Reinforcement and Rheology of Epoxy‐Mediated Lignin Integration in Carboxylated Nitrile Rubber

Lignin has attracted increasing interest as a renewable filler for rubber composites; however, it shows weaker reinforcing efficiency than conventional fillers such as carbon black and silica, primarily due to its limited compatibility with elastomers. Here, the dispersion and reinforcement of lignin in carboxylated nitrile butadiene rubber (XNBR) were investigated using two curing systems: trimethylolpropane triglycidyl ether (TMPTE) and dicumyl peroxide (DCP). TMPTE effectively reacted with XNBR during compounding and simultaneously promoted interfacial interactions between lignin and XNBR, as evidenced by FTIR analysis showing carbonyl band shifts associated with epoxide ring opening and ester formation. At a lignin loading of 30 phr, both curing systems achieved comparable tensile strength, whereas the epoxy‐cured system exhibited higher modulus at 100% strain, indicating enhanced stiffness and reinforcement. Fourier‐transform rheology further revealed that increasing the lignin loading up to 30 phr in epoxy‐cured XNBR reduced the nonlinear viscoelastic response and improved the network continuity of the composites compared with their DCP‐cured counterparts. These findings suggest the formation of a dual‐network structure in which epoxide groups facilitate interactions between lignin and XNBR, thereby enhancing lignin dispersion and reinforcement. The epoxy‐mediated lignin/XNBR system provides a high‐performance, accelerator‐free, and more sustainable alternative to conventional peroxide crosslinking in rubber processing.