A perspective on the inherent oxidation sensitivity of epoxy materials

There has long been awareness that epoxy materials are not inert to high temperature-induced degradation and therefore have performance limits. As organic polymers, epoxy thermo-sets and composite materials will degrade under thermo-oxidative conditions with loss of useful properties in adhesion or mechanical toughness, and this degradation may initiate at the surface. While high temperature empirical data on the depth of degradation profiles have been made available for some materials, precise data on the intrinsic oxidation sensitivity of epoxy materials do not yet exist. Two different epoxy material types were chosen: a DGEBA based resin (Epon 828) was cured with either a cycloaliphatic amine (Ancamine 2049) or a polyether amine (Jeffamine D230) for comparison. Due to generally low O2 permeability in epoxies any oxidation will be limited to the surface; hence experiments were conducted for thin films under non-diffusion limited conditions to obtain unbiased rate data. We have succeeded to show for the first time that oxidation behavior of epoxies at moderate temperatures can be approached experimentally. This study offers an overview on the intrinsic oxidation sensitivity of two cured epoxy thermo-set materials between 25 and 140 °C. Excellent Arrhenius behavior was observed with activation energies in the 70-80 kJ/mol range. These epoxy materials are surprisingly reactive and show higher oxidation rates than anticipated, even at moderate temperatures. In any applications where epoxy materials are used at elevated temperatures under non-inert conditions, oxidation will result in material chemistry changes and this will affect their performance with regard to adhesion or properties in composites.