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Extensional rheology of coating suspensions in relation with misting in film coaters

Coating suspensions are submitted to very high strain rates in the transfer nip of film coaters, which leads under certain conditions to misting. The extensional viscosity of coating suspensions is mainly responsible for this major runnability issue. An orifice flowmeter was used for measuring the extensional properties of complex rheology fluids such as coating suspensions. 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 corresponding curve was then used to determine the apparent extensional viscosity of coating suspensions. Results of extensional properties of paper coating suspensions are presented and compared to shear viscosity. The Trouton ratio (ratio of extensional to shear viscosity) for some coating suspensions was shown to exceed considerably the theoretical value of 3 expected for Newtonian fluids. The runnability of four coating suspensions was assessed using a pilot coater in terms of the transfer ratio and the coat weight. Results showed that a pronounced strain hardening reduces considerably the amount of misting but the drop size becomes smaller.