Continuous Conversion of Cellulose and Lignin-Cellulose Precursors to Carbon Fibers: The Importance of Stretching during Low-Temperature Carbonization

Cellulose-lignin-based fibrous carbon materials have received significant attention as sustainable alternatives to petroleum-derived carbon fibers. While prior studies have mainly focused on spinning parameters and thermal processing to enhance mechanical performance, limited research has addressed the influence of stretching these biocomponent filaments during carbonization on molecular alignment, crystalline structure, and resulting mechanical properties. In this study, continuous cellulose-lignin mixed filaments containing 0, 30, and 50% lignin were produced using a dry-jet wet spinning method. The filaments underwent tailored stabilization and carbonization using a lab-scale continuous line, with varying draw ratios (DR = 1–1.5) applied during carbonization at 850 °C. The effects of stretching and lignin content on the carbon fibers’ properties were evaluated through tensile testing, wide-angle X-ray scattering (WAXS), and Raman spectroscopy. Lignin-containing filaments demonstrated improved stretchability compared to pure cellulose fibers. Especially, carbon fibers derived from precursor fibers with 50 wt % lignin showed a 25% increase in Young’s modulus under DR = 1.5 (49 GPa) relative to DR = 1 (41 GPa). After high-temperature carbonization at 1100 °C, the modulus further increased to 58 GPa compared to 46 GPa for unstretched fibers, corresponding to an enhanced molecular orientation factor (0.6 vs 0.5) confirmed by WAXS. This approach provides a promising route toward higher performance of bioderived carbon fibers.