Ink-jet printability of aqueous ceramic inks for digital decoration of ceramic tiles

• Water-based inks having properties within the inkjet printing range were obtained.
• A second vehicle (MEG) was used to modulate the ink viscosity and surface tension.
• Different ink rheological behaviors were found (Newtonian or slightly pseudoplastic).
• A proper ink colloidal stability (z-potential) was achieved by using dispersants.
• Prolonged milling tends to deteriorate the ink stability (colloidal and over time).

Digital decoration of ceramic tiles has turned to be a prevalent and dynamic technology in the last decade. Current printers use drop-on-demand (DOD) ink-jet print heads (IJP) fed with solvent-based inks containing ceramic pigments as coloring agents. However, due to environmental constraints, water-based systems are envisaged as a green alternative for ceramic tiles decoration. Nevertheless, aqueous suspensions are difficult to be managed because physical properties of water are far away from the DOD-IJP operating window. Thus, the control on the stability, homogeneity and rheology of such water-based systems is an important factor to achieve better product performances. This study was aimed at exploring both the rheological behavior and stability of three inks based on micronized pigments dispersed in water and water-MEG solutions: (V,Zr)SiO4 (turquoise zircon, TZ), (Cr,Sb)TiO2 (orange rutile, OR) and (Co,Mn) (Fe,Cr)2O4 (black spinel, BS). The pigments were ground to submicronic size in water using a circulation type agitator mill and by changing the main parameters affecting the particle-size distribution (rotation speed, type and amount of dispersants). The stability of aqueous pigment suspensions was studied by measuring the zeta potential as function of pH and by sedimentation tests. The zeta potential was sufficiently strong (lower than −30 mV) to render the inks stable in the 7–10 pH range. Rheological measurements of the suspensions showed a Newtonian flow behavior for zircon and rutile inks and slightly pseudoplastic for the spinel one. An attempt was also made to evaluate the inks printability with the help of dimensionless numbers based on the relevant physical and rheological properties affecting the jetting, i.e. viscosity, surface tension and density. It was observed that the ground aqueous systems fall in the printable fluid region obtained with semi-empirical models.