Biobased Polyurethane Coatings for Corrosion Protection of Carbon Steel ^†

Polyurethane (PU) coatings are becoming increasingly important due to their unique properties, including excellent mechanical resistance, high polymer healing ability, good adhesion to steel and adequate surface tolerance. Despite these advantages, commercial PU formulations often do not comply with “green chemistry” requirements. This study explores the full replacement of fossil-based isocyanates and polyols in PU coating formulations by their bio-derived counterparts for the development of bio-PU coatings, which contribute to a green, circular, bio-based economy.

A commercially available biobased pentamethylene diisocyanate trimer was used as the isocyanate source, the hardener component, while a biopolyol was produced by a process of liquefaction of lignocellulosic biomass, namely pinewood shaves and Stipa Tenacissima, a species of grass endemic to Portugal. The liquefaction process was conducted from the biomass together with an organic solvent (2-ethylhexanol) or bio-solvent (propylene glycol) and an acidic catalyst at mild temperatures and atmospheric pressures. A biomass conversion of ca. 78% m/m was achieved. The resulting biopolyols present a good number of hydroxyl functional groups, which makes them a good candidate for bio-PU formulations. They were introduced into one component PU formulations, fully replacing petroleum-derived polyols in the formulation.

The bio-PU coatings on carbon steel were prepared with varying molar ratios (NCO/OH), solvent combinations and dilutions. Coating thickness, interface coating steel, and the presence of voids and defects were assessed by SEM, while their chemical structure and thermal stability were studied by FTIR-ATR and TGA, respectively. Surface hydrophobicity, water uptake, solvent resistance, and adhesion were also tested. The barrier properties of the coating applied to steel coupons were studied by electrochemical impedance spectroscopy (EIS), revealing good, long-lasting barrier properties for corrosion prevention, suggesting a possible self-healing ability. Preliminary localized electrochemical tests were performed to confirm self-healing.

These coatings will serve as a matrix for the testing of different anti-corrosion additives and corrosion-sensing species.