Design of a versatile lignin-based synergistic anti-aging reagent for natural rubber composites

This research designed a versatile lignin-derived synergistic anti-aging reagent for natural rubber (NR) composites. Soda lignin (SL) was modified via Mannich reaction producing tryptophan-grafted lignin (TL). The molecular structure of TL was confirmed through FTIR, XPS, 2D-HSQC and elemental analysis. Thermal analysis showed an elevated glass transition temperature (186°C) for TL, indicating enhanced molecular rigidity. NR composites with varying amounts of SL and TL were analyzed for vulcanization characteristics, mechanical properties and anti-aging performance. The NR-TL series exhibited a substantial increase in the vulcanization torque and hardness compared to the NR-SL series, reflecting a superior stiffening effect of TL. The NR-TL series demonstrated remarkable anti-aging performance, with NR-TL10.0 (containing 10.0 phr of TL) achieving improvements of 63.7% in tensile strength (after thermal-aging) and 86.4% in the anti-aging coefficient compared to the NR-SL counterpart. These enhancements are attributed to improved dispersion of TL and enhanced radical scavenging efficiency. The anti-aging mechanism of TL was clarified, pointing to an intramolecular synergism with a favorable radical transfer route from aminyl to phenoxy radical. This work presents a facile approach to prepare a novel lignin-based multifunctional reagent with excellent anti-aging effectiveness, significant mechanical reinforcement, and high vulcanization efficiency for rubber materials.