Slot die coating of polybenzimiazole based membranes at the air engulfment limit Kanthi Latha Bhamidipati, Sima Didari, Tequila A.L. Harris * George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA highlights graphical abstract  Air engulfment in shear-thinning so- lutions is investigated.  A semi-empirical model is derived to predict air engulfment velocity.  Smaller coating gap or higher oper- ating temperature enable larger coating speed.  Air engulfment can be delayed by employing solutions with higher surface tension. article info Article history: Received 14 December 2012 Received in revised form 21 March 2013 Accepted 22 March 2013 Available online 6 April 2013 Keywords: Slot die Shear-thinning Coating window Semi-empirical model Air engulfment abstract The objective of the current study is to analyze the slot die coating process of highly viscous (1e20 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 under- stand 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 ve- locity within 10% accuracy. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Slot die coating classified as a pre-metered coating technique, is successfully being used for manufacturing a broad range of thin films such as photographic films, papers [1], coatings on glass substrates [2], optical films for liquid crystal displays (LCD) [3], fuel cell membranes [4e6], etc. As shown in Fig. 1 , in this method, a solution with a predetermined flow rate, (Q in ) is suspended from a slot die onto a substrate moving at speed (u w ). The coated film attains a constant thickness downstream of the die, which is called the wet thickness (h). One of the main advantages of slot die coating is that the coating thickness is pre-metered and controlled, meaning the final coating thickness is determined from the flow rate through the slot die and the substrate speed [7,8]. A limited range of operating conditions, for which high quality films are produced, exists for slot die coating as defined by the “coating window” as shown in Fig. 2. Outside the coating window, various types of defects such as dripping, air entrainment, and break lines will be formed [9e11]. Dripping corresponds to the top most boundary of the coating window and occurs when the flow * Corresponding author. Tel.: þ1 404 385 6335; fax: þ1 404 894 9342. E-mail address: tequila.harris@me.gatech.edu (T.A.L. Harris). Contents lists available at SciVerse ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour 0378-7753/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jpowsour.2013.03.132 Journal of Power Sources 239 (2013) 382e392