Misting droplet size analysis in a metered film coater

The high process speed as well as the low stress at which the web is submitted are two good reasons for considering roll coating over other technologies for paper coating such as slot coating or blade coating. However, due to the film splitting some filaments of coating fluid are formed and broken up at the exit of the transfer nip resulting in small droplets ejected to the surroundings. Such phenomenon, known as misting, appears usually at high speed when using low concentrated coating fluids. The runnability of two coating colors has been evaluated by coating bond paper in a pilot roll coater at process speeds similar to those encountered in the paper industry. Misting droplets size and distribution determined by image analysis have been related with the coating transfer efficiency and the extensional viscosity. It was demonstrated that high solids content suspensions having large extensional viscosity produce less misting especially at high speed resulting in higher coating effectiveness. (c) 2007 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.