Applied study and modeling of penetration depth for slot die coating onto porous substrates
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A distinctive field in the coatings industry is the coating of porous media, with broad applications in paper, apparel, textile, electronics, bioengineering, filtration and energy sector. A primary industrial scale process that can be used to coat porous media in a fast and flexible manner is slot die extrusion. A major concern when coating porous media with a wetting fluid is fluid penetration into the substrate. Although some level of penetration is desirable to obtain specific material properties, inadequate or excessive fluid penetration can negatively affect the strength, functionality or performance of the resulting material. In spite of its apparent industrial importance, limited modeling and experimental work has been conducted to study fluid penetration into porous media during fabrication. The effects of processing parameters on the penetration depth, the effects of penetration on material quality, and the method to predict and control the penetration depth are not well understood. This dissertation is composed of two parts. Part I is an applied study for coating onto porous media. This part focuses on the first objective of this dissertation which is to elucidate clearly the feasibility, advantages and disadvantages of the direct coating method as a potential fabrication route for membrane electrode assembly (MEA). MEA samples are fabricated using both traditional and the direct coating methods. Then, the quality and performance of the MEA samples are examined. Experimental results in Part I demonstrate that it is feasible to fabricate MEAs using the direct coating method. However, Nafion® solution penetrates into the catalyst layer during the coating process and causes lower performance of fuel cells, which is the motivation for Part II of this thesis. The objective of Part II is to fundamentally understand the fluid penetration process and predict the penetration depth when directly coating porous media, using a comprehensive approach. A series of computational and analytical models are developed to predict the penetration depth for both Newtonian and non-Newtonian fluids with or without capillary pressure. Finally the accuracy of developed models are validated through experiments. The relative error between the predicted and experimentally measured penetration depth is generally lower than 20%.