Aminomethylated lignin as multifunctional bio-based reinforcer for natural rubber with enhanced mechanical strength, steel cord adhesion, and antiaging performance

The activity of lignin, a sustainable reinforcing filler for rubber composites, is often hindered by its inherent hydrophilicity, and poor compatibility and functionality. This study addresses these limitations by developing a multifunctional bio-based reinforcer by grafting aminomethyl groups onto lignin via a hexamethylenetetramine-mediated Mannich reaction. Comprehensive characterization confirmed the successful synthesis of lignin@hexamethylenetetramine (HMT), which exhibited enhanced hydrophobicity, thermal stability, and radical-scavenging capability. Lignin@HMT incorporated into natural rubber (NR) imparted pronounced multifunctional performance, simultaneously acting as a reinforcing agent, adhesion promoter, and antioxidant. Compared with pristine NR, the NR–lignin@HMT composites exhibited an 83.8% increase in stress under 100% tensile strain, 20.7% enhancement in the steel cord adhesion force, and 87.0% improvement in the antiaging coefficient. Density functional theory calculations revealed that the introduction of aminomethyl groups strengthened interfacial interactions with the rubber matrix while effectively quenching free radicals, elucidating the synergistic mechanisms underlying the enhanced mechanical reinforcement and thermo-oxidative aging resistance. This study provides a viable strategy for transforming lignin into a high-value, multifunctional reinforcer for sustainable and high-performance rubber composites, particularly for tire-related applications.