Functionalizing lignin with introducing benzene rings and adjusting intramolecular polarity for improving bituminous photooxidative aging resistance

Lignin (LG) is utilized to strengthen the photooxidative aging resistance of bitumen due to its ability to absorb ultraviolet (UV) radiation and scavenge free radicals. However, the anti-aging performance of LG is constrained by a dilemma: at low dosages, the functional structures are insufficient; while at high dosages, uneven dispersion occurs within the bituminous matrix. Herein, we demonstrate a strategy to functionalize LG for improving bituminous photooxidative aging resistance through synergistically introducing benzene rings and adjusting intramolecular polarity. Specifically, 4,4′-diphenylmethane diisocyanate (MDI) functionalized LG (MFLG) was synthesized via covalent bonding between hydroxyl and isocyanate groups, and its structural properties were analyzed using multiple characterization techniques. The results revealed that the introduction of benzene rings enhanced the particle size, hydrophobic, thermal stability, and UV absorption capacity of LG. Meanwhile, bitumen modified with MFLG (MFLG-MB) demonstrated superior resistance to photooxidative aging compared to LG modified bitumen (LG-MB) and pristine bitumen (PB). Furthermore, molecular simulations (MS) elucidated the anti-aging mechanism. The incorporation of MDI improved the dispersion of LG within the bituminous matrix, as evidenced by binding energy calculations, which was beneficial for anti-aging protection of bitumen. Snapshots, electrostatic potential, and dipole moment values further demonstrated that MFLG exhibited stronger intramolecular polarity and higher affinity for asphaltenes compared to LG. These properties underscored MFLG's critical role in inhibiting asphaltene gelation for enhancing the photooxidative aging resistance significantly. This finding highlights a novel strategy to improve the anti-aging effectiveness of LG in bitumen by increasing aromatic groups and optimizing intramolecular polarity.