Influence of Elongational Viscosity on Coat Weight in Paperboard Coating With a Smooth Rod, 1999 Coating Fundamentals Symposium Proceedings

Annaleena Kokko, Stefan Kuni, Tom Grankvist

The aim of this paper is to investigate the rheological properties of some coating colors and their influence on coat weight development in a rod coating application of paperboard. The focus is on utilizing the elongational properties of coating colors with different co-binders and pigments. Like the commonly used steady shear viscosity, the elongational viscosity is a rheological parameter, which uniquely characterizes a material. Highly concentrated suspensions in a polymer solution like coating colors have, due to the presence of polymeric thickeners or co-binders, elongational viscosity as well as shear viscosity. We use these rheological parameters to show their importance in characterizing paper coating colors for coat weight development.

The elongational viscosity of the coating color describes its tendency to resist flow under accelerating velocity profiles. To a certain degree, elongational viscosity depends on pigment-pigment interactions, but most important are the pigment-polymer interactions. With high molecular weight polymer coating colors, it is possible to enlarge the shear flow region, which normally exists only in the high shear region under the rod, to also include part of the entrance region before the rod nip. Elongational viscosity is shear stress and strain dependent and therefore may be relevant to the assessment of runnability and coat weight under process flow conditions where a certain shear is applied on the coating color in a converging rod or blade nip. The results show that coating colors containing high molecular weight PVA, which appear to be dilatant, have a strong elongational viscosity, which is caused by the strong PVA network that can resist high shear, whereas the coating colors containing CMC have a very weak elongational viscosity.