Critical bubble size and surfactant for curtain disruption

Multi-layer curtain coating has the potential to produce novel grades of paper and products that make economical use of resources. The stability of the curtain with regard to the operational window is well understood, but the influence of bubbles and non dissolved surfactant droplet in the coating formulation can be problematic. Air removal systems can be designed to remove bubbles of a certain size, but it is not clear at present, the size of bubble that can be tolerated in this coating system and design parameters for equipment to remove air. Surfactant are often added into coating color in excess; the influence of non-dissolved surfactant to curtain stability is also not clear. Experiments were performed on a semi-pilot scale coating unit at KCL. Natural bubbles or bubbles that were injected into the fluid upstream of the curtain were observed with high speed cameras. To test possible mechanisms, finite volume methods were used to describe the stretching of the curtain and the behavior of a large bubble in the curtain flow field. In addition, a thin film model is proposed to calculate the drainage of the film between a bubble and the curtain side. A front-tracking volume of fluid (VOF) method is used to model Marangoni forces that induce curtain break. The film thinning event is faster for small bubbles than for larger bubbles. Therefore, in curtain flow, when a small bubble is brought near the surface due to flow, the potential to rupture the curtain and disrupt the flow is large. Larger bubbles, over 0.5 mm, will not rupture the surface in the time scale of the flow. The breaking time is in millisecond order for a typical curtain coating color. A thinner film is easier to break than a thicker film. In equipment to remove bubbles from coatings, the design should focus on getting small bubbles to the surface and not the residence time of the bubble being at the surface. Non-dissolved surfactant droplet may causes curtain break if the change of surface tension is sufficiently high, e.g. 0.01N/m.