• Fully bio-based epoxy coatings developed from renewable soybean oil and tannins. • Strong adhesion and barrier performance achieved on steel substrates. • Effective corrosion protection obtained for steel in aggressive saline environments. • Petroleum-based resins replaced with sustainable and non-toxic resources. • An eco-friendly pathway shown for high-performance protective epoxy coatings. The development of bio-based epoxy thermosets with durable anticorrosion performance is essential for replacing petroleum-derived systems in protective applications. In this work, fully bio-based epoxy coatings were formulated by curing epoxidized soybean oil (ESO) with hydrolysable tannins. A plant-derived hydrolysable tannin (HT) was systematically investigated to evaluate the influence of tannin source and formulation on curing behavior, network structure, and corrosion protection performance, using commercial tannic acid (TA) as a reference. Differential scanning calorimetry (DSC) and ATR-FTIR confirmed efficient epoxy ring-opening reactions and progressive network formation across varying hydroxyl-to-epoxide molar ratios. The resulting ESO-HT coatings exhibited thermal stability up to 260 °C, and limited water uptake (<3% after 7 days), consistent with their moderately hydrophobic surface character (water contact angle ∼ 90 °). Anticorrosive performance, assessed via salt spray exposure test and electrochemical impedance spectroscopy (EIS) in 3.5 wt% NaCl solution, revealed durable barrier protection, with high impedance and coating resistance maintained over 70 days of immersion. Compared with the TA-based system, ESO-HT coatings exhibited slightly reduced mechanical performance but improved long-term barrier stability. These results demonstrate that tannin chemistry and source play a key role in governing network formation and corrosion resistance. This work provides a practical strategy to design fully bio-based epoxy coatings with balanced thermal, mechanical, and anticorrosive properties, supporting their application as sustainable alternatives to conventional epoxy systems.
Qi et al. (2026) studied this question.