Local oxidation of the buried epoxy-amine/iron oxide interphase

Abstract

Epoxy-amine resins continue to find widespread use as the binders of corrosion protective organic coatings. In service, exposure to the environment ultimately results in oxidative deterioration of epoxy coatings, limiting the performance lifetime. Whilst the mechanisms underpinning surface oxidation have been characterized, few studies have examined polymeric degradation in the polymer/metal interphase, the integrity of which is central to performance. This is in part due to the inaccessibility of the buried interphase, which lies beyond the resolution limits of most organic analysis techniques. In this study, two innovative approaches are used to examine early stages of oxidative degradation of the buried polymer/iron oxide interphase for diglycidyl ether of bisphenol-A (DGEBA) coatings cross-linked with an aliphatic amine hardener, triethylenetetraamine (TETA). First, high fractions of iron oxide interfaces are introduced into the resin systems using synthetic hematite, Fe2O3, magnetite, Fe3O4 and goethite, Fe(O)OH powders, and the oxidation process is monitored using conventional infrared spectroscopy. Next, the buried interphase of coatings applied to iron is examined directly using nanoscale cross-sectional infrared analysis (via the photothermal infrared, PTIR, atomic force microscopy technique, known as AFM-IR). Diffusion limited oxidation is shown to initiate at the buried polymer-metal interface and progress slowly into the polymer, in accordance with pre-established mechanisms, resulting in chemical gradients of <400 nm after 28 days exposure to mild thermal aging conditions (70 °C, 14% RH).