Hybrid shell microcapsules containing isophorone diisocyanate with high thermal and chemical stability for autonomous self-healing of epoxy coatings

Abstract

Silica-polyurea/polyuretane hybrid shell microcapsules (MCs) loaded with isophorone diisocyanate (IPDI) with long shelf-life and high thermal and chemical stability are prepared via emulsification followed by interfacial polymerization at the surface of oil droplets of the oil-in-water emulsion. The resultant MCs are aimed at self-healing performance in epoxy coatings. A commercially available, highly reactive polyisocyanate named tris(p-isocyanatophenyl) thiophosphate is successfully employed as shell forming agent, while triethoxyoctylsilane and hexadecyltrimethoxysilane (HDMS) are tested as “latent” active hydrogen sources. The resulting MCs display core–shell morphology, spherical shape with diameter of 5–20 μm, shell thickness ca. 1–2 μm, and an IPDI core fraction of 69 and 65 wt %, when HDMS and triethoxyoctylsilane are employed, respectively. MCs exhibit an increased thermal stability, comparing with pure IPDI, which makes them robust enough to resist the thermal cycles involved in the coating's preparation. Stability of MCs inside specific solvents and chemicals, their chemical composition and shelf-life as well as effect of MCs on the epoxy curing are evaluated by Fourier transformed infrared spectroscopy. MCs, remarkably, show excellent environment stability and a long shelf-life of more than 3.5 months. Their addition to an epoxy formulation is found to heal damaged zones in the epoxy coating, as shown by scanning electron microscopy and electrochemical impedance spectroscopy.