Numerical and experimental investigation of the unexpected thickening effect during PolyActive™ coating of TFC membranes

In this work, we used a previously described computational fluid dynamics (CFD) model of a roll-to-roll coating process for the fabrication of thin-film composite membranes to predict the final coating thickness for PolyActive™, a commercially available multiblock copolymer utilized for CO₂ removal membranes. We found a strong thickening effect that could not be explained by our previous model. We investigated the process experimentally. In addition, we conducted a variety of simulations with extensions of the initial CFD model. Based on our findings, we conclude that the Marangoni effect, that is, gradients of surface tension that induce secondary flow patterns in the meniscus region, is the most likely source of the observed thickening. We explain the simulation results in order to understand the physical mechanisms at play and to show how especially surface tension gradients that arise from the particular flow structure in the meniscus may explain the additional transfer of polymer solution to the membrane. Finally, we draw some conclusions on future research and give ideas on future improvements of the process. To our knowledge, Marangoni effects for the coating of PolyActive™ were not described so far in the literature, even though it is a well-known polymer for gas separation membranes targeted at CO₂ removal. Roll-to-roll coating is a well-established coating method and often believed to be suitable for the scale-up of membrane production, therefore we think that this work will help membrane researchers who are using similar coating devices to be cautious about possible complications in the process.