Isocyanate-Free Hybrid Polyhydroxyurethane-Siloxane-Epoxy Coatings: A Sustainable Route toward High-Performance Anticorrosion Protection

Metal corrosion causes significant economic losses and safety risks, necessitating the use of effective protective coatings. Nonisocyanate polyurethanes offer a sustainable alternative to conventional PU coatings, avoiding toxic isocyanates. Incorporating siloxanes, particularly polydimethylsiloxane, into polyurethane coatings can enhance both corrosion resistance and water resistance. However, its poor adhesion can be significantly improved by combining it with urethanes. This study focuses on developing hybrid polyhydroxyurethane-siloxane-epoxy (HPHU-Sil-EP) coatings derived from cyclic carbonate-functionalized polysiloxanes. The precursors have been synthesized via the cycloaddition of epoxy-functionalized polysiloxanes with CO 2, followed by amine termination to create an amine-terminated prepolymer. The resulting prepolymer has been cured with three different commercial epoxy compounds (di-, tri-, and tetra-epoxy) to form the final HPHU-Sil-EP coatings. The chemical structure of the synthesized materials has been characterized using FTIR and 1 H NMR spectroscopy. All three formulations exhibited thermal degradation near 300 °C with single glass transition temperatures slightly above room temperature. The diepoxy formulation showed the highest tensile strength (6.6 MPa) with 29% elongation, while the tri- and tetra-epoxy systems displayed 4.9 MPa (20%) and 3.7 MPa (15%), respectively. Adhesion tests revealed lap shear strengths of 6.2 MPa (Al–Al) and 4.1 MPa (ST-ST), along with a cross-hatch rating of 5B for all systems. Coating resistance to harsh environments was evaluated by exposing the samples to acidic, alkaline, and saline media. Further, the corrosion resistance of HPHU-Sil-EP coatings was investigated using electrochemical impedance spectroscopy. The developed environmentally friendly HPHU-Sil-EP coatings have been found to show great potential for high-performance anticorrosion coating applications, which could be explored in space missions.