Operational Issues in High-Speed Curtain Coating of Paper, 2001 Coating Conference Proceedings

Nick Triantafillopoulos, Johan Grön, Iiro Luostarinen, Petri Paloviita--The curtain coating method features a liquid sheet that falls freely over a certain height before it impinges onto the substrate to be coated. A high impinging velocity allows the method to be used to coat at relatively high web speeds. The non-contact method does not either put any mechanical stress on the substrate to be coated. In spite of the many advantages, curtain coating is susceptible to operational conditions, e.g. onset of heel formation, air-entrainment, curtain contraction. These issues relate to both air boundary and air content control. The thin air layer entering the curtain impingement zone on the paper surface needs to be removed to avoid air intrusion, which disrupt the color transfer. By removing the air layer at speeds exceeding 400 m/min, it has been possible to coat at web speeds up to 1500 m/min. Air voids in the coating color might break the curtain at the slot die exit. The macrovoids are easily removed, but microvoids may cause harm to the coating layer. Coating at high speed requires a color with a low air void volume. An increased knowledge regarding color formulations has made this possible. The curtain coater as a contour type coating method gives possibilities to apply low coat weights (5.8 g/m²) with a high coverage (85 %), even at a low solids content (54 %). At the same coat weight of 5-6 g/m², there is almost a 40 % advantage in coating coverage using the curtain coater instead of the metered size press. The curtain coater could be a practical coating method, especially for light paper grades with low coat weights.