Flow-induced orientation of flakes in metallic coatings - II. The orientation mechanism

In part I of this article, we derived an expression for the time-dependent orientation of flakes in a metallic coating. Here, this expression is used to investigate the mechanism that makes metallic flakes to orient predominantly parallel to the coating surface. Using numerical examples representative for basecoats, we show that flakes are not oriented already during the impact of paint droplets on the substrate, because the resulting flake motions are quickly damped. Instead, the orientation process takes place mainly during the subsequent spreading of the droplets over the surface. The associated velocity of paint flow was shown to be large enough to orient all flakes parallel to the substrate surface. We also show that in a later stage of the application process the convection flows due to evaporation of solvent are strong enough to affect the orientation of flakes, especially for small flakes.