High‐Performance Polyurethane Coatings From Cardanol‐Derived Polyols: A Sustainable Approach for Mild Steel Protection

ABSTRACT This paper presents a study of the ring‐opening process for production of polyols, including diols (two hydroxyl groups), triols (three hydroxyl groups), and tetra‐ols (four hydroxyl groups). These cardanol‐based polyols were subsequently used as important precursors in the formulation of PU coatings, which were cured on mild steel substrates using a polyisocyanate hardener. The synthesized PU coatings had exceptional mechanical qualities, including maximum pencil hardness (6H), high flexibility with minimal breaking during conical mandrel bending, and superb impact resistance. The coatings were also highly chemically resistant, withstanding immersion in acid (0.5 N HCl), alkali (0.5 N NaOH), and organic solvents (xylene, MEK) without degradation. Notably, the PU coating made from tetra‐ol polyol had the maximum thermal stability ( T 50% = 445°C), gel content (96%), and water resistance (1.4% absorption). This enhanced performance is due to the increased cross‐linking density caused by the four reactive hydroxyl groups of tetra‐ol. The salt spray analysis demonstrated that the tetra‐ol‐based PU coating outperformed its diol and triol counterparts, with negligible pitting and blistering after 500 h of exposure to a 3.5% NaCl solution. This indicates its superior barrier qualities and tolerance to corrosive conditions.