Lignin-containing cellulose nanofibrils for paper reinforcement with exceptional strength and minimal bulk sacrifice

• LCNF with tunable lignin contents were prepared from pulp rejects. • LCNF with 3.8% lignin content obviously enhances paper strength while minimizing bulk loss. • Addition of LCNF breaks the traditional trade-off between strength and bulk in paper. • The preparation of LCNF enables high value-added utilization of pulp rejects. Driven by the dual demands of sustainability and functionality, the pulp and paper industry requires innovative bio-based reinforcements that simultaneously offer high efficiency, structural integrity, and environmental compatibility. This study addresses this need by valorizing high-yield pulp rejects into lignin-containing cellulose nanofibrils (LCNF) via an integrated process of NaClO 2 delignification, TEMPO-mediated oxidation followed by high-pressure homogenization. Lignin content critically regulates LCNF properties: reduced lignin raises crystallinity and narrows fibril dimensions but lowers thermal stability. As a paper reinforcement, LCNF with 3.8% lignin content at a 5.0% loading achieved an optimal balance, dramatically enhancing the strength of both corrugated containerboard (OCC) and printing & writing paper while minimizing bulk loss. Specifically, tensile index improvements of up to 263% were accompanied by bulk reductions of only 3–13%, successfully decoupling the traditional strength–bulk trade-off. This synergy is mechanistically explained by lignin’s dual function: its steric hindrance modulates inter-fibrillar hydrogen bonding, while its rigid framework resists structural compaction. This LCNF presents an economically promising strategy that overcomes the common trade-off between strength and bulk, highlighting its considerable application potential for papermaking.