Slot die coating of polybenzimiazole based membranes at the air engulfment limit

The objective of the current study is to analyze the slot die coating process of highly viscous (1-20 Pa s), shear-thinning solutions. The upper and lower coating boundaries are determined with respect to various non-dimensional parameters. Polyphosphoric acid doped polybenzamidazole (PPA/PBI) solutions are used as the test solutions. Simulations are performed using 2D, volume-of-fluid (VOF) model available in FLUENT 6.3.26 to predict the coating windows. The numerically predicted coating window compared within +/- 4% of the experimental data. Design of Experiments (DOE) is performed to understand the impact of operational and processing conditions on the coating windows. It is observed that the air engulfment could be delayed by using smaller coating gaps and higher processing temperatures, which would facilitate faster processing of thin films, an important attribute during industrial scale processing. One major contribution from the study is the development of a semi-empirical model, in which operational and processing parameters are supplied as inputs to predict the air engulfment velocity within +/- 10% accuracy. (C) 2013 Elsevier B.V. All rights reserved.