Continuous Stabilization and Carbonization of a Lignin-Cellulose Precursor to Carbon Fiber.

Abstract

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.