Valorization of Agricultural Waste into Borate-Modified Cardanol for Recyclable Thiol−Ene Thermoset Coatings

Cardanol (CD), derived from cashew nut shell liquid (CNSL), is a promising bio-based feedstock for sustainable polymer development due to its renewable origin, cost-effectiveness, and inherent functional versatility. In this study, a borate ester-functionalized cardanol, tris(3-((8Z,11Z)-pentadeca-8,11-dien-1-yl)-phenyl) boratetermed tricardanyl borate (TCB), was developed as a recyclable crosslinker for thiol−ene photo-curable thermoset coatings. Unlike conventional cardanol modifications that require harsh conditions and halogen substituents, TCB was synthesized via a solvent-free transesterification reaction, preserving the native unsaturation and functionality of cardanol. The blocked phenolic hydroxyl groups in TCB eliminated radical-quenching behavior, enabling efficient thiol−ene click polymerization. Thermosets prepared with varying thiol/ene ratios (BT-25, BT-50, BT-100) were compared to reference networks based on unmodified cardanol (CT-25, CT-50, CT-100). Thermal analysis revealed that TCB-based networks exhibited higher glass transition temperatures and char yields, attributed to increased crosslinking density and boron oxide formation. Anti-corrosion performance was evaluated via salt spray testing, electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization (PDP). BT-50, formulated with an equimolar thiol/ene ratio, exhibited the highest low-frequency impedance (2.9 MΩ cm2), indicating superior barrier performance on galvanized steel. Recyclability was demonstrated by alkaline degradation of BT-50, yielding thioether-functionalized cardanol and free phenolic groups, as confirmed by 1H NMR. These results highlight the multifunctional role of TCB in enabling robust, photo-curable, and recyclable thermosets, reinforcing its potential in sustainable material design aligned with green chemistry and circular economy principles.