Ink technology for digital decoration of ceramic tiles: an overview

The rapid and widespread diffusion of digital printers is turning inkjet printing into the leading technology in ceramic tile decoration. The purpose of this overview is to outline the evolution of ink technology in the last decade and to highlight its role in the development of successful digital printing of ceramic tiles. Indeed, the quality and reliability of inkjet printing on ceramic surfaces largely depend on ink performance. For this reason, the technological requirements of inks extend well beyond the prescriptions of DOD printhead manufacturers, encompassing the storage, interaction with substrate and firing stages. Ink behaviour is theoretically governed by its density, rheological and surface properties in a wide and dynamic range of stress regimes: from the very high stress rates during jetting, drop flight and impact on the tile; moderate-low stress during drop spreading and penetration into the porous substrate; to minimal stress during footprint drying or ink storage. However, the peculiar conditions occurring in ceramic tile application have gradually led to specific fields for ink properties and performances, where particle size distribution, sedimentation rate, agglomeration phenomena and colour strength are particularly highlighted. This situation has generated original pathways in the criteria of ink formulation and pigment processing, entailing different technological solutions concerning colorants, solvent/carriers and additives, which will be briefly reviewed. Relevant parameters (e.g., viscosity, surface tension, Zeta potential, solids load, fluid mechanics dimensionless numbers: Reynolds, Weber, Ohnesorge) acting on stability over time, jettability, footprint formation and colouring performance will be outlined and discussed to focus on the peculiarities of ceramic ink technology and the challenges for the near future.