Air-gap spinning of lignin-cellulose fibers

Doctoral thesis, 2021

Co-processing of lignin and cellulose, the two main constituents of wood, has previously been identified as a potential route for the production of inexpensive and bio-based carbon fibers. The first step in this process is to spin a precursor fiber. This can be done with different techniques, and the specific characteristics of air-gap spinning of solutions containing lignin and cellulose were investigated in this work. Studies on how the addition of lignin to a cellulose solution affect the spinnability, the coagulation process, and the fiber structure and properties were performed.

In accordance with the hypothesis, it was found that it was possible to gain advantages from both materials, by combining cellulose and lignin. Cellulose is a stiff and linear polymer that contributed to the strength of the fiber, while lignin, with its high carbon content, enhanced the final yield after conversion into carbon fiber.

Additionally, solutions that contained both lignin and cellulose could be air-gap spun at substantially higher draw ratios than pure cellulose solutions. This improvement could not be explained with the shear rheology results, however, based on measurements of the take-up force during spinning it was proposed that lignin stabilize against diameter fluctuations during spinning. To analyze how lignin affects the coagulation of lignin-cellulose fibers the total mass transport during coagulation was studied. Different coagulation baths were used, and it was found that minor parts of the lignin leached out, the amount correlated to the lignin solubility in the coagulation bath. Nevertheless, from the results it could also be concluded that the addition of lignin to a cellulose solution did not hinder the coagulation of the fibers. Regarding the fiber structure, it was possible to analyze the molecular order of cellulose and lignin separately and the lignin was found to be completely disordered also in a stretched fiber. In contrast, cellulose attained a preferred molecular orientation even in fibers with high lignin ratios. To further assess the full potential of the system, the lignin-cellulose fibers that were produced were also converted into carbon fibers, and the mechanical properties are promising. In summary, it was considered that lignin-cellulose based carbon fibers have great potential in becoming commercially available if efforts are continued in increasing the strength and stiffness of the fibers even further together with the implementation of efficient solvent recycling.