Synthesis of pH sensitive microcapsules containing ZAPP, SAPP, and 8-HQ, and evaluation of their anti-corrosion performance, and mechanical enhancement of epoxy coating

Self-healing coatings containing microcapsules have a remarkable ability to preserve metal surfaces for a long time, which is why these smart coatings have received a lot of attention. In this study, pH-sensitive poly melamine-formaldehyde microcapsules were synthesized by in-situ polymerization and loaded into the epoxy coating. 8-HQ, ZAPP, and SAPP were used as inhibitors in the core of microcapsules (named MF8HQ, MFZAPP, and MFSAPP, respectively). The microcapsules were synthesized at different stirring times and rates to obtain the optimum state (800 rpm and 2.5 h), then characterized by SEM, TEM, and FT-IR tests. Also, the mechanical properties of coatings with different weight percentages of loaded microcapsules were investigated using tensile measurement, DMTA, and Pull-off tests. The results revealed that the presence of 5 wt% of microcapsules in coatings is along with more appropriate Tg, modulus, and adhesion strength compared to more or fewer amounts. Also, EIS and salt spray tests were used to evaluate the anti-corrosion properties of the coating. The results suggested that epoxy coating containing MF8HQ showed the best anti-corrosion performance after 35 days of immersion in 3.5 wt% of NaCl solution (delamination index = 45.7 %), followed by MFZAPP (delamination index = 69.5 %) and MFSAPP performance (delamination index = 84.2 %) which were better than neat epoxy, respectively. Finally, the Quantum analysis was used to ensure the 8-HQ inhibitory performance and analyze the mechanism of inhibiting action at the interface.