The scope of this work is the development of a new process for the synthesis of special effect pigments by generating oxidic films on metallic effect pigments based on aluminum. In order to overcome the disadvantages of established state-of-the-art processes, the approach of thermal degradation in solution is chosen, enabling environmentally friendly, uncomplicated process
management without toxic components and pH control. The precursors used are urea nitrate complexes, which, through the combination of urea and nitrate as fuel and oxidizer, provide
optimal conditions for transformation in a combustion reaction at comparatively low temperatures and, thanks to their wide variety, make a whole range of metal oxides accessible. Specifically, the formation of iron, zinc, cobalt and copper oxide derived from corresponding molecular precursors is successfully demonstrated in this work. For the protection of the aluminum substrates to be coated, the reaction takes place in an organic solvent (1-methoxy-2-propanol) at moderate temperatures (120 ° C). The analysis of the obtained precipitates by infrared spectroscopy, X-ray diffractometry and thermogravimetry confirms the degradation of the precursors accompanied by the formation of varying intermediates. If the chemical nature of the cation present allows it, the decomposition of the urea ligands leads to the formation of ammine complexes. If this is not possible, as is the case with hexakis(urea)iron(III)nitrate, a mixture of iron hydroxides and largely intact, but no longer complexing urea is formed. When heated further, urea polymers and triazines are observed. These reactions can be inhibited by adding water to the initial reaction mixture. A thermal treatment process in a temperature-stable organic solvent (diphenyl ether, 259 ° C) proves suitable for the reduction of the organic impurities and the formation of the final oxidic phases. The purity of the oxides obtained ranges from almost 80 % (iron and zinc oxide) to 88 % (copper oxide) up to 96 % (cobalt oxide), depending on the reaction mechanism and the decomposition temperature of the intermediates formed. If the precursor degradation takes place in the presence of aluminum substrates, a dense and largely homogeneous film is formed, as can be demonstrated by means of scanning electron microscopy, energy-dispersive X-ray spectroscopy and X-ray diffractometry. The optical characterization of the pigments obtained is carried out by means of spectroscopic measurements of the particles applied out of a lacquer by doctor blade technique. Since the process developed here allows the direct coating of aluminum substrates only 10-30 nm thick, a so far unmatched hiding strength for such pigments is achieved, accompanied by a color difference of 0.3 between white and black background measured with a degree of pigmentation of only 3%. A gloss of up to 101 and a flop index in the range of 20-30 indicate great potential for high-quality optical applications; only the color saturation with a value of 0.2-0.4 could still be improved. Besides the coloristic applications, the precipitated oxidic films also have the potential to produce functional metal-organic framework (MOF) coatings. The suitability of this approach, in this work exemplary shown for of ZIF-8 and HKUST-1, can be verified by means of infrared spectroscopy,
X-ray diffractometry and scanning electron microscopy. The BET surface area of the HKUST-1 coated pigments is 132.5 m2/g, in the case of the ZIF-8 coatings it even reaches a value as high
as 736.2 m2/g | English |