Preparation of environmentally-friendly inorganic pigments

A series of compounds that have potential as environmentally-friendly inorganic pigments have been prepared using solid-state methods. The materials have been characterised using a variety of techniques including powder X-ray diffraction, colour measurements, EDX, Mössbauer spectroscopy and solid state NMR. Materials based on the industrial pigment, ultramarine blue, have been investigated using a variety of methods to determine the source of the favoured red-hue. Industrial materials of different shades as well as laboratory synthesised and ion-exchanged ultramarine materials have been analysed for colour, impurities using bromine insoluble method and structure by Rietveld analysis. ICP analysis has been shown to have limited success in determining the AI:Si ratio for ultramarine materials. The role of small levels of potassium in the formation of red-shade blue and the introduction of impurities by the use of feldspar in the industrial formulation has been confirmed. The investigation of a green pigment based on CuO-B203 mixtures has been investigated by varying the ratios of the two components. In the copper-rich region, the materials produced have similar colour properties to the industrial green standard, chrome oxide. The blue boron-rich materials have relatively poor pigment properties and the colour weakens dramatically on grinding and mixing with diluents such as titanium oxide. The poor acid and water stability of the products suggest that the use of these materials as pigments would be limited. A solid solution in the CoO-A1203-GeO2 spinel system has been investigated. As the germanium level increases, the firing temperature required to produce a pure phase decreases. The samples range in colour from blue at the aluminium-rich end through purple to pink at the germanium-rich end. The use of mineralisers appears to have a favourable effect on the colour of the materials. Rietveld refinement of the materials suggests that the series shows disordering of the cations over both tetrahedral and octahedral sites and that the materials become cobalt deficient as the aluminium level increases. Doping, annealing and mineralisation of the SnO-Nb2O5 series using cation (V, Ca, Ta) and anion (Cl, S) doping has produced two different phases with very different properties. The foordite structure, SnNb2O6 is a very stable stoichiometric yellow phase that shows good temperature and gaseous environment resistance. The pyrochlore phase, SnII2x_N, b2y.S n7yO7_7h, owever is very susceptible to changes in atmosphere and readily undergoes substitution at both anion and cation sites to produce phases that vary in colour from yellow through to red. The product phases have mixed divalent and tetravalent tin across two different sites. Substitution of some of the oxygen with sulfur (to a limit of 0.3 per unit cell) leads to a reddening of the sample and stabilisation of the divalent tin within the structure. Mössbauer analysis indicates the presence of both divalent and tetravalent tin in the compound where the level of tin (IV) decreases on substitution of sulfur. Thermal treatment of both tin samples facilitates a reversible temperature dependent change in the colour of the samples from yellow/orange through to bright red. Temperature dependent XRD analysis has shown that the colour change does not result from a change in the structure.