Colour variation in drying paint films
Three paints with differing concentration (0.5%, 2% and 4% by weight) of spherical blue pigments were applied with varying film thickness (310, 490, 730 μm) on stainless-steel substrates and dried in an oven while the paint surfaces were photographed. Weight loss due to solvent evaporation from the paint was recorded. The colour of the dried paint films was measured. Surface tension driven convection was observed in drying paint films, which caused movement of pigment particles.
Amount of pigment or paint thickness changes colour
An analytical model was used to predict solvent concentration gradients in the paint films and calculate Marangoni numbers, which determined the velocity of fluid motion. All motion stopped when the Marangoni number became small. Pigment particles tended to cluster around the edges of convection cells and produce changes in colour, especially when the paint films were thin and the pigment fraction low.
Increasing either the amount of pigment or the paint thickness made the paint colour lighter, as a larger number of pigment particles increases reflection of light. Paints with high pigment concentration are less likely to see changes in colour.
The study is published in: Progress in Organic Coatings Volume 136, November 2019, 105173.