Functionalized Lignin Nanoparticles for Sustainable Rubber Compounds

The transition toward sustainable tire manufacturing requires renewable alternatives to fossil-derived reinforcing fillers, such as carbon black (CB). Lignin is a promising biobased candidate; however, its application in rubber is limited by poor dispersion and weak interfacial compatibility. In this work, lignin nanoparticles (LNPs) with tunable size were prepared via a solvent-shifting process and incorporated into natural rubber (NR) using an optimized latex-based predispersion strategy. The nanostructured lignin exhibited uniform morphology and enabled improved dispersion and filler–matrix interfacial area compared to bulk lignin, resulting in enhanced mechanical reinforcement. To further promote interfacial interactions, LNPs were surface-functionalized with cystamine, a bioderived diamine containing a disulfide moiety active during vulcanization. Surface modification was confirmed by spectroscopic and elemental analyses, and the curing curve revealed accelerated vulcanization. Rubber compounds in which CB was partially replaced by cystamine-functionalized LNPs displayed tensile and dynamic mechanical properties comparable to carbon-black-filled references while maintaining low hysteresis. This study demonstrates that combining lignin nanostructuring with targeted surface functionalization enables effective partial substitution of CB, offering a scalable pathway toward more sustainable rubber reinforcement systems.