Toward Sustainable Rubber Composites: Black Liquor‐Derived Lignin as a Hybrid Filler in NBR

ABSTRACT This study explores the potential of lignin isolated from black liquor, a waste byproduct of the paper industry, as a sustainable partial replacement for carbon black (CB) in nitrile butadiene rubber (NBR) composites. Fourier transform infrared (FTIR) and nuclear magnetic resonance spectroscopic analyses confirmed the presence of characteristic lignin functional groups, supporting its suitability as a reinforcing filler. Extracted lignin, after ball milling, was incorporated into NBR composites through an industrially viable dry compounding process, with the total filler loading maintained at 50 parts per hundred rubber (phr). The mechanical properties of the composites were evaluated by systematically varying the lignin‐to‐CB ratio. All formulations exhibited curing times under 4 min, aligning well with industrial requirements. Although tensile strength declined with increasing lignin content, values above 7 MPa were retained even at 80% CB replacement. Hardness showed minimal reduction, indicating lignin's potential to maintain stiffness in the rubber matrix. Morphological analysis revealed uniform filler dispersion, attributed to the inherent polarity and good compatibility of lignin with NBR. These hybrid composites demonstrate moderate tensile strength and excellent hardness, making them promising candidates for oil‐resistant rubber products such as O‐rings, hydraulic seals, and hoses. Moreover, utilizing lignin from black liquor offers a dual benefit‐reducing industrial waste and promoting sustainable material development.