Weeping phenomenon in blade coating

This paper describes a flow visualization examination of the blade coating defect known as `weeping.' High-speed video and still micro-photography demonstrate the dynamics of weeping. With few exceptions, weeps all share a common structure: large, fluid, bulbous crowns on top of a comb of separate columns or `trunks' that join, two-by-two, in their upper reaches. The crowns are about 1-2 mm in diameter and have smooth surfaces. The trunks are thin (less than 100 μm across) and crooked, with narrow channels on the sides that face the web and smooth surfaces on the other side. The crowns oscillate rapidly away from and back toward the web as they grow. The trunks of young weeps are already partly dry, and they anchor to the blade the wet crowns during their oscillating motion. SEM of microtome-sectioned weeps showed an excess number of booklets and lumps larger than 10 μm across in the trunks compared with crowns of the weeps and drops of air-dried formulation. The high shear rheology of the coating colors was examined using a capillary rheometer. Variation in pigment type, binder type and level yielded changes in the critical coating speed required for the onset of weeping, which could not be accounted on the basis of the bulk rheological properties of the coating. The results of this study suggest that the ability of the binder to enhance the structural integrity of the coating, at slightly elevated solids levels, may have significant influence on the initiation of the weeping process.