Predicting fluid penetration during slot die coating onto porous substrates

Slot die coating onto porous media is a distinct field in the coating industry and has broad applications. One important technical issue related to coating porous media is how to predict and control the fluid penetration depth, which directly affects the appearance, properties, and performance of the resulting material. Up to now, the analytical relationship between processing parameters and the final penetration depth are still not well understood. Existing modeling work either uses oversimplified assumption of pressure distribution in the coating bead, or does not provide a simple expression to predict the penetration depth and often requires a complex calculation procedure. Furthermore, previous models are limited to Newtonian fluids. In this study, new computational fluid dynamics (CFD) and analytical models to study the penetration of fluid into porous media during slot die coating were developed for Newtonian and non-Newtonian fluids. Penetration driven by the pressure in coating bead was considered in these models. To this end, basic analytical models that can easily and rapidly calculate the penetration depth were developed and the results were compared to CFD models. It was found that there exists good agreement between the analytical and CFD models when coating Newtonian and non-Newtonian fluids, having an overall relative error of less than 6% and ranging between 12-26%, respectively. Results from a parametric study, when coating Newtonian fluids, showed that viscosity has a negligible effect on the penetration depth; whereas coating speed, flow rate, permeability and porosity are more important. Based on the experimental analysis, similar trends exist for the measured and predicted values of the penetration depth when coating non-Newtonian fluids.