Modeling the effect of temperature-induced surface-tension gradient in coating processes

In the application of fluorescent lamp coating, clip marks formed during drying can adversely affect product quality, resulting in higher production cost and/or lower customer satisfaction. It is believed that these defects are caused by gradient in coating surface tension but their mechanisms are still not well understood.; To facilitate a more systematic approach to coatings design, it is important to have a better understanding of the roles of surface tension in defect formation. A one-dimensional mathematical model, which describes the flow of drying coating on horizontal planar substrates, was, developed in this study to investigate the formation of defects, particularly in the fluorescent-lamp coating process. A partial differential equation was derived based on the Navier-Stokes equation, using the lubrication approximation for thin layers. The effect of temperature distribution on surface-tension gradient was incorporated into the model, which enhances our ability to quantify defect formation in drying coatings.; The results show that, temperature-induced surface-tension gradient plays a major role in defect formation. The effect of pressure gradient is negligible compared to the surface-tension gradient in defect formation. A linear relation is observed between defect peak growth and time between t = 10 s and t = 500 s. Defect formation time also varies linearly with viscosity in the range between μ = 0.1 P to 2 P. Parametric studies show that all the parameters studied have an effect on the defect. Temperature shows the greatest influence in defect formation, followed by viscosity. This model can be used as a process analysis tool in industrial applications.