Carbon Fibers from Wet-Spun Cellulose-Lignin Precursors Using the Cold Alkali Process

Abstract

In recent years, there has been extensive research into the development of cheaper and more sustainable carbon fiber (CF) precursors, and air-gap-spun cellulose-lignin precursors have gained considerable attention where ionic liquids have been used for the co-dissolution of cellulose and lignin. However, ionic liquids are expensive and difficult to recycle. In the present work, an aqueous solvent system, cold alkali, was used to prepare cellulose-lignin CF precursors by wet spinning solutions containing co-dissolved dissolving-grade kraft pulp and softwood kraft lignin. Precursors containing up to 30 wt% lignin were successfully spun using two different coagulation bath compositions, where one of them introduced a flame retardant into the precursor to increase the CF conversion yield. The precursors were converted to CFs via batchwise and continuous conversion. The precursor and conversion conditions had a significant effect on the conversion yield (12–44 wt%), the Young’s modulus (33–77 GPa), and the tensile strength (0.48–1.17 GPa), while the precursor morphology was preserved. Structural characterization of the precursors and CFs showed that a more oriented and crystalline precursor gave a more ordered CF structure with higher tensile properties. The continuous conversion trials highlighted the importance of tension control to increase the mechanical properties of the CFs.

Boric Acid as A Low-Temperature Graphitization Aid and Its Impact on Structure and Properties of Cellulose-Based Carbon Fibers

“…Almost all CFs used in these applications are made from either polyacrylonitrile copolymers (PAN) or mesophase pitch which together account for a market share of between 96 and 98% [3]. However, CFs based on natural resources such as cellulose have recently attracted much attention due to their high potential for having a significantly lower carbon footprint and lower possible costs [3][4][5].…”

“…Moreover, the progressive exothermic cyclization reactions occurring in stabilization carry the risk of accidents during production [1]. Therefore, cellulose-based precursors, as an alternative, came into focus again [4]. Nevertheless, the drawbacks of cellulose-based CFs like poor yield and the need for stretch-graphitization still need to be overcome.…”

“…Moreover, suitable additives like Lewis acids and flame-retardants, e.g., ammonium tosylate, ammonium dihydrogen phosphate [3,9,15], diammonium hydrogen phosphate, phosphoric acid, ammonium sulfate, boric acid (BA) [9,13,16,17], or carbon-rich fillers, such as carbon black [12] and lignin [18][19][20][21], were tested. The latter cellulose-lignin precursors show outstanding potential due to their excellent spinnability at high lignin levels [4,22,23] and a high mass yield after thermal conversion of up to 44 wt% [4,[24][25][26][27][28].…”