Simultaneous multilayer coating of water-based and alcohol-based solutions

Multilayer continuous liquid coating is the most efficient way to manufacture films that require more than one layer for optimal performance. Two-layer slot coating is one of the different coating methods largely used to deposit two thin, uniform liquid layers onto a moving substrate. In recent years, the operating conditions and solution properties which are required to avoid various coating defects have been discussed in the literature. However, these past studies have not focused on the problem of top layer dewetting, which may occur at some operating conditions. It is important to include in the operating window of the process, the region of the parameter space at which the top layer dewetting is avoided. In this work, we used water-based and alcohol-based solutions to create two-layer films over a moving substrate using a two-layer slot coating die. The critical operating condition at which top layer dewetting is observed is determined experimentally for different liquid viscosities, surface tensions, and wet thicknesses. A physical mechanism that explains the reported phenomenon is proposed. The results show how dewetting can be avoided by controlling the liquid properties (surface tension and viscosity) and wet thickness of each layer and will certainly help the design of two-layer slot coating processes.