The influence of the healing agent characteristics on the healing performance of epoxy coatings: assessment of the repair process by EIS technique

The effects of the healing agents' molecular characteristics were studied on the self-healing performance of the epoxy coatings via corrosion evaluation techniques. Methylene diphenyl diisocyanate (kept constant) and different polyetheramine healing agents were encapsulated separately in poly(styrene-co-acrylonitrile) through the electrospray method and added to the epoxy matrix to prepare a polyurea-based dual capsule extrinsic healing system. Commercial grades of polyetheramine, Jeffamine D230, Jeffamine D400, and Jeffamine T403, were used to study the effects of molecular weight and functionality. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images revealed the formation of spherical shape with multicore morphology for the prepared polyetheramine containing microcapsules (MCs). Successful encapsulation was evaluated by Fourier transform infrared spectroscopy (FTIR), while the encapsulation yield was measured by thermogravimetric analysis (TGA). Electrochemical Impedance Spectroscopy (EIS) was employed to monitor the corrosion behaviour of a series of coated carbon steel samples through the evolution of the impedance spectra, and the numerical values of the related electrical equivalent circuit components (e.g., corrosion resistance), of the scratched coatings at different exposure times in a near-neutral 3.5 wt% NaCl solution. The results revealed the adverse effect on the corrosion protection ability by increasing the healing agent's molecular weight, while an increase of its functionality improved the final healing efficiency of the coating. According to the EIS results, the maximum healing efficiency was determined to be 85%, 72%, and 90% for Jeffamine D230, Jeffamine D400, and Jeffamine T403, respectively.