Abstract
Epoxy-amine resins continue to find widespread use as the binders in protective and decorative organic coatings, as the matrix in composite materials, and as adhesives. In service, exposure to the environment ultimately results in oxidative deterioration of these materials, limiting the performance lifetime. Defining this auto-oxidation process is therefore a key challenge in developing more durable high-performance materials. In this study, we investigate oxidative degradation of a model resin based on diglycidyl ether of bisphenol-A (DGEBA) and an aliphatic amine hardener, triethylenetetraamine (TETA). Using infrared spectroscopy, we find that prior to the expected detection of formate groups (corresponding to the well-known radical oxidation mechanism of DGEBA), a band at 1658 cm−1 forms, associated with amine cross-linker oxidation. Infrared microspectroscopy, in-situ heated ATR-infrared, Raman spectroscopy and AFM-IR techniques are thus employed to investigate the early stages of resin oxidation and demonstrate strong parallels between the initial stages of cured resin degradation and the auto-oxidation of TETA cross-linker molecules.
Highlights
Epoxy-amine resins continue to find widespread use as the binders in protective and decorative organic coatings, as the matrix in high performance composite materials, and as adhesives
We examine the early stages of oxidative degradation of a model epoxy-amine resin based on diglycidyl ether of bisphenol-A (DGEBA) and an aliphatic amine hardener, triethylenetetramine (TETA), since this system falls within the most widely studied group of epoxies and amine hardeners (i.e., those based on ethylene diamine (EDA) building block; including EDA, diethylenetriamine (DETA) and TETA)
A combination of Fourier Transform Infrared (FTIR) microspectroscopy, in-situ heatcontrolled FTIR spectroscopy, Raman spectroscopy and the recently developed AFM-IR technique were applied to investigate the auto oxidation of epoxy-amine resins under mild conditions (70 C), producing chemical changes representative of those induced by prolonged natural weathering
Summary
Epoxy-amine resins continue to find widespread use as the binders in protective and decorative organic coatings, as the matrix in high performance composite materials, and as adhesives. For epoxies based on the diglycidyl ether of bis-phenol-A (DGEBA) unit, several characteristic spectral changes have consistently been identified after photo- or thermal aging, both in the absence of cross-linkers (phenoxy resins), and in networks formed using anhydride or amine hardeners. A second, more minor pathway to formate generation has been identified via CH2 hydrogen abstraction from isopropylidene sites [12] Despite these advances in understanding, attempts to fully characterise oxidation in cross-linked epoxy networks remain hampered by the broad, ill-defined infrared bands emergent after oxidation, and the accepted contribution of complex alkoxy radical chemistry, which yields an abundance of possible mechanisms. Oxidation of the molecular amine hardener is fully characterised, and we find significant parallels between the aliphatic amine oxidation products and those found in the resins during the early stages of thermal aging
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