Role of Physicochemical Structure of Organosolv Hardwood and Herbaceous Lignins on Carbon Fiber Performance

Lignin was extracted from Alamo switchgrass ( Panicum virgatum ) and yellow polar ( Liriodendron tulipifera ) by organosolv fractionation at different pretreatment temperatures, and its chemical structure was studied by means of elemental analysis and spectroscopy. Thermal properties of lignins were investigated using thermogravimetric analysis and differential scanning calorimetry. Lignin fibers were produced via melt-spinning by a twin-screw extruder with a custom spinneret. Fibers were thermostabilized at different rates and finally carbonized. In both species, lignin obtained from higher severity organosolv fractionation had fewer impurities, higher content of phenolic hydroxyl groups, and more condensed structures as a result of extensive cleavage of aryl ether linkages. Higher organosolv severity improved the ability to spin fibers; in the case of switchgrass, only the high severity sample was spinnable. High severity also decreased thermostabilization time and increased tensile strength and modulus of carbon fibers. A decrease in the ratio of ether linkages to condensed units appears to be the main reason for faster stabilization. Switchgrass lignin had less thermal stability at low temperature that results in formation of volatiles, mainly due to the presence of ester-linked phenolic acids. These volatiles are more prevalent at low severity and prevent forming continuous fibers during spinning. As a result, pores forming on the surface of switchgrass fibers led to lower strength. Tuning the severity of fractionation is recommended as an easy method to change lignin characteristics, to find the proper severity range, and to produce lignin suitable for carbon fiber production.