Wettability of waterborne and solvent-based epoxy coatings on contaminated steel panels

Hypotheses: Surface tension components, such as polar, dispersive, and acid-base contributions, can be quantified for liquid coatings, and it is expected that a coating with a surface tension lower than the surface free energy of a solid will provide adequate wettability and strong adhesion. However, under certain conditions, interactions between surface tension and surface free energy components can lead to unexpected wetting mechanisms.

Experiments: Adhesion of liquid epoxy coatings to oil and rust contaminated steel panels was investigated using contact angle and pendant drop measurements, as well as model-based estimations of the ideal and roughness-corrected work of adhesion. For quantification of the theoretical wettability, the Owens, Wendt, Rabel and Kaelble (OWRK) approach, coupled with the acid-base theory, was used to determine the dispersive, polar, and acid-base components of the surface tension of liquid coatings and the surface free energy of oil and flash rust contaminated substrates.

Findings: Surface tension measurements of six coatings showed minimal variation, with waterborne coatings having slightly higher values. Furthermore, the roughness-corrected work of adhesion, the surface tension, and the surface free energy results, pointed to the importance of compatibility between polar, dispersive, and acid-base components in understanding wettability. Due to increased polar contributions from additives in the IQ-X EKSTRA 10W/40 engine oil, such as zinc dialkyl dithiophosphate and dimethyl sulfoxide (DMSO) extracts, coatings, contrary to expectations, demonstrated better wettability on oil-contaminated substrates. In addition, the acid-base approach was particularly useful for quantifying the wettability of non-pigmented coatings. However, estimation of the wettability through the OWRK and acid-base theories faced difficulties due to the chemical heterogeneities in pigmented coatings and the rust layer, which led to the violation of some of the underlying model assumptions.

The mechanistic understanding of this work can help to enhance coating wettability, particularly for oil and rust contaminated substrates.