Bonding process efficiency and Al-flake orientation during the curing of powder coatings

The orientation of aluminium flakes in powder coatings has been studied. Three effect powder samples were prepared by variation of the bonding time and the temperature associated with the production of the powders. SEM micrographs show that only smaller flakes (<10 μm) bond to the surface of the powder. The influence of bonding parameters on the microstructure of the cured coatings was studied using various optical methods. The colour differences measured in four perpendicular reflectance planes and the lightness flop were practically the same for all the coatings. The scattering of infrared light shows that longer bonding times give coatings in which the flakes are distributed more uniformly, parallel to the coating surface. The topmost layer of flakes was seen on SEM micrographs of selectively etched samples. The film forming process and reorientation of flakes were monitored on samples that were prepared at curing temperatures between 80 and 180 °C. It was observed that the flakes reorient from their initial position in the bonded powder to the final position in the effect coating almost completely during the formation of the continuous film. This process was accomplished at 100 °C.