Characterization of synthetic hydrous hematite pigments

The application of synthetic hematite pigments, prepared by the direct precipitation process from an iron(II) sulfate solution, leads to problems in the processing of plastic tubes by injection moulding and extrusion. Undesirable flaws are formed. A characterization by thermogravimetry (TG) and infrared spectroscopy (IR) proved an incorporation of water (∼1.5 mass%) in the crystal lattice. X-ray investigations of tempered hydrous hematite pigments in a temperature range between 100 and 800 °C showed an decrease of the cell volume between 400 and 500 °C. The absence of superstructure reflections by powder X-ray diffraction (XRD) investigations and the small dehydration enthalpy of ∼6.1 kJ mol−1 determined by differential scanning calorimetry (DSC) indicates a statistical distribution of hydroxyl groups. Pores of hexagonal geometry were observed at the crystal surface by transmission electron microscopy (TEM) after dehydration. The so-called “hydrohematite”, described in the literature, with ordered incorporation of hydroxyl groups in the hematite lattice was not found.