Mechanism of Yellowing: Carbonyl Formation during Hygrothermal Aging in a Common Amine Epoxy

Abstract

Epoxies are often exposed to water due to rain and humid air environments. Epoxy yellows during its service time under these conditions, even when protected from UV radiation. The material’s color is not regained upon redrying, indicating irreversible aging mechanisms. Understanding what causes a discoloration is of importance for applications where the visual aspect of the material is significant. In this work, irreversible aging mechanisms and the cause of yellowing were identified. Experiments were performed using a combination of FT-NIR, ATR-FT-IR, EDX, HR-ICP-MS, pH measurements, optical microscopy, SEM, and DMTA. Such extensive material characterization and structured logic of investigation, provided the necessary evidence to investigate the long-term changes. No chain scission (hydrolysis or oxidation-induced) was present in the studied common DGEBA/HDDGE/IPDA/POPA epoxy, whilst it was found that thermo-oxidation and leaching occurred. Thermo-oxidation involved evolution of carbonyl groups in the polymeric carbon–carbon backbone, via nucleophilic radical attack and minor crosslinking of the HDDGE segments. Four probable reactive sites were identified, and respective reactions were proposed. Compounds involved in leaching were identified to be epichlorohydrin and inorganic impurities but were found to be unrelated to yellowing. Carbonyl formation in the epoxy backbone due to thermo-oxidation was the cause for the yellowing of the material.

Prediction of Orthotropic Hygroscopic Swelling of Fiber-Reinforced Composites from Isotropic Swelling of Matrix Polymer

“…One of the main effects of water on the mechanical property deterioration of polymers is swelling, even more so if the polymer is not affected by hydrolysis or chain scission, such as the epoxy in this study [1,9]. Swelling is a specific response accompanying moisture diffusion in polymers and polymer-based composites [2].…”

“…Fiber-reinforced composite (fiber-reinforced polymer; FRP) laminates are used for structural applications in marine, offshore and oil and gas industries due to their light weight and corrosion resistance [1][2][3]. Composites offshore have been implemented in such applications as risers, tethers, repair patches and ship hulls [4][5][6][7][8].…”

“…Composites offshore have been implemented in such applications as risers, tethers, repair patches and ship hulls [4][5][6][7][8]. In these applications, FRPs are exposed to water and experience subsequent water-induced or hygroscopic swelling [1][2][3].…”

Thermal mending in novel epoxy/cyclic olefin copolymer blends

“…The thermal treatment for 1 hour at 190°C affects the color, as all thermally treated samples turned their color to an orange coloration. The change in color is a known effect of thermal aging for epoxy resin systems and it can be attributed to oxidation processes [46]. However, considering that the prepared blends are intended to be used as matrices for composites materials, the effects of the COC addition and of the thermal treatment do not represent a relevant aesthetic issue.…”

Yellowing of laser-cleaned artworks: Formation of residual hydrocarbon compounds after Nd:YAG laser cleaning of gypsum plates covered by lamp black

“…Since the residual carbon compounds yellow the surface, they include specific species not absorbing in the 560-590 nm wavelength range. A possible candidate is the carbonyl group -C=O, which causes the yellowing of organic polymers [34]. Similar to the hydrogenation reactions, -C=O groups can form by oxidation of the carbon precursors.…”