Abstract
In this paper, we report on the use of amorphous lignin, a waste by-product of the paper industry, for the production of high performance carbon fibers (CF) as precursor with improved thermal stability and thermo-mechanical properties. The precursor was prepared by blending of lignin with polyacrylonitrile (PAN), which was previously dissolved in an ionic liquid. The fibers thus produced offered very high thermal stability as compared with the fiber consisting of pure PAN. The molecular compatibility, miscibility, and thermal stability of the system were studied by means of shear rheological measurements. The achieved mechanical properties were found to be related to the temperature-dependent relaxation time (consistence parameter) of the spinning dope and the diffusion kinetics of the ionic liquids from the fibers into the coagulation bath. Furthermore, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and dynamic mechanical tests (DMA) were utilized to understand in-depth the thermal and the stabilization kinetics of the developed fibers and the impact of lignin on the stabilization process of the fibers. Low molecular weight lignin increased the thermally induced physical shrinkage, suggesting disturbing effects on the semi-crystalline domains of the PAN matrix, and suppressed the chemically induced shrinkage of the fibers. The knowledge gained throughout the present paper allows summarizing a novel avenue to develop lignin-based CF designed with adjusted thermal stability.
Highlights
Carbon fiber (CF) reinforced thermoplastics and thermosets combine high stiffness and high strengths with low density, enabling them as light-weight construction materials for space and aviation sectors, wind energy, and automotive industry.The high tensile strength and high tensile modulus of carbon fibers (CF) are owing to the unique preferably along with the fiber axis oriented turbostratic or graphite-like carbonaceous crystal structure
In order to gain CF based on lignin/PAN in combination, main attention should be paid to the adjustment of the thermal stability of the starting material, as well as the ordering and orientation of the lignin oligomers alongside the fiber axis
Softwood Kraft lignin was selected, as Softwood Kraft lignin is industrially available as lignin-type and, upon the thermal conversion process, shows lower void formation tendency, as the content of the methoxy groups is lower than the Hardwood organosolv lignin
Summary
Carbon fiber (CF) reinforced thermoplastics and thermosets combine high stiffness and high strengths with low density, enabling them as light-weight construction materials for space and aviation sectors, wind energy, and automotive industry. The high content of hydroxyl groups and the low molecular weight (as a molecular origin) increase the agglomeration tendency of the lignin fractions even in strong solvents, that is, in the water-based binary solvent system with N, N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), or dimethylacetamide (DMAc) This leads in the wet-spinning process with polyacrylonitrile to leaching phenomena in the coagulation bath [11]. Softwood lignin is highly susceptible to any thermal processing, which initiates radical polymerization and increases the molecular weight owing to the cross-linking This mechanism shortens the stabilization time required during the oxidative stabilization of softwood lignin-based precursor fibers [18]. In order to gain CF based on lignin/PAN in combination, main attention should be paid to the adjustment of the thermal stability of the starting material, as well as the ordering and orientation of the lignin oligomers alongside the fiber axis This fundamental research methodology will be the scope of the present research paper. The impact of lignin on the stabilization kinetics and the kinetics of the thermo-mechanical behavior of the fibers will be discussed in depth throughout the present paper
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