Crosslinked succinic acid based non-isocyanate polyurethanes for corrosion resistant protective coatings

Non-isocyanate polyurethanes (NIPUs) are considered as safer alternatives to toxic isocyanate-based polyurethanes (PUs). However, NIPU coatings are generally known for their poor water resistance due to the presence of pendant hydroxyl groups in the structure unlike traditional PUs. In this work, solvent-borne crosslinkable NIPU coating formulations were developed in a non-toxic solvent, diethylene glycol diethyl ether, and their coatings properties were evaluated. The crosslinking chemistry involved acid catalyzed transetherification of a commercially available crosslinking agent hexa(methoxymethyl) melamine (HMMM) with the NIPU hydroxyl groups. The crosslinked NIPU coatings showed good thermal stability (up to 245 °C), high pencil hardness (up to 3H), strong adhesion to metal substrates (adhesive strength up to 9.04 MPa), high resistance to water and good salt-water corrosion resistance. The formulations were also applied successfully for dip coating of copper specimens with complex and hard to reach surfaces. Finally, pigmented solvent-borne coating formulations were also demonstrated. All these results highlight the potential of NIPU resins for the application in industrial coatings substituting traditional petroleum-based polyol resins used currently.