A travel into ceramic pigments micronizing

An increasing number of ceramic tiles are decorated by inkjet printing, utilizing in most cases pigmented inks. These inks are manufactured by micronizing conventional ceramic pigments, starting from 3-10 ?m in size and going down to a median diameter usually ranging from 0.2 to 0.6 ?m. The theoretical framework predicts significant changes in both optical and fluid mechanical properties during such a size reduction of pigment particles. However, not all the expected advantages occur and still unanswered questions concern colour strength and particle size distribution of micronized pigments, as well as efficiency and actual yield of the milling process. The present contribution is thought as a travel along progressive steps of pigment micronizing, that is aimed at disclosing what happens in terms of particles size, shape and composition in the submicrometric field. For this purpose, industrial pigments were selected to represent crystal structures with different density, hardness, cleavage and fracture toughness: Cr-Sb-doped rutile (orange-yellow), Co-Cr-Fe-Mn-Ni spinel (black), and V-doped zircon (turquoise). Pigments were micronized in a pilot plant (Netzsch Labstar LS1) keeping carrier, solids load, type and concentration of dispersant, rotation speed, amount and size of grinding media, and milling time under control. For each pigment, sampling was carried out at increasing milling time in order to get "instantaneous pictures" at progressive stages of micronizing. Pigments were characterized for particle size distribution (laser diffraction and dynamic light scattering), particles morphology (SEM), phase composition and unit cell parameters (XRD-Rietveld), and colour after application in glazes for porcelain stoneware tiles fired at 1200°C (CIE L*a*b*). Preliminary results highlight a different behaviour during micronization: the milling efficiency decreases from zircon to rutile to spinel, in partial agreement with literature data. Crystal structural and optical features are substantially changed once pigment particles turn into submicronic size. A gradually lower particle dimension is accompanied by increasing frequency of lattice defects (inferred from variation of unit cell parameters) and sometimes amorphization. The occurrence of the amorphous phase may significantly reduce the pigment yield with loss up to 75% by wt. These structural changes are associated to decreasing colour strength and increasing brightness through the submicrometric field, as outlined by zircon pigments taken as an example. Microscopic observations reveal changes in particles size and shape during micronization that are able to influence both pigment milling yield and ink performance.