LAUSR

The demand for carbon fibers (CFs) based on renewable raw materials as the reinforcing fiber in composites for lightweight applications is growing. Lignin–cellulose precursor fibers (PFs) are a promising alternative, but so far, there is limited knowledge of how to continuously convert these PFs under industrial-like conditions into CFs. Continuous conversion is vital for the industrial production of CFs. In this work, we have compared the continuous conversion of lignin–cellulose PFs (50 wt % softwood kraft lignin and 50 wt % dissolving-grade kraft pulp) with batchwise conversion. The PFs were successfully stabilized and carbonized continuously over a total time of 1.0–1.5 h, comparable to the industrial production of CFs from polyacrylonitrile. CFs derived continuously at 1000 °C with a relative stretch of −10% (fiber contraction) had a conversion yield of 29 wt %, a diameter of 12–15 μm, a Young’s modulus of 46–51 GPa, and a tensile strength of 710–920 MPa. In comparison, CFs obtained at 1000 °C via batchwise conversion (12–15 μm diameter) with a relative stretch of 0% and a conversion time of 7 h (due to the low heating and cooling rates) had a higher conversion yield of 34 wt %, a higher Young’s modulus (63–67 GPa) but a similar tensile strength (800–920 MPa). This suggests that the Young’s modulus can be improved by the optimization of the fiber tension, residence time, and temperature profile during continuous conversion, while a higher tensile strength can be achieved by reducing the fiber diameter as it minimizes the risk of critical defects.