Nucleation of bubbles during drying of sprayed paint films

Experiments were done to study the growth of bubbles in paint films sprayed on steel substrates. High-speed video images were taken of the impact of paint spray droplets, generated with an air-atomizing nozzle, on a stainless steel surface. Paint droplets stuck to the surface after impact and were not dislodged by subsequent droplets landing on them. Droplets hitting patches of paint merged with them to form an uneven layer. Once the spray was turned off after 0.5 s the paint masses settled and formed a uniform film in a few seconds. The paint film contained a large number of entrapped air bubbles that migrated to the paint surface and disappeared in 4-5 min. Painted substrates were placed in an oven at 130 °C to cure. Any air bubbles present in the film at the start of baking acted as nucleation sites that expanded as paint volatiles diffused into them. Waiting 10 min prior to curing ensured that the cured paint film was free of bubbles. The diffusion of volatiles out of the paint film was modelled using a one-dimensional analytical solution of the mass diffusion equation. The model was fit to experimental measurements of the mass decrease of painted films to estimate the diffusivity of volatiles in paint, Dv = 1.5 × 10−12 m2/s. The concentration of volatiles in the paint film after a flash-off time of 10 min was estimated to be less than 50% of the initial value.