Microstructure and morphology of amine-cured epoxy coatings before and after outdoor exposures—An AFM study

Abstract

Atomic force microscopy (AFM) has been used to study the morphology and microstructure of an amine-cured epoxy before and after outdoor exposure. Measurements were made from samples prepared in an essentially CO 2 -free, H 2 O-free glove box and from samples prepared in ambient conditions. For those prepared in a CO 2 -free glove box, AFM imaging was conducted on (1) an unexposed air/coating surface, (2) an unexposed coating bulk, (3) an unexposed coating/substrate interface, and (4) a field exposed air/coating surface. For samples prepared in ambient conditions, only the unexposed air/coating surface was investigated. The same regions of the exposed samples were scanned periodically by the AFM to monitor changes in the surface morphology of the coating as UV exposure progressed. Small angle neutron scattering and Fourier transform infrared spectroscopy (FTIR) studies were performed to verify the microstructure and to follow chemical changes during outdoor exposure, respectively. The results have shown that amine blushing, which occurs only under ambient conditions, had a significant effect on the surface morphology and microstructure of the epoxy. The surface morphology of the samples prepared under CO 2 -free, dry conditions was generally smooth and homogeneous. However, the interface and the bulk samples clearly revealed a two-phase structure consisting of bright nodular domains and dark interstitial regions, indicating an inhomogeneous microstructure. Such heterogeneous structure of the bulk was in good agreement with results obtained by small angle neutron scattering of unexposed samples and by AFM phase imaging of the degraded sample surface. The relationship between submicrometer physical changes and molecular chemical degradation is discussed.

Insights into Epoxy Network Nanostructural Heterogeneity Using AFM-IR

“…28 More recently, examination of fracture surfaces has become possible under ambient conditions (i.e., circumventing any sample preparation effects) using the atomic force microscope (AFM), and numerous reports have emerged of nodular morphologies detected in tapping mode height or phase images. 26,29,30,31 However, Duchet et al countered that such features may be attributed to imaging artefacts generated by a blunt SPM probe tip scanning over rough surfaces, and a similar argument has recently been made for peakforce tapping mode modulus maps showing an apparently heterogeneous nanostructure for epoxy amine fracture surfaces. 32,33 These differences in interpretation are unsurprising, given that the evidence for heterogeneous network formation has largely been limited to topographical measurements of fracture interfaces.…”

Toward a computational and experimental model of a poly-epoxy surface

“…Our experimental approach is based on the method described in [17,18] for forming model poly-epoxy surfaces. It consists in the polymerization of the poly-epoxy in an Ar gloves box at ambient temperature for at least 24 h, followed by a postcuring at elevated temperature (polymer-dependent).…”

“…It consists in the polymerization of the poly-epoxy in an Ar gloves box at ambient temperature for at least 24 h, followed by a postcuring at elevated temperature (polymer-dependent). Gu et al [17] synthesize samples from a stoichiometric mixture of DGEBA + 1,3-di(aminomethyl)-cyclohexane, with a small amount of toluene for decreasing viscosity and favoring an homogeneous stirring (7 min). Samples are then stored for 24 h at ambient temperature, and postcured for 2 h at 130 • C in an air furnace.…”

Model-based analysis of photoinitiated coating degradation under artificial exposure conditions

“…The approximate thickness of this zone and the surface morphology in the top 10-30 nm 13,56 have been studied for epoxy-amine coatings, but the porosity, glass transition, and crosslink density profiles, phase behavior, and oxygen solubility and diffusion rates are needed to verify some of the assumptions made in this work. It would also be interesting to find out how the oxidation zone of densely crosslinked epoxy-amine networks differ from that of other networks and full commercial coating systems.…”

“…55 Part of the epoxy backbone structure is gone and oxidation products are present. The diffusion coefficient of oxygen in the oxidation zone is most likely a strong function of position because the porosity is high at the very outer surface 13,56 and, in the model, falls to zero at the oxidation front. Possibly, the oxidation zone is only porous in the outer part-the low oxygen diffusion coefficient used in the model is an indication of this-but investigations are needed to quantify the hypothesis.…”