Extensional viscosity of curtain coating colors evaluated as Euler number

Abstract

The aim of this study was to evaluate different additives and their effect on the Euler number, and thus the extensional viscosity of barrier coating dispersions. The Euler number was measured over a wide range of flow rates, with an emphasis on a proposed standard of 15 m/s. The Euler number was measured using a high shear capillary viscometer, using an orifice geometry. Additives used in this study were carboxymethyl cellulose, polyvinyl alcohol and polyethylene oxide. The results show significant differences in the effect on the Euler number. The addition of high molar mass polyethylene oxide increased the Euler number significantly. Polyvinyl alcohol and lower molar mass polyethylene oxide showed a moderate increase in Euler number. Carboxymethyl cellulose and the barrier coating dispersion with no additives showed insignificant increases. The extensional viscosity is, however, also affected by the shear viscosity, which must be taken into account when comparing measurements. Extensional viscosity plays an important role in several coating methods. In curtain coating, it plays a key role in the falling curtain and the impingement point. Extensional viscosity can slow down the expansion of holes in the curtain, and even prevent them from forming altogether. Insufficient extensional viscosity can also lead to uncoated areas on the surface due to inadequate stretching of the coating fluid onto it.