Role of sulfuric acid in thermostabilization and carbonization of lyocell fibers

Abstract

Carbonized fibers with quantifiable mechanical properties were prepared from environmentally benign lyocell fibers (LFs) through sulfuric acid-catalyzed continuous thermostabilization and following carbonization. The pristine LF was decomposed into small molecules which can be evaporated at high temperatures during thermal treatment, leading to a low mass yield (similar to 12 wt%) after carbonization. In contrast, the impregnated LF with sulfuric acid (ILF) underwent various chemical reactions at low temperatures, catalyzed in the presence of protons and water within the fiber. These catalyzed reactions encompassed not only thermal dissociation of glycosidic bonds, but also elimination reactions, the dehydration of decomposed products to 5-hydroxymethylfurfural, and its subsequent conversion. A series of chemical analyses revealed that linear cellulosic macromolecular chains were transformed into highly crosslinked network structures consisting of decomposed products. As a consequence of this chemical transformation, the mass yield of ILF increased by more than double compared to that of LF. In addition, the density of the carbonized ILF was higher than that of the carbonized LF, indicating that impregnation with sulfuric acid contributed to the development of carbon structures. Unlike the highly brittle carbonized LFs, the carbonized fibers derived from the thermostabilized ILFs exhibited superior tensile strength and modulus with values of 0.92 GPa and 41 GPa, respectively.