Renewable resources, such as kraft lignin, have shown great potential as precursors for carbon fiber production. This manuscript reports an investigation into the stabilization of softwood kraft lignin (SKL) fibers and the determination of the difference in stabilization between hardwood‐ and softwood‐based kraft lignin fibers. The stabilization was achieved either thermally by using only heat or oxidatively in the presence of air, at various heating rates. A heating rate of 4°C min−1 and a holding time of 30 min at 250°C were successfully used for the thermal stabilization experiments. Faster stabilization was achieved using oxidative conditions at a heating rate of 15°C min−1 and 30 min holding time at 250°C. Furthermore, stabilization and carbonization in a one‐step process was performed on SKL fibers, which show great potential to reduce both production time and costs. The stabilized and carbonized fibers were evaluated using thermal, spectroscopic, and microscopic methods. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Kraft lignin obtained from the pulping of wood is an interesting new precursor material for carbon fiber production because of its high carbon content and ready availability. However, continuous spinning of softwood kraft lignin (SKL) has been impossible because of its insufficient softening characteristics and neat hardwood kraft lignin (HKL) has required extensive pretreatments to enable fiber formation. Softwood kraft lignin permeate (SKLP) and hardwood kraft lignin permeate (HKLP), fractionated by membrane filtration, were continuously melt spun into fibers. To improve the spinnability of SKL and HKL, HKLP was added as a softening agent. SKLand HKL-based fibers were obtained by adding 3-98 wt % HKLP. A suitable temperature range for spinning was 20-85 C above the T g of the lignin samples, and this range gave a flawless appearance according to the SEM analysis. Smooth, homogeneous fibers of SKLP, HKLP, and SKL with HKLP were successfully processed into solid carbon fibers.