Influence of the carbonization temperature on the properties of carbon fibers based on technical softwood kraft lignin blends

Abstract

Lignin is an interesting renewable resource for carbon fibers. In this work we investigate the usage of unpurified softwood kraft lignin Indulin AT for melt-spinning of lignin-fibers and the properties of resulting lignin and carbon fibers. Indulin AT is blended with 10, 15 and 20 wt% polyethylene glycol (PEG) and melt-spun on a pilot plant. The obtained as-spun lignin fibers are stabilized in air and afterwards transformed to carbon fibers at carbonization temperatures of 900 °C, 1500 °C, 2000 °C and 2300 °C. Along the process chain the fibers are characterized by mechanical testing, scanning electron microscopy, X-ray diffraction (XRD) and Raman spectroscopy to provide insight into the structure and quality of the lignin and carbon fibers. Additionally, analytical studies show strong intermolecular interactions between Indulin AT and polyethylene glycol. Despite the high ash content the resulting carbon fibers exhibit promising mechanical properties. XRD and Raman show the increase in ordered carbon structures with an increasing carbonization temperature. Blends of unpurified Indulin AT and PEG are successfully used for melt-spinning of lignin fibers and production of low-cost carbon fibers thereof.