CO₂-Induced Amine Blushing and Heterogeneity in DGEBA/m-XDA Epoxy Coatings: FTIR and EIS Insights

Epoxy-amine coatings are widely used for metal protection because they provide strong interfacial bonding, long service life, and excellent corrosion resistance. In this work, Bisphenol A diglycidyl ether (DGEBA) cured with m-xylylenediamine (m-XDA) was used as a model system to investigate how environmental CO₂ affects coating heterogeneity and optical appearance. Particular attention was given to amine blushing, since m-XDA can react rapidly with atmospheric CO₂ to adsorb and to form carbamated species on the coating surface, leading to reduced transparency of both the bulk polymer and the film.

DGEBA/m-XDA specimens were cured at room temperature under either N₂ or a CO₂-containing ambient atmosphere, and characterised using Fourier-transform infrared spectroscopy (FTIR) and electrochemical impedance spectroscopy (EIS). The CO₂-exposed samples showed pronounced inhomogeneity throughout the bulk, obvious surface amine blushing, and strong bands at 2400–2300 cm⁻¹ attributed to CO₂ species trapped in the polymer matrix, which correlated with decreased transparency, whereas the N₂-cured samples appeared more homogeneous and showed no evidence of CO₂ capture. Subsequent high-temperature treatment at 160 °C greatly reduced the CO₂-related FTIR bands, indicating that CO₂ near the surface could desorb, although cloudiness remained in much of the bulk and only localised regions showed disappearance of carbamate/carbamic acid features with concurrent strengthening of bands near 1700 cm⁻¹ and in C–N/N–H regions.

Free-standing films cured at 60 °C and post-cured at 100 and 160 °C were also evaluated by EIS, which revealed no significant differences in coating resistivity between CO₂-exposed and N₂-cured samples. These results suggest that CO₂ uptake and carbamate formation at the surface mainly influence visual appearance and micro-scale heterogeneity rather than global barrier performance. The study highlights the critical role of cure atmosphere in controlling amine blushing and optical properties of epoxy-amine coatings without severely compromising their electrochemical corrosion protection.