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.2018 Sep 13;10(9):1017.
doi: 10.3390/polym10091017.

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

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

Andrey E Krauklis et al. Polymers (Basel)..

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.

Keywords: carbonyl formation; epoxy; leaching; mechanism; thermo-oxidation; yellowing.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Molecular structures of epoxy and hardener components: (A) DGEBA monomer; (B) DGEBA oligomer (n = 1–2); (C) HDDGE; (D) POPA; (E) IPDA.
Figure 2
Figure 2
Visual inspection of an epoxy resin showing discoloration due to hygrothermal aging.Top: initial (blueish grey);bottom: redried after conditioning (yellow).
Figure 3
Figure 3
Digital optical (top) and SEM (bottom) micrographs of initial dry (left) and redried after hygrothermal aging (right).
Figure 4
Figure 4
FT-NIR spectra of initial resin and redried epoxy after hygrothermal aging. (Left): visible light region. (Right): NIR region.
Figure 5
Figure 5
ATR-FT-IR spectra of initial (bottom) and redried after hygrothermal process resin (top).
Figure 6
Figure 6
Ca, Cl, K, Na and S release from neat resin during hygrothermal aging at 60 °C.
Figure 7
Figure 7
pH measurements of distilled water samples after contact with the resin.
Figure 8
Figure 8
Temperature sweep for glass transition temperature determination for initial (dry); saturated; and redried. (Left): temperature sweep of tensile storage modulus. (Right): temperature sweep of tensile loss modulus.
Figure 9
Figure 9
Schematic representation of the logic during investigation.
Figure 10
Figure 10
Chemical structure of the studied DGEBA/HDDGE/IPDA/POPA amine epoxy network (mixing ratios are not considered). Marked sites represent “weak points” for radical attack in the network. Sites marked in red are excluded based on experimental evidence and literature. Sites marked in green are the main reactive sites.
Scheme 1
Scheme 1
Crosslinking reaction of the HDDGE segments via reactive sites δ+HDDGE-IV.
Scheme 2
Scheme 2
Carbonyl formation involving polyoxypropylene moiety on reactive site δ+POPA-I.
Scheme 3
Scheme 3
Carbonyl formation reaction involvingi-propanol moiety on reactive sites δ+DGEBA-II and δ+HDDGE-III.
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