Experimental Studies of Air Entrainment in a Blade Coating System - Part I: The Adverse Effects of Air Entrainment and Associated Coating Defects, 1998 Coating Conference Proceedings

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In conventional blade coating systems, coating nonuniformity, such as streaks, is often a problem limiting both productivity and product quality. It is reported [1] that one type of streak is caused by factors other than entrapment of particles under the blade or blade defects. Air entrainment from a solid/liquid/gas interfacial line, known as the dynamic contact line, is considered [2-8] to be one of the main sources of coating defects. Due to the complexity of the air entrainment process, however, little information is known about the physics of air entrainment at high-speed coating.

This study, presented in two parts, examines the link between air entrainment and coating defects. Part I presents the adverse effects of air entrainment on coating formation and the mechanisms of air entrainment in a pressurized coating pond. Optimization of the operating conditions to minimize air entrainment effects is presented in Part II.

In this study, the detrimental effects of air entrainment and the associated coating defects are investigated. The flow visualization experiments document, for the first time, the shape of the dynamic contact line and its stability in a pressurized coating pond. The recorded images reveal that the occurrence of certain coating nonuniformities is a direct consequence of the inception of a larger-scale air entrainment. The mechanisms of air entrainment affecting coating formation are presented and discussed.