Extensional Properties of Coating Colors at High Strain Rates

Abstract

Paper coating fluids also called colors are concentrated aqueous suspensions composed mainly of mineral pigments, thickeners, binders and dispersing agents. They are applied onto moving paper web for improving the optical and printing properties. Roll coating is one of the most used technologies for paper coating, however jet coating is currently a promising technology for high-speed processes. Coating colors are submitted to high strain rates in both roll or jet coaters, therefore the extensional viscosity plays a major role in the process. An orifice flowmeter was used for measuring the extensional properties of complex rheology fluids such as coating colors. The principle of this flowmeter is based on the relationship between pressure drop and the flow rate of fluid passing through a small orifice. The flowmeter was firstly calibrated in terms of a dimensionless Euler number as a function of the Reynolds number with Newtonian fluids. The calibration curve was then used to determine the apparent extensional viscosity of coating colors. Results of extensional properties of paper coating colors are presented and compared to shear viscosity. The ratio of extensional to shear viscosity (Trouton ratio) for some coating colors was shown to exceed considerably the theoretical value of 3 expected for Newtonian fluids.

Extensional Viscosity of Coating Colors and its Relation With Jet Coating Performance

“…These values of the Trouton ratio are comparable to those obtained by Della Valle et al [9] and Ascanio et al [16] for clay suspensions in a Newtonian fluid containing 50 -65 wt% solids content. Furthermore, these results are similar to the value obtained with coating colors based on delaminated kaolin clay (52 and 62 wt%) and ground calcium carbonate (55 and 65 wt%) by Arzate et al [17] who found a Trouton ratio between 8 and 20. However, it has been found that the Trouton ratio could reach values of 85 for coating colors based on delaminated kaolin clay [16].…”