Advancing self-stratifying coatings: A systematic approach based on spreading coefficients

Despite the high performance of multilayer coating systems in industrial applications, they still face ecological and economic challenges. The technology of self-stratifying coatings, representing the spontaneous formation of multiple layers within a single coating application step, is a promising approach to overcome some of these drawbacks. However, designing such coatings requires detailed understanding of the underlying driving forces and mechanisms resulting in a controlled layer alignment during film formation. In this study, we propose a novel approach based on spreading coefficients to systematically describe the binder phase behavior in self-stratifying coatings. Starting from the coating application, the introduced spreading coefficients assess the potential interactions of the continuous and dispersed binder phase with the surrounding interfaces and connect them with the occurring self-stratification mechanisms. A previously described prediction model is additionally considered in the analysis and compared to the presented approach. To validate the introduced considerations, an epoxy/silicone-based coating composition was applied in two mixing ratios to various substrates. Surface and interfacial tensions of the binder phases and substrates were estimated by van Oss-Chaudhury-Good theory, and the resulting coating morphologies were analyzed by Scanning Electron Microscopy with Energy Dispersive X-ray spectroscopy. The novel spreading coefficient approach could successfully explain the different coating morphologies and phenomena observed, including self-stratified coatings, various non-stratified structures, and binder phase inversion due to substrate dewetting, and showed superior accuracy compared to the previous model. This study provides valuable new insights into the fundamental understanding and formation pathways of self-stratifying coatings, advancing their development and practical implementation.