Sustainable bio-based isocyanate-free poly (urethane amic acid) coatings for the corrosion protection of mild steel

Abstract

In pursuit of sustainable and environmentally benign coating technologies, we report the synthesis and evaluation of isocyanate-free/non-isocyanate poly (urethane amic acid) (NIPUAA) coatings developed from a renewable source, castor oil. NIPUAA was prepared using a step-growth polymerization strategy using functionalized castor oil (carbonated castor oil, CCO), isophorone diamine and pyromellitic dianhydride, eliminating the need for toxic isocyanates. The chemical structure of the synthesized NIPUAA was confirmed by FTIR and NMR spectroscopy, indicating the successful formation of urethane and amic acid linkages. The thermal transitions and stability were analyzed using DSC and TGA. The results showed a moderate level of thermal endurance, suitable for coating applications. The NIPUAA coatings were coated on mild steel substrates and subjected to a thorough performance evaluation, including Tafel polarisation, electrochemical impedance spectroscopy (EIS), and salt spray testing for corrosion resistance. Pencil hardness, abrasion resistance, adhesion test, and cone mandrel flexibility tests were used to assess mechanical durability. The coatings demonstrated competitive performance in comparison to conventional systems, with satisfactory corrosion inhibition and mechanical integrity. These findings demonstrate the potential of NIPUAA derived from castor oil as a promising candidate for isocyanate-free protective coatings, providing a more environmentally friendly option for multipurpose metal surface protection.