Tailoring structure–property relationships of fractionated lignin-containing poly(urethane-methacrylate) coatings

Abstract This study presents a sustainable valorization strategy for Lignoboost kraft lignin by fractionating it into low-molecular-weight (LW) and high-molecular-weight (HW) fractions, alongside unfractionated softwood kraft lignin (SK). These lignins were incorporated at 0.1–1.0 wt% into UV-curable oligo (urethane-methacrylate) (OUM) protective coatings. The OUM resin was synthesized from isophorone diisocyanate, tolylene diisocyanate, poly (oxypropylene) diol, hexane-1,6-diol, and end-capped with 2-hydroxyethyl methacrylate, with 2-ethylhexyl acrylate as reactive diluent. Coatings were prepared by blending the OUM with diurethane dimethacrylate (DUDM) as comonomer, adding lignin fractions, and curing under UV light. Fractionation significantly improved lignin compatibility with the urethane-methacrylate matrix. LW lignin fraction exhibited the best overall performance. At 0.5 wt% LW lignin, the coatings achieved the highest tensile strength (2.27 MPa), maximum elongation at break (63.2 %), and balanced Young’s modulus (2.85 MPa), while maintaining good thermal stability ( T max = 394 °C) and markedly improved surface homogeneity (Ra = 166.6 nm at 1 wt%) compared to unfractionated SK lignin. In contrast, unfractionated SK lignin provided the highest stiffness at low loading (Young’s modulus 3.98 MPa at 0.2 wt%) but caused greater surface heterogeneity, more voids, and the largest reduction in Shore hardness. HW lignin showed intermediate behavior. The results demonstrate that solvent fractionation, particularly toward lower molecular weight, enables precise tuning of thermal stability, surface topography, mechanical strength, elasticity, and hardness of bio-based UV-curable coatings. This approach transforms an abundant industrial by-product into a high-value additive, substantially increasing the renewable content of high-performance protective coatings while delivering superior structure–property control.