| Many industrial processes involve the coating of a substrate with layers of paint. Although surfactants are only a small part in a coating system, they play a key role in the application process and greatly influence coating quality. Previous studies of surface-tension gradient driven flow only approximated the relation between surface tension and surfactant concentration using empirical expressions. In the present study, a molecular model for surface tension was integrated with a coating leveling model to predict coating film quality and provide better physical insight into surfactant behaviour during the coating leveling process. The surface-tension model treats surfactants at the molecular level based on a well-established molecular-thermodynamic theory, which considers hydrophobic interactions, hydrocarbon-chain packing, interfacial effects, and steric interactions between heads. The model was applied to model commercial surfactants, Igepal CO-630 and CO-720, and the predicted surface tension agreed well with experimental data.; The coating leveling model accounts for surface-tension gradient as one of the driving forces. This model is based on a simplification of the Navier-Stokes equation using the classical lubrication theory for a drying paint layer, which consists of a non-volatile resin, a small amount of surfactant, and water as solvent. A one-dimensional numerical code was developed to predict the transient coating thickness, and the effects of temperature-induced surface-tension gradient, viscosity, and solvent evaporation rate on the drying paint film on a horizontal substrate were studied using this integrated model. For a uneven temperature distribution, higher evaporation rate and higher viscosity decrease defect formation. However, surfactant concentration has little effect on defect shape and magnitude, especially when the concentration exceeds the CMC. |
