Why the use of yellow iron oxides in high-temperature-cured coatings is limited
Standard yellow iron oxides are the ideal inorganic pigments to develop a wide range of color shades due to the advantages in performance and cost provided by their high hiding power and opacity, excellent weather, light and chemical fastness, and reduced price. But their use in high-temperature-cured coatings such as coil coating, powder coatings or stoving paints is limited. Why?
When yellow iron oxides are submitted to high temperatures, their goethite structure (FeOOH) dehydrates and partially turns into hematite (Fe2O3), which is the crystal structure of red iron oxide. This is why the standard yellow iron oxide that exists before curing becomes darker and browner .
This change can take place from temperatures close to 160ºC, depending on the curing time, the binder system and the coating formulation itself.