What happens to ceramic pigments during ink micronization? A tale of phase composition, grindability, microstructure and optical properties

Nowadays, ceramic decoration is being industrially carried out mainly by inkjet printing (IJP). The advent of digital decoration has changed the technological requirements of pigments and the way they are obtained. Ceramic pigments must be micronized (median particle diameter d50 ~300 nm) to fulfil IJP and shelf-life requirements. Particle size distribution plays a crucial role for ink compatibility and the final application. In this respect, pigment grindability is a key parameter which has strong repercussions on tinctorial strength, mechanical properties, and degree of amorphization of the pigment crystal structure. The reasons behind different pigment behavior during milling are still unknown. In order to deeply investigate the relationship between crystal properties and milling effects, five pigments with distinct physical properties and different phase compositions (zircon, spinel, malayaite, olivine, and eskolaite as the main constituting phase) were selected. Model inks, characterized by a different starting particle size, were prepared and underwent lab-scale stirred media milling reproducing industrial micronization conditions. Milling evolution was followed by determining particle size distribution (laser diffraction) and energy demand (mill electricity consumption). The effect of the process on phase composition and crystal structure parameters was followed by XRPD-Rietveld analysis. Furthermore, to define the changes in color saturation, micronized pigments were characterized by optical spectroscopy (DRS). Finally, to point out the possible involvement of different mechanisms during the milling process, a detailed microstructural characterization was performed (SEM). Every pigment exhibits a distinct grindability with non-linear trends of comminution rate and specific energy consumption with particle size. Regardless of the amount of energy involved, only a limited amorphization is observed, and a correlation between the crystallite size and microstrain is highlighted. Optical spectra suggest a specific effect of milling for each pigment, sometimes entailing the formation of new chromophore environments (e.g., in Cr-doped malayaite and Co-olivine). Microstructures hint at different mechanisms acting during comminution, with particle shapes proper of brittle fracture versus plastic deformation or fragments agglomeration. Preliminary results draw a complex dependence of grinding efficiency on the pigment mean bulk modulus, with a different relationship of particle size reduction, optical properties and tinctorial strength on milling cycles.