Influence of lignins on processing and surface features of bio-based carbon fibres from Melt-Spun Lignin/TPU precursors

Carbon fibres (CFs) are valued for their high specific strength and stiffness, making them indispensable in structural composites. Lignin-derived CFs offer a sustainable alternative to their petroleum-based counterparts, with lignin structure strongly influencing fibre properties. In this work, two lignins, a sulphur-free lignin (BL) with a purity of 98% and a Kraft lignin (KL) with 98% purity, were melt-spun with a bio-based thermoplastic polyurethane (TPU) and subsequently converted into CFs. Processability of each blend was assessed using rheological and thermal analyses, while fibre surface and mechanical properties were characterised via atomic force microscopy (AFM) and single-fibre tensile testing. The Kraft lignin (KL) blend displayed superior spinnability, attributed to its lower molecular weight/polydispersity and broader thermal softening window, whereas BL produced fibres with higher peak strength but poorer spinning stability, consistent with its higher molecular weight/heterogeneity and sharper thermal transitions. CFs derived from KL exhibited significantly rougher surfaces (59% higher roughness) but lower tensile strength than BL-based CFs. These differences were attributed to variations in lignin structure, highlighting how lignin feedstock selection can be tailored to control surface morphology, which can potentially influence fibre–matrix interactions in composite applications.