Dynamic Surface Tension Measurement in Liquid Curtain Coating Systems
Dynamic surface tension at millisecond surface ages dictates liquid curtain stability and dynamic wetting limits in paperboard coating operations.
Curtain Necking
Transverse Contraction
Geometric deformations occurring in a falling sheet of liquid cause the width of the coating to shrink before it contacts the substrate. In the process of curtain necking, surface tension pulls the edges of the liquid inward as it accelerates under gravity. This narrowing results in a coating layer that is narrower than the die from which it was extruded.
Edge Control
Edge guides or physical barriers are used to minimize the inward pull by providing a surface for the liquid to cling to as it falls. Without these interventions, the reduction in width would create thick beads at the margins and uneven coverage across the web. The extent of the shrinkage depends on the flow rate, the height of the fall and the surface tension of the fluid.
Higher speeds often increase the stability of the curtain, which helps to counteract the effects of necking.
Width Maintenance
Precision in the final width of the application is necessary to prevent wastage and to ensure the coating reaches the edges of the paperboard. Operators adjust the formulation of the coating to lower the surface tension, which reduces the force driving the contraction. This adjustment allows for the use of wider substrates without changing the die configuration.
Consistent width management is a requirement for high-speed curtain coating operations.