Network formation in ultra-thin epoxy films on magnesium

The network formation in thin spin coating epoxy films (DiGlycidylEther of Bisphenol A + DiEthyleneTri-Amine) on PVD magnesium substrates with varied chemical surface state is studied by FTIR External Reflection Absorption Spectroscopy. It is found that the degree of epoxy conversion at room temperature falls below the bulk value and decreases with decreasing epoxy film thickness. This impeded network formation results from a demixing of the reactive components forced by the magnesium substrate. Increasing the DETA content (beyond the stoichiometric oxirane-amine ratio) generally leads to a higher degree of conversion in the films but the values are still lower than in the corresponding bulk. An increase of curing temperature (10, 23, 40 deg C) diminishes that hindrance of epoxy conversion and its dependence on film thickness. It is concluded that the substrate-induced demixing is more and more suppressed by the accelerated network formation at higher temperature. The network formation in thin epoxy films on dried Mg substrates is studied by FTIR spectroscopy. As compared with the bulk, the epoxy conversion is impeded and this effect extends over 10(exp 2) - 10(exp 3) nm into the films. This is attributed to a phase separation of amine and epoxy which is forced by the metal substrate. An increased amine content and higher curing temperatures reduce the tendency to phase separation and this results in increased degrees of conversion, i. e. stiffer epoxy networks. Thus, tailored interphases can be produced on Mg substrates by the choice of the adhesive composition and the appropriate curing regime with respect to temperature and time. Epoxy film formation on Mg surfaces is critical, however, since adsorbed water or CO2 result in strong dewetting.