(594h) What Makes Paint Stick? Insights from Computer Simulations of Epoxy Resin Binding on Iron Oxide / Oxide-Hydroxide Surfaces

The infrastructure of our modern society relies on large scale metal structures, which have a very large embedded carbon dioxide footprint. Protective organic coatings are key to sustainability as they extend the life of assets in aerospace, automotive, and freight by providing protection against corrosion. Epoxy resins are widely used due to their good heat and chemical resistance, favourable mechanical properties and good adhesion to a range of substrates. In all cases, the performance of the final solid-polymer system is dependent on the physicochemical properties of the interface and the interaction between the polymer and the solid substrate. However, experimental methods to characterize this interaction are limited and mostly are destructive of the interface. Computer modelling provides an excellent tool to investigate the surface-polymer interface at an atomistic level.

Here we perform atomistic molecular dynamics simulations to investigate the binding of common epoxy resin components to different iron oxide/ oxide-hydroxide surfaces. We calculate the binding energy, a measure of the energy difference between the bound adsorbate and the adsorbate at infinite distance from the surface, alongside free energy calculations which are computationally more demanding but provide valuable insight into the underlying free energy landscape that drives adsorption.

In epoxy resin applications the composition of the solid substrate is highly varied, with pre-treatments and production processes leading to a non-uniform surface chemistry and roughness. To reflect this, we investigate three different iron oxide/ oxide-hydroxide surfaces; hematite (Fe₂O₃) and magnetite (Fe₃O₄) and goethite (FeOOH) and consider different surface hydroxylation levels. We compare our findings with experimental results and find that the binding of epoxy components to each surface is quantitatively different. From these results, we suggest reasons for this trend based on the surface termination and gain fundamental understanding on how epoxy resins bind to metal substrates.