Computer simulations of water diffusion through thermoset polymers: applications to corrosion protection coatings Charlie Wand 1 , Simon Gibbon 2 , Peter Visser 3 and Flor Siperstein 2 1 University of Exeter, UK, 2 The University of Manchester, UK, 3 AkzoNobel, The Netherlands 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. It has been considered that polymer coatings provide corrosion protection by forming a barrier to prevent mass transfer between the metal surface and the surroundings. However, it has been shown that water can permeate through these coatings and reach the metal surface [1]. Therefore the mechanism for corrosion protection is significantly more complex than originally assumed, with the transport of species through the coating playing a significant factor in corrosion protection properties of the coating. Here we perform atomistic molecular dynamics simulations of a model epoxy-amine coating comprising of Bisphenol A diglycidyl ether (DGEBA) and, m-Xylylenediamine, (MXDA) and investigate the diffusion of water through the coating and the influence of water on adhesion to an iron oxide surface as a proxy for a steel substrate [2].For the diffusion of water through a thermoset polymer, we investigate the effect of temperature, cross-linking (cure), and moisture content. We find that the free volume takes the form of isolated voids. The fraction of voids with a diameter larger than a water molecule increases with increasing temperature, cross- linking percentage, and moisture content. Conversely, the diffusion coefficient of water decreases with increasing cross-linking percentage. From the simulations we find that water diffusion is via a polymer-assisted hopping mechanism between isolated free volume nanoscale pores and that the polymer mobility is the determining factor for the speed of water diffusion.In the case of water on an iron oxide surface, we find that the water layer between the coating and the solid surface is qualitatively different for different iron oxide/ oxyhydroxide surfaces and that in some cases the presence of water can mediate surface adhesion, demonstrating the complex fundamental science behind corrosion protection coatings. References 1. C. Grave, I. Mcewan, and R. A. Pethrick, Influence of Stoichiometric Ratio on Water Absorption in Epoxy Resins, J. Appl. Polym. Sci. 69 , (1998). 2. C. R. Wand, S. Gibbon, P. Visser, and F. R. Siperstein, Water-Mediated Epoxy/Surface Adhesion: Understanding the Interphase Region, J. Eur. Chem. (2022) e202202483
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