Model analysis of a multilayer curtain coating

High speed multilayer curtain coating is becoming important in paper industry due to its higher efficiency, low impact on the substrate and good uniformity. Although the single layer case has been studied, documentation regarding multilayer in paper industry condition has been scant. In the curtain impingement region, the stability of multilayer coating in different operation conditions is relevant to process optimization. Understanding the role of individual layers is especially important in designing functional layers without compromising the coating speed. In this work, a pilot multilayer curtain coating impingement region is modeled with a Finite Volume Method. The curtain and web speed is 2 m/s and 16 m/s respectively. Coating colors are modeled as shear thinning fluids. The reference case has capillary number Ca=8. Apparent contact angle is used as the criteria for air entrainment tendency. The influence of individual parameters on the angle is analyzed systematically. The multiphase simulation reveals a small but intensive air vortex close behind the curtain near the web. Within the investigated variable space, the results show that higher limiting viscosity increases air entrainment tendency, and the effect is stronger in the upper coating layer. Coating density, curtain speed and thickness have positive influence on reducing air entrainment, while web speed, curtain impingement angle has negative influence. Static surface/interface tension does not have apparent influence on air entrainment due to the high capillary number. Dynamic surface tension is likely not important in high speed curtain impingement region.