Abstract Pilot-scale kraft lignin, unmodified (UKL) or tall-oil/rosin-modified (MKL), was assessed as a sustainable alternative to carbon black in nitrile–butadiene rubber (NBR). NBR composites with 30–50 phr UKL or MKL were compounded and vulcanized, and their mechanical, thermal, and viscoelastic properties were evaluated. Lignin produced a strong reinforcing effect: tensile strength more than doubled to ~14 MPa at 30–50 phr with MKL, comparable to conventional carbon-black-filled NBR, while elongation at break increased alongside modulus and hardness. MKL also improved thermal stability, shifting the oxidative decomposition onset to ~365°C (versus ~290°C for UKL), and caused a slight upward shift in T g . Contact-angle analysis showed higher surface free energy for MKL (≈62.4 versus ≈55.3 mJ/m 2 ), consistent with modified interfacial behavior, reduced lignin self-association, and improved dispersion/interfacial contact. Thus, simple chemical modification turns kraft lignin into an application-ready, bioderived filler for demanding NBR applications. Impact statement This work shows that industrial kraft lignin, a byproduct of pulp mills, can move from the boiler to the product and act as a viable, tunable alternative to carbon black in nitrile rubber. By combining pilot-scale LignoBoost™ lignin with a simple tall-oil/rosin modification, we demonstrate that lignin can deliver carbon-black-level reinforcement in nitrile–butadiene rubber while simultaneously improving thermal robustness and maintaining elasticity. Scientifically, the study links lignin structure, glass transition, surface free energy and interfacial energetics to filler dispersion and composite performance, offering a design framework that is transferrable to other polar elastomers and polymer blends. Practically, the use of pilot-scale lignin and standard sulfur vulcanization shows that integration into existing rubber compounding lines is realistic without radical process changes. At the policy and societal level, replacing a portion of fossil-derived carbon black with bioderived lignin supports the circular bioeconomy, opens higher-value product streams for pulp mills, and reduces the CO 2 footprint of seals, hoses, gaskets, and other industrial rubber components. The results point to lignin-based fillers as an immediately relevant lever for decarbonizing rubber technology. Graphic abstract Schematic overview of kraft lignin valorization into nitrile–butadiene rubber (NBR) composites: lignin isolated from hardwood black liquor is used either unmodified (UKL) or after tall-oil/rosin modification (MKL), compounded into NBR, and evaluated for interfacial compatibility and composite performance (thermal stability and mechanical reinforcement, benchmarked against carbon black).
Nadányi et al. (Tue,) studied this question.