glass. Because no absorption or penetration occurs, the printed image relies on quick evaporation of the ink solvent to be fixed onto the substrate.

Figure 32. A SEM of a fused phase-change ink drop on a paper that results from a offset drum transfer process.

Table III. Phase-change Ink Composition

Component	Function	Concentration, %
Solid wax mixture	Ink vehicle	40 to 70
Viscosity modifier	Lowers viscosity	5 to 20
Tackifier	Imparts adhesion	1 to 15
Plasticizer	Provides flexibility	1 to 15
Dye or pigment	Provides color	1 to 10
Antioxidant	Heat stability	0.05 to 2

Another type of nonaqueous-based ink is oil-based. This type of ink was recently used in several large-format ink-jet printers including the Raster Graphics PiezoPrint 5000 and the Xerox ColorgrafX. Both of these printers utilize Nu-Kote piezo shear-mode printheads. This printhead requires its ink to be compatible with the PZT electrode that is located on the walls of its ink chambers. The use of nonpolar oil-based ink minimizes the effect of electrical fields on ink and printhead materials. However, the ink manufacturer (Zeneca) for the Nu-Kote shear-mode ink-jet printhead also claims that the advantages of oil-based inks, when compared with water-based are faster drying time and the absence of cockle on paper substrates.³⁷ With the proper paper coating design, the above claim can be realized.

Image quality and durability for water-based, phase-change, and oil-based ink-jet inks are generally acceptable when they are printed on ink-jet papers or coated substrates. However, when printing on nonabsorbent substrates such as metal, glass, and plastic, the above ink systems are not adequate to produce durable and sharp images. To solve this problem, the idea of using a UV curable ink system for ink-jet printing has been discussed for a long time. However, many factors (such as ink-jet printhead capability, photoinitiator and low-toxicity monomer availability, and market needs) have hindered the progress of UV curable ink-jet ink development. Today ink-jet printheads are more capable and available; UV photoinitiators, monomers, and oligomers are readily available at economic scale; and market needs are strong. Successful development of UV curable inks for ink-jet applications are predicted in the near future.

Drying mechanisms for various ink-jet ink systems are summarized in Table IV.

Another major development in the ink-jet printing industry is the successful implementation and commercialization of pigment-based inks in color printing applications. Many companies including 3M, Dupont, and Kodak have already had pigmented ink-jet ink products on the market. With such focus by the industry, the color quality, image durability, and jetting reliability of the inks will be improved. In addition, severe competition for market share will likely result in significantly reduced cost for pigmented ink-jet inks.

Table IV. Drying Mechanisms for Different Ink-Jet Inks

Ink	Printhead	Drying Mechanism
Aqueous	Thermal/Piezo	Continuous Absorption/ Penetration Evaporation
Oil	Piezo continuous	Absorption/Penetration
Solvent	Continuous piezo	Evaporation
Hot melt	Piezo	Solidification
UV curable-based	Piezo continuous	Polymerization
Reactive-based	Piezo Continuous	Oxidation Polymerization

Figure 33. Comparison between dye solution and pigment dispersion behaviors on paper.

 

Figure 33 compares the ink and paper interactions between dye solution and pigment dispersion. Dye molecules are dissolved into an ink base that tends to penetrate or absorb into a paper or coating substrate. Pigment particles are dispersed into the ink base. While the ink base penetrates into the bulk of the paper substrate, pigment particles tend to remain on the surface of the paper. Figure 34 is a cross section light microscope photograph showing the behavior of pigmented black and cyan dye ink-jet inks throughout the thickness of a bond paper.

 

Figure 34. 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,^37–40 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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