Sustainable Dual Cross-Linking Strategy of Lignin with Oxazoline-Modified Nitrile Rubber for Mechanically Robust...

Lignin, the most abundant aromatic biopolymer in nature, represents a highly promising renewable feedstock for the development of value-added polymer composites. However, its inherently poor compatibility with elastomeric matrices limits its effective utilization in high-performance and multifunctional lignin-filled systems. In this study, we report a cost-effective and sustainable cross-linking strategy to compatibilize and chemically integrate lignin within an acrylonitrile butadiene rubber (NBR) matrix through interfacial oxazoline chemistry, without the use of conventional sulfur- or peroxide-based curing agents. A small fraction of the pendant nitrile (-CN) groups of NBR were selectively converted into oxazoline rings via reactive melt compounding. Furthermore, interfacial compatibility was enhanced by incorporating 4 phr ZnCl₂, which promotes strong metal-ligand coordination interactions between Zn²⁺ ions and oxazoline functionalities in the modified NBR. This approach results in the formation of a double cross-linked network comprising (i) permanent covalent linkages generated through oxazoline ring-opening reactions initiated by nucleophilic attack of lignin phenolic hydroxyl (-OH) groups and (ii) dynamic sacrificial networks formed via Zn²⁺-oxazoline, Zn²⁺-cyano, and Zn²⁺-phenolic hydroxyl/carboxylate coordination interactions involving residual nitrile groups of NBR and phenolic hydroxyl/Carboxylic acid groups of lignin. The presence of this dual cross-linking architecture significantly enhances mechanical performance. At 60 phr lignin loading, the oxazoline-modified NBR/ZnCl₂ composite exhibits an approximately 129% increase in tensile strength relative to the NBR/ZnCl₂ system without oxazoline modification. In contrast, the unmodified NBR/ZnCl₂ composite containing the same lignin loading shows a comparatively lower tensile strength improvement of 86%. The pronounced reinforcing effect of lignin in the oxazoline-modified system is attributed to its dual function as both a reinforcing filler and reactive cross-linking component, as well as to the synergistic contribution of permanent covalent bonds and reversible coordination interactions within the double cross-linked network.