Functionalized Lignin Derivatives as Melt-Spinnable Precursors for Carbon Fiber Production without Stabilization

Abstract

Softwood kraft lignin, produced during industrial pulping processes, is investigated as a carbon fiber precursor with the distinct advantage of having smaller carbon footprint compared to polyacrylonitrile. Despite its advantageous high carbon content and aromatic structure for carbon fiber production, lignin presents challenges due to its brittleness and limited thermal processability. To overcome this intractability, a two-step chemical modification method is applied to softwood kraft lignin, converting nearly all available hydroxy groups into cinnamate esters. NMR analysis revealed a high-degree substitution of ester bonds, while FT-IR showed significant reduction in hydroxyl stretching. The bulky aromatic groups allowed for stable melt spinning, while adding additional flexibility to the fiber, creating 100% lignin-derivative fibers at an uptake speed of 50 m min−1. Thermal and chemical oxidation methods are compared prior to carbonization. After carbonization to 1000 °C, both stabilization methods are equally effective. Spun samples are stretched under heat to achieve a diameter of 11 µm, which significantly enhanced the mechanical properties of the resulting carbon fiber. Upon carbonization, the resulting carbon fiber exhibited superior mechanical properties compared to most lignin-based carbon fibers in the literature, reaching values over 120 GPa modulus and nearly 1 GPa strength.