SUMMARY

Defect generation and propagation in thin films, such as separation membranes can lead to premature or catastrophic failure of devices such as polymer electrolyte membrane fuel cells (PEMFC). It is hypothesized that defects (e.g., air bubbles, pin-holes, and holes) originate during the manufacturing stage if precise control is not maintained over the coating process, and they propagate during operation. The manufacturing process used for this dissertation is slot die coating which involves the wetting of a substrate by displacing the air between the die gap and the substrate. The primary objectives are to establish the upper and lower processing boundaries or coating window for relatively high viscosity (> 1 Pa-s), non-Newtonian, shear thinning solutions and to understand the mechanism(s) by which defects ensue at or along the dynamic contact line. Simulations and experiments were used to meet these objectives for a range of materials. To capture the two-phase flow field of the coating process; numerical simulations were solved using a volume-of-fluid multiphase method, available in FLUENT 6.3.26. To validate the numerical model and to investigate the mechanism(s) that causes defects, a customized roll-feed imaging system was used. The effect of coating gap, slot gap, solution viscosity, and surface tension were investigated to understand their impact on the size of the coating window and defects. A semi-empirical model correlating the maximum coating speed to a solution’s material properties, geometric parameters and processing conditions was developed. Such a predictive model will enable engineers to determine the maximum coating boundary without the need for simulations or experiments, for both shear-thinning and Newtonian solutions.