Micronizing ceramic pigments for inkjet printing: Part II. Effect on phase composition and color

Drop on demand ink-jet printing is turning to be the leading technique in the decoration of ceramic tiles. This technology makes use in most cases of pigmented inks which are manufactured by micronizing conventional ceramic pigments in the 0.2-0.6 µm range (median diameter). Although significant improvements to optical properties are in theory put forth by reducing the pigment particle size, not all the expected advantages occur and still unanswered questions concern colour strength of micronized pigments. This is the second part of a study aimed at disclosing what happens during pigment micronizing; it is focused on phase composition and colour in the submicrometric field. For this purpose, representative industrial pigments were selected: Cr-Sb-doped rutile (orange-yellow), Co-Cr-Fe-Mn-Ni spinel (black), and V-doped zircon (turquoise-blue). Such pigments were micronized in a pilot plant and characterized for particle size and morphology (SEM, HR-TEM), phase composition, crystallite size and unit cell parameters (XRD-Rietveld), optical properties (DRS) and colour after application in glazes for porcelain stoneware tiles fast fired at 1200°C (CIE L*a*b*). Results highlight a different behaviour during micronization: crystal structural and optical features are substantially changed once pigment particles turn into submicronic size. A gradually lower particle dimension is accompanied by reduction of crystallite size and increasing frequency of lattice defects (inferred from variation of unit cell parameters and optical properties) up to amorphization, that may attain 75 wt%. The formation of amorphous phase takes place below a critical crystallite size (30-40 nm) which discriminates two regimes with fast and slow comminution rates. These structural changes are associated to decreasing colour strength and increasing brightness through the submicrometric field.