Physics of coating tensioned-web over slot die.

Abstract

Coating tensioned-web running over slot die is an efficient way of depositing thin and uniform layers on flexible web at high speed with a relatively simple set-up, a combination of slot die pre-metering and carefully controlled web transport. It is different from conventional slot die coating, where the coating gap is maintained by the clearance between backup roll and slot die, in that coating gap is maintained by elastohydrodynamic balance between the tension-resultant force of the curved and tensioned web and the pressure in the coating bead; with this setup much thinner coating gap is made than is possible in conventional slot die coating. For successful operation in tensioned-web coating system, properly designed die is necessary. In conventional slot die coating, pressure profile in the coating bead is well understood and efficiently predicted by considerations based on the one-dimensional viscocapillary model. The model to describe the pressure and gap profiles in tensioned-web slot die coating needs to incorporate the interaction between the liquid flow and the web deformation, leading to a nonlinear system of equations. In this thesis, coating bead flows and gap profiles in both single-layer and two-layer tensioned-web slot die coating were studied by theoretical model based on shell theory and lubrication approximation, hybridized with two-dimensional flow model when two-dimensional flow model is necessary, and visualization using bench-top web transport system with slot die mounted. The results from visualization and theoretical model study show that a tensioned-web slot die coating die can be viewed as a combination of component foil bearings. Interpreting the results in this manner, it is easier to predict how the change in die shape and process condition will affect the pressure and gap profiles and therefore easier to design the die shapes required to achieve desired process states, as thin and fast coating, stable two layer coating, or to predict and remove the appearance of microvortexes. By the two-dimensional model on the flow of the two-layer coating, positioning of separation line and appearance of the microvortexes inside the feed slot were studied with the changes in operation condition.