Surface-acylated lignin-rich fines as strength and functional fillers for micro/nano fibrillated cellulose

Surface acylation of softwood thermomechanical pulp (TMP) fines (<76 μm) using succinic anhydride (SA) and imidazole effectively introduced carboxyl groups to both lignin and carbohydrate components, increasing the total carboxyl content to 0.6 mmol/g and up to 1.2 mmol/g in lignin. This chemical modification facilitated fibrillation during high-pressure homogenization, producing a fine fraction with uniform microscale morphology and cellulose nanofibrils containing 12 wt% lignin (LCNF). Films produced from these SA-modified fines exhibited enhanced mechanical properties, including a tensile strength exceeding 31 MPa and a Young's modulus over 3.9 GPa. When incorporated into micro/nanofibrillated cellulose (M/NFC) matrices at 30-40 % loadings, the modified fines significantly improved the mechanical performance of M/NFC-derived films, nearly doubling tensile strength (from 36 MPa to over 65 MPa) and modulus (from 1.8 to ∼3.6 GPa), while concurrently enhancing UV-blocking performance without compromising water vapor transmission. Additionally, incorporation of SA-modified fines in M/NFC-based cryogels (from 1 wt% suspensions) increased compressive strength (from 19 to 29 kPa) and modulus (from 0.18 to 0.35 kPa), demonstrating their value as reinforcing agents in lightweight, bio-based materials. Importantly, the inclusion of these fines preserved the ultralow density and exceptionally low thermal conductivity (0.0307 W/m·K) of the cryogels, underscoring their potential in thermal insulation applications.