Sustainable thermosetting coatings from urushiol and alkali lignin: Oxidative radical thermal curing behavior, structural interactions, and functional properties

Urushiol is a catechol-based natural polymer known for its self-curing ability and water resistance. Its blends with lignin have been studied mainly to enhance hydrophobicity and mechanical strength. However, the thermal curing behavior and reaction kinetics of urushiol/lignin blends remain inadequately understood. This study systematically investigates the thermal curing mechanisms and structure-property relationships of urushiol/lignin blend coatings, aiming to develop sustainable, high-performance polymer materials. Urushiol was blended with varying amounts of alkali lignin (0-75 wt%) and thermally cured at 140 °C in air for 2 h to form functional coatings. The curing kinetics were evaluated using non-isothermal differential scanning calorimetry under air atmosphere, revealing that lignin accelerated the curing reaction at lower temperatures, though excessive lignin content (≥50 wt%) hindered curing due to steric effects. Spectroscopic analyses revealed the first systematic evidence of oxidative radical polymerization between urushiol and lignin, elucidating the role of lignin's hydroxyl groups in promoting ether linkage formation and directly participating in the curing reaction of urushiol. Thermogravimetric analysis demonstrated that the thermal stability of the uncured and cured urushiol blends with 50 wt% lignin was enhanced, with decomposition temperatures reaching ~517 °C and reduced weight loss compared to neat urushiol. SEM and AFM images revealed lignin aggregation at higher loadings, which impacted surface roughness and contact angle. Shore D hardness increased with lignin content up to 50 wt%, indicating improved mechanical integrity, but plateaued at 75 wt% due to network heterogeneity. These findings suggest that urushiol/lignin blends, particularly at 25-50 wt% lignin, offer promising potential for thermally curable, bio-based coatings with enhanced performance and sustainability.