Wettability of contaminated steel panels by water- and solvent-borne epoxy coatings: Interactions among surface tension and surface free energy components

This study explores the wettability of liquid epoxy coatings on clean, oil contaminated, and surface-corroded steel substrates, emphasizing the role of surface tension components and their interaction with surface free energy. Contact angle and pendant drop measurements, along with model-based estimations of ideal and roughness-corrected work of adhesion, were utilized. The Owens, Wendt, Rabel, and Kaelble (OWRK) method, combined with acid-base theory, quantified the dispersive, polar, and acid-base components of surface tension and surface free energy. Results showed minimal variations in surface tension among ten coatings, with waterborne formulations having slightly higher values than solvent-borne ones. The roughness-corrected work of adhesion, combined with surface tension and interfacial tension, effectively explained wettability. Contrary to expectations, both water- and solvent-borne coatings demonstrated better wettability on engine oil contaminated steels compared to clean substrates, due to polar additives like zinc dialkyl dithiophosphate. While the interfacial tension from the OWRK theory explained wettability on non-polar silicone oil contaminated and surface-corroded steels, it was less effective for polar engine oil contaminated surfaces. The interfacial tension from the acid-base approach, using standard parameters for water (γ⁺ = γ⁻ = 25.5) and glycerol (γ⁺ = 3.92, γ⁻ = 57.4), clarified wetting on polar engine oil contaminated and corroded steels. However, it could not explain the wetting performance on non-polar silicone oil contaminated steels. The mechanistic understanding of this work helps to enhance coating wettability and surface tolerance, particularly for oil contaminated and surface-corroded steel surfaces.