Coating Rheology: Component Influence on the Rheological Response and Performance of Water-Borne Coatings in Roll Applications

Abstract

Even though much of the published coating-flow literature --- including many chapters in this book --- focuses on Newtonian liquids, most coating liquids in commercial applications are strongly non-Newtonian: the viscosity is not simply a constant, but depends on the deformation rates the liquid experiences; viscoelasticity gives rise to additional stresses that can significantly alter the local force balances in the flow; and, at sufficiently low deformation rates, coating liquids can exhibit solid-like yield behavior preventing flow altogether. In some instances, non-Newtonian characteristics of the formulation are adjusted for the specific purpose of enhancing coating process performance. In the pre-metered processes discussed in Chapters 11a-11c, for instance, small amounts of polymeric additives can enhance the stability and hence the operating latitude of the process. Another example comes from paints and varnishes, for which much research has been invested to achieve ease of flow during application yet resistance to sagging and other defects once the coating is applied. In many instances, on the other hand, the non-Newtonian behavior is the result of major formulation components necessary to achieve the ultimate performance of the coated film, and can give rise to serious difficulties in applying uniform layers at acceptable speeds. A primary case in point is that of adhesives, for which the desired `tackiness' in the final product is often associated with substantial viscoelasticity during processing.