Self-Healing Coatings Containing Core–Shell Nanofibers with pH-Responsive Performance

The micro-nanofibers prepared by the electrospinning technique can be used as a good container for loading healing agents. The core-shell electrospun nanofibers with polyacrylonitrile as the outer shell and tannic acid (TA) and tung oil as the core healing agents were synthesized by a coaxial electrospinning method and exhibited pH-sensitive ability. The nanofibers as additives were added to an epoxy resin coating as a self-healing coating. The morphological stability of the electrospun nanofibers were observed by a scanning electron microscope and a transmission electron microscope. Fourier transform infrared spectroscopy and fluorescence microscopy reveal that the successful synthesis and uniform distribution of core-shell fibers. The mechanical properties test revealed that the tensile properties of the coating could be improved by adding nanofibers. The infrared mapping test, energy-dispersive spectrometry, and X-ray photoelectron spectroscopy, which were carried out on the scratched part of the coating, proved the release of the healing agent in the damaged part. TA forms a protective film on the exposed metal surface through molecular adsorption under acidic conditions. Meanwhile, the curing of tung oil can effectively compensate into the microcracks to form a TA protective film, which could improve the self-healing performance. As the tung oil dries and solidifies in the alkaline solution, the cross-linking effect of the molecules is combined to form a tight film and strength the self-healing ability. TA as an acidic healing agent and tung oil as an alkaline healing agent played the role of pH-sensitive products in healing the cracked coating. The self-healing rates of coating immersing in 3.5 wt % acidic NaCl solution and alkaline solution were 81.6 and 71.2%, respectively. The composite coating shows a great pH-sensitive self-healing ability to heal the cracked coating.