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. 2025 Apr 23;18(9):1906.
doi: 10.3390/ma18091906.

In Situ Evaluation of Epoxy Self-Healing Coating by Encapsulated Linseed Oil in Poly(Urea-Formaldehyde-Melamine) Microcapsules

Lucas Henrique de Oliveira Souza  1 , Michele Fedel  2 , Fernando Cotting  3 , Wagner Reis da Costa Campos  1
Affiliations

In Situ Evaluation of Epoxy Self-Healing Coating by Encapsulated Linseed Oil in Poly(Urea-Formaldehyde-Melamine) Microcapsules

Lucas Henrique de Oliveira Souza et al. Materials (Basel). .

Abstract

The development of self-healing coatings represents a promising approach to enhance the durability of metal substrates exposed to corrosive environments, demanding thorough in situ investigations. In this study, poly(urea-formaldehyde-melamine) (PUF) microcapsules containing linseed oil (LO) were synthesized via in situ polymerization to act as healing agents in protective coatings. The microcapsules were characterized using scanning electron microscopy (SEM), optical microscopy (OM), Fourier-transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA). The capsules exhibited a regular spherical morphology with an average diameter of 96 µm and an LO encapsulation efficiency of 81 wt%. TGA confirmed their thermal stability up to 200 °C, while FTIR verified the successful encapsulation of LO. For performance evaluation, 10 wt% of the microcapsules was incorporated into an epoxy matrix and applied to carbon steel. Corrosion resistance was evaluated using electrochemical impedance spectroscopy (EIS) in 0.1 mol/L of NaCl solution over 500 h. The coating with microcapsules exhibited a |Z|0.01 of 106 Ω·cm2, higher than the 104 Ω·cm2 observed for the coating without microcapsules, indicating improved barrier properties. Raman spectroscopy confirmed the auto-oxidation of LO at damaged areas, evidencing the self-healing mechanism. Although full barrier recovery was not achieved, the system effectively delayed corrosion progression.

Keywords: Raman spectroscopy; corrosion protection; electrochemical impedance spectroscopy; in situ characterization; linseed oil; organic coatings; polymeric microcapsules; self-healing.

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Conflict of interest statement

The authors declare no conflicts of interest.

Figures

Figure 1
Figure 1
FTIR spectra of poly(urea–formaldehyde–melamine) (PUF) resin, linseed oil, and microcapsules containing core material.
Figure 2
Figure 2
(a) Thermogravimetric (TGA) and (b) differential thermogravimetric (DTG) of poly(urea–formaldehyde–melamine) (PUF) resin, linseed oil (LO), and microcapsules containing core material.
Figure 3
Figure 3
Histogram of the particle size distribution of poly(urea–formaldehyde–melamine) microcapsules containing the linseed oil.
Figure 4
Figure 4
Optical microscope images of microcapsules synthesized, (a) 50× and (b) 100×.
Figure 5
Figure 5
Optical microscopy images after microcapsules being pressed.
Figure 6
Figure 6
SEM images of microcapsules with rough surfaces, (a) 150× and (b) 1000×.
Figure 7
Figure 7
SEM image from cross-section of embedded (10 wt.%) microcapsules in epoxy coating, (a) 150× and (b) 200×.
Figure 8
Figure 8
EIS diagrams of the intact coatings during 1000 h in 3.5 wt% NaCl. (a,b) Pure epoxy resin coating (EP0) and (c,d) composite coating with 10 wt% microcapsules (EP10).
Figure 9
Figure 9
Nyquist plots for intact coating in the immersion of 3.5 wt% NaCl solution. (a) EP0; (b) Zoom EP0; (c) EP10; and (d) Zoom EP10.
Figure 10
Figure 10
Equivalent circuits used for fitting the EIS spectra: (a) one-time constant, (b) two-time constants, and (c) two-time constants self-healing.
Figure 11
Figure 11
EIS diagrams of the damaged coatings during 500 h in 0.1 mol/L NaCl. (a,b) Pure epoxy resin coating (EP0); (c,d) composite coating with 10 wt% microcapsules immediate immersion (EP10-I); and (e,f) composite coating with 10 wt% microcapsules five-day curing (EP10-D).
Figure 12
Figure 12
Nyquist plots for damaged coating in the immersion of 0.1 mol/L NaCl solution. (a) EP0; (b) Zoom EP0; (c) EP10-I (d) Zoom EP10-I; (e) EP10-D; and (f) Zoom EP10-D.
Figure 13
Figure 13
Immersion studies performance without microcapsules (EP0) and with microcapsules coatings (EP10) at different exposure periods.
Figure 14
Figure 14
Raman spectrum of fresh linseed oil and the scratched region.
See this image and copyright information in PMC

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