Dielectric Characterization of Pigment Inks for Electrohydrodynamic

Oct 16, 2014 - This article presents a characterization method that could be useful for evaluating the performance of ink in electrohydrodynamic jet p...
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Dielectric Characterization of Pigment Inks for Electrohydrodynamic Jet Printing Ayoung Lee,†,‡ Hiroshi Watanabe,‡ Yumi Matsumiya,‡ Kyung-Hyun Choi,§ Kyung Hyun Ahn,*,† and Seung Jong Lee† †

School of Chemical and Biological Engineering, Institute of Chemical Process, Seoul National University, Seoul 151-744, Korea Institute for Chemical Research, Kyoto University, Kyoto 611-0011, Japan § School of Mechatronics Engineering, Jeju National University, Jeju 690-756, Korea ‡

ABSTRACT: This article presents a characterization method that could be useful for evaluating the performance of ink in electrohydrodynamic jet printing. The ink contains pigment particles and polymer (stabilizer for pigments) and has a low viscosity and a low elasticity. Depending on the medium (hydrocarbon for oil-based inks and glycol ether for polar solvent-based inks) and the type of pigment (exhibiting different colors), vast differences were found in the dielectric properties even when the viscosities of the inks were almost identical. These differences between the two series of inks were related to differences in the ion concentration n and ion mobility μ estimated from an analysis of the complex dielectric constant ε* (= ε′ − iε″) on the basis of Macdonald theory. These differences between the polar solvent- and oil-based inks are discussed in relation to the performances of the inks in electrohydrodynamic inkjet printing.

1. INTRODUCTION Commercially available inks are shifting from dye-based types to pigment-based types, as the latter offer more environmentally friendly products with better durability and compatibility in packaging as well as in direct printing of conductive patterns with metal particles.1−4 Inks for jet printing must satisfy two requirements simultaneously: They are required to manifest color more clearly without causing any clogging problems when they flow out of a nozzle head.5 That is, inks should contain a variety of components to manifest color tones completely, and at the same time, they should minimize fatal nozzle clogging. Electrohydrodynamic (EHD) jet printing is a complex process in which the polarization of the fluid induced by an electric field influences the momentum flux of the fluid.6,7 One practical advantage of EHD jet printing is that the probability of nozzle clogging is minimized and the charged particles become self-dispersed, mitigating the drawback of piezo-type inkjet printing.8 In EHD jet printing, a jet of conical shape can be generated from a balance between the flux strength and electric field strength of the system, which makes it possible to form patterns of submicrometer length scale with high resolution. Characterization of inks and their respective applications in terms of an appropriate jetting method is necessary to achieve optimal performance in jet printing. However, the inks for jet printing are usually designed to have a low viscosity (not very different from the viscosity of the suspending medium), and thus, it is not easy to estimate the EHD jet printing performance from a rheological standpoint, which leads to the necessity for material characterization from a nonrheological point of view. In this article, we propose a dielectric method as a new tool for ink characterization. This method is not disturbed by the low viscosity and low elasticity of inks or by ink opacity. Dielectric measurements at frequencies of 1).

Thus, for each ink examined in this study, we varied the dimensionless flow rate α at fixed dimensionless voltage β to examine the ink jetting performance. Figure 11 summarizes the results for different β and α values. In Figure 11a,b, the ranges

of β and α surrounded by the thick line (−) specify classical EHD systems having Tq/Th < 1 (as deduced from eq 5). This range, which is much wider for solvent-based inks than for oilbased inks, matches the experimentally observed jetting

β=

Va Vc

with

Va =

γd ε0

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dx.doi.org/10.1021/ie5031437 | Ind. Eng. Chem. Res. 2014, 53, 17445−17453

Industrial & Engineering Chemistry Research

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behavior (thin cone jetting). In addition, in the range of β and α satisfying the relationship Tq/Th ≈ 1 [surrounded by the dotted line (---) in Figure 11a], No1 M and No1 C (for α = 1) and No1 K (for α = 0.1) are expected to exhibit reasonably thin cone jetting, which again appears to be consistent with the observations. Thin jetting cannot be expected theoretically and, indeed, was not observed experimentally in the rest of the regimes (nonclassical EHD systems having Tq/Th > 1), as shown in Figure 11. As demonstrated above, the analysis based on eqs 5−7 satisfactorily identifies the jetting performance of the inks on the basis of their dielectric parameters ε′ and K (involved in eqs 5 and 6). Specifically, the ion concentrations n of the inks containing yellow and black pigments are much larger in the polar solvent-based inks than in the oil-based inks, thereby giving a larger regime of good EHD performance in the polar solvent-based inks. In addition, the μ values of the inks containing magenta and cyan pigments are much larger in the polar solvent-based inks than in the oil-based inks (having similar n values), which also results in a larger regime of good EHD performance in the polar solvent-based inks. It should be emphasized that both the n and μ values of the ions affect the ε′ and K values involved in eqs 5 and 6 and specify the ink jetting performance. On the basis of the above analysis, we propose a strategy for high-throughput EHD jet printing performance. First, the jetting condition of Tq/Th < 1 should be satisfied to generate a thin jet in cone-jet mode. This means that the charged thin layer should be formed by charge accumulation in the direction normal to the liquid−gas interface, thereby amplifying the electric normal force FE,n shown in Figure 1b. The ion concentration n should be on the order of 1023−1024 m−3 so as to generate such a thin charge layer. In addition, the surface charge layer formed with the concentrated ions is driven toward the cone apex by the electric tangential force FE,t,25 and the ion mobility μ needs to be sufficiently large to ensure rapid motion of the charged layer. In this way, the dielectric properties of the inks affect the jet printing performance.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +82-2-880-8322. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF; Grant 2013R1A2A2A07067387) funded by the Korean government (MEST).

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REFERENCES

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4. CONCLUDING REMARKS We have examined the dielectric behaviors of oil-based and polar solvent-based inks in an effort to characterize inks having almost identical rheological properties. Electrode polarization due to the ions concentrated near the electrodes at low ω was observed for both series of inks, but the magnitude of electrode polarization was much larger for the polar solvent-based inks. Thus, the dielectric method can be used to characterize rheologically similar inks. It is possible to quantify the differences between inks according to differences in the ion concentration n and ion mobility μ. These differences could have an important impact on the electrohydrodynamic jet printing performance of the inks. The ion concentrations n of the inks containing yellow and black pigments were found to be much larger in the polar solvent-based inks than in the oilbased inks, thereby giving a larger regime of good EHD performance in the polar solvent-based inks. In addition, the μ values of the inks containing magenta and cyan pigments were much larger in the polar solvent-based inks than in the oilbased inks (having similar n values), which also resulted in a larger regime of good EHD performance in the polar solventbased inks. 17452

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dx.doi.org/10.1021/ie5031437 | Ind. Eng. Chem. Res. 2014, 53, 17445−17453