Low-flow limit in slot coating of dilute solutions of high molecular weight polymer

Abstract

Slot coating is a common method in the manufacture of a wide variety of products. It belongs to a class of coating method known as premetered coating: the thickness of the coated liquid layer in principle is set by the flow rate fed to the die and the speed of the substrate moving past, and is independent of other process variables. Thus, premetered methods are ideal for high precision coating. An important operating limit of slot coating is the minimum thickness that can be coated at a given substrate speed, generally referred to as the low-flow limit. The mechanism that defines this limit balances the viscous, capillary and inertial forces in the flow. Although most of the liquids coated industrially are polymeric solutions and dispersions that are not Newtonian, previous analyses of the low-flow limit in slot coating dealt only with Newtonian liquids. In this paper, the low-flow limit in slot coating of an extensional thickening polymer solution is examined both by theory and by experiment. The continuity and momentum equations coupled with an algebraic non-Newtonian constitutive equation that relates stress to the rate-of-strain and relative-rate-of-rotation tensors were solved by the Galerkin/finite element method to model the flows. The flows themselves were visualized by video microscopy and the low-flow limit was found by observing, at given substrate speed, the feed rate at which the flow becomes unstable and breaks up. Various solutions of low molecular weight polyethylene glycol (PEG) and high molecular weight polyethylene oxide (PEO) in water were used in order to evaluate the effect of mildly viscoelastic behavior on the process. At the concentration level of the high molecular weight polymer explored here, the viscoelastic behavior of the solutions could not be accessed by oscillatory tests; the only measurable response to the addition of PEO was the rise of the apparent extensional viscosity.