Curing study and optimization of a polyurethane-based model paint coated on sheet molding compound - Part II: Drying defects related to curing conditions

This study deals with the bubbling phenomenon which appears in some drying conditions on paint films applied as thin layers on SMC substrates. The bub bles and craters origin can be due either to reactions which may occur at t he interface paint / SMC either to inadequate drying conditions which affec t the polymerization reaction (gelation, viscosity) and the solvent vaporiz ation. Firstly, we reported data describing the solvent interaction with the SMC s ubstrate - namely the contact angle, the interfacial tension, the absorptio n kinetics and the sorption isotherms - and the relationship between the bu bbling defects and the numerous process variable (operating conditions, pai nt formulation and support properties). Then, we gathered experimental data concerning the influence of different p arameters (state and type of substrate, temperature, heating mode, film thi ckness, paint formulation, etc.) on the bubbling phenomenon and we tried to establish some quantitative interpretation. It proved that this defect was mainly due to the SMC heterogeneity expressed by the porosity of this comp osite material.