Orange peel formation due to surface tension-driven flows within drying paint films

Paint films with uniform thicknesses ranging from 150 to 820 μm were applied on stainless steel substrates using a model paint consisting of a resin dissolved in butanol and the test samples were cured in a natural convection oven at a temperature of 140°C. Photographs of the paint surface were taken during drying, and the weight loss was measured. Cellular structures appeared on the paint surface, induced by surface tension-driven flows due to solvent concentration variations. For thin films (<500 µm), the patterns disappeared in a few minutes and the dried paint surface was smooth, while for thicker paint films, wave-like structures remained on the hardened paint layer, creating an uneven surface. An analytical solution of the mass-diffusion equation was used to model solvent evaporation from the paint film and to calculate the concentration gradient and surface tension variations in the paint films. In thin films, all the solvent was depleted, and surface tension gradients disappeared before curing was complete, allowing the surface to become smooth. In thicker films, concentration gradients that drove cellular flows persisted until the paint dried, leaving orange peel on the surface.