Ceramic pigments for digital decoration inks: an overview

The field of ceramic colorants is one of the most conservative in the tilemaking sector. Reduction of cost and impact on health and the environment has been the driving force for innovation in pigment manufacturing, where the main technological goals were fast synthesis routes and minimization of hazardous components and additives. The advent of digital decoration is overviewed with its paradigm shift from colorant to ink. The requirements for ink-jet printing are reviewed: rheological properties, surface tension, zeta potential, sedimentation, drop size and shape, kinetics of penetration, particle size, control on electrical properties, stability in organic media, improved colorant strength. As conventional pigments and dyes proved to be unsuitable for digital decoration, colorant manufacturers were forced to upgrade processing (micronizing step for pigments) and to get involved in ink design (improving colorant strength for dyes). By this way, different classes of colorants for digital ink have been developed: organo-metallic complexes, micronized pigments, colloidal metals, nanopigments and reactive sol precursors for in situ synthesis. The main challenges for ink manufacturers are still the stability over time (sometimes limited to a few weeks) and the gamut (much narrower than that of conventional ceramic colorants). As a matter of fact, typical quadrichromy or hexachromy is still a hard task to be achieved with ceramic colorants and tilemakers are often choosing unconventional colour sets based on brown shades. This circumstance is revitalizing the industrial interest towards alternative routes for pigment synthesis (chimie douce or bottom-up approach) and technological solutions to improve the colour performance (pigment coating, core-shell structures, use of primers and buffers). Physico-chemical properties of inks, which affect the stability over time, are turning critical with increasing diffusion of digital decoration. From this standpoint, technologies able to control colloidal suspensions and to design hybrid organic-inorganic composites are rapidly gaining interest and application potential.