Two-Layer Slot Coating: Study of Die Geometry and Interfacial Region

Abstract

<p>The two-layer slot coating method is commonly used in industry to apply multiple layers of miscible liquids onto a moving substrate. Previous experimental work using a standard flat die face geometry has shown that tiny recirculations exist within the coating bead. Recirculations near the interfacial region may cause convective mixing of the two liquid layers and thus, defects in the final coated material. Previous studies on the interfacial region between the two miscible liquids have assumed that diffusional and convective mixing of the two layers can be ignored and that an 'interface' of zero surface tension exists between the two layers. This work attempts to show that observable mixing does occur between layers of miscible liquid and that the convective mixing can be minimized by changing the geometry of the die face between the two slots.</p> <p>Flow visualizations using particles and two different coloured dyes were performed on the standard square die face geometry. Result show that vortices exist in the coating bead under all the test conditions, either at the downstream shoulder of the die or in the interfacial region near upstream feed slot. The presence of the vortices at these positions, as well as the position of the separation line, was partially determined by the ratio of top layer viscosity to bottom layer viscosity, among other factors.</p> <p>Visualizations were also done using various geometries for the centre block: knife, groove, and bullet referring to the shape of the different blocks. The knife geometry decreased the performance of the two slot coater, causing problems such as oscillations of the upstream meniscus and invasions of the top layer into the bottom layer feed slot. The performance of the groove geometry was very similar to the standard square geometry except for the occurrence of an extra tiny vortex inside the groove itself. The bullet centre block eliminated the vortices in the interfacial region and provided limited control of the region's shape and position.</p> <p>The commercial CFD package FIDAP^TM was used to simulate a two dimensional Newtonian model of the two slot coating bead under the same operating conditions used in the experiments. Variations in liquid properties across the interfacial region were calculated by defining viscosity as a function of false species concentration, where the isoconcentration contours were then used to define the interfacial region. The simulation showed excellent agreement with the square and groove geometries, and reasonable agreement with the knife and bullet geometries The simulations indicate that a vortex forms in the interfacial region near the upstream feed slot for viscosity ratios of 4:1 to 10:1. The vortex caused convective mixing between the liquid layers for all geometries; however, the mixing was reduced or eliminated by the knife, groove and bullet geometries at a viscosity ratio of 10:1.</p>