Slot die coating of dilute suspensions

An experimental study was carried out to investigate slot die coating of dilute suspensions. Polymethyl methacrylate (PMMA) particles of different sizes were added to glycerol solutions as test fluids. The coating flow experiment was performed on a lab coater. The operating window, which is a domain in which only a stable and uniform coating solution is possible, was determined experimentally. It was found that adding particles has little effect on the physical properties of glycerol solutions, as only the fluid viscosity increases slightly. Two coating defects are commonly observed; ribbing which is caused by flow instability refers to lateral waves on the coated surface, and air entrainment which implies that air bubbles are trapped in the coating solution. Adding particles has little effect on the operating window if the coating defect is ribbing. However, the operating window can be expanded substantially if the coating defect is air entrainment. Higher the particle concentration, or the smaller the particle size, more significant is the expansion of the operating window. The maximum coating speed may increase to 80% for high suspension particle density number. A previous study reported that there exists an optimal particle size which has the largest operating window for a given pure solution; this was not found in the present work. Copyright © 2008 Curtin University of Technology and John Wiley & Sons, Ltd.