Microcapsules containing dehydrated castor oil as self-healing agent for smart anticorrosive coatings

In this work, novel poly(urea-formaldehyde-melamine) microcapsules containing dehydrated castor oil (DCO) were synthesized by in situ emulsion polymerization to promote self-healing for epoxy coatings. DCO is a green and low-cost drying oil with intermediate drying between linseed oil and tung oil, providing self-healing through oxidative polymerization. Scanning electron microscopy (SEM) showed microcapsules with a spherical and smooth morphology with a shell thickness around 500 nm. Microcapsules obtained a mean diameter of 24 μm by laser diffraction. The core content and the encapsulation yield were 86 wt% and 81 %, respectively, as determined by Soxhlet extraction. Microcapsules were doped at 15 wt% loading to a high solids epoxy mastic primer. Abrasive blasted carbon steel substrates were coated with the epoxy primers (with or without microcapsules) and a polyurethane topcoat. Despite decreasing the pull-off force, adhesion of coatings containing microcapsules was still high and above 10 MPa and failure mode was cohesive in the primer. Electrochemical impedance spectroscopy (EIS) results of coatings without defect, after 1 year of immersion in 0.1 mol/L NaCl solution, showed that microcapsules preserved original coating barrier properties. Coatings doped with microcapsules containing an artificial defect showed higher impedance values by EIS and lower electrochemical activity by scanning vibrating electrode technique (SVET), pointing out the self-healing protection provided by dehydrated castor oil film by undergoing oxidative polymerization. Self-healing performance was confirmed after 4200 h of ISO 12944-9 cyclic corrosion test for offshore structures, where microcapsules reduced 30 % of the corrosion around the scribe, bridging the gap between the development of novel smart particles and their long-term performance in self-healing coatings. This work demonstrates that microcapsules containing dehydrated castor oil can provide long-term self-healing protection to epoxy coatings even under very corrosive atmospheres.