Weeping in Blade Coating Part I: High-Speed Video and Still Macro-Photography, 1993 Coating Conference Proceedings

This is the first of two papers on visualization of the phenomenon in blade coating of paper known as ‘weeping.’ a localized bleeding of the coating suspension into millimeter-scale wet deposits called ‘weeps’ that appear and grow on the otherwise dry tip zone of the outer surface of the blade. Here we report results from recording the short time-scale dynamic phenomena of weeping with an EktaPro 1000 high-speed video system (Eastman Kodak) equipped with a long-distance microscope, and stopped-motion photographs with a 35 mm camera equipped with a 150 mm macro lens stacked on a 200 mm telephoto lens. Web speed ranged up to 3000 fpm; blade angles were around 45 degrees, Beloit S-matic pilot coater load settings were around 17 degrees, and solids content was about 69%. Still photographs show that, with few exceptions, the weeps all share a common structure: a bulbous crown atop a comb of separate columns or ‘trunks’ that join, two-by-two, in their upper reaches. Videotapes at 1000 frames/s show how weeps initiate and then appear to oscillate rapidly against the web as they grow, partially solidify, often merge, and finally detach. Our weeps grow in at least two ways, as seen when dyed formulation was added to the coating pan after weeping had begun. Trunks acquire coating formulation that flows up their web sides. Crowns acquire formulation during temporary contacts with the moving web, as seen by swirls of dye that appeared in the crowns. It also appeared that the weeps initiated at preferred sites, although only beyond critical limits of web speed, blade angle and loading, and solids content.