(329f) Study of the Effects of Carbonization Parameters on the Structure of Carbonized Electrospun Lignin

Carbon fibers with various carbon contents, dimensions, and wide range of properties have been developed for multiple applications such as light-weight advanced composite reinforcements for aerospace applications, automotive parts, oil exploration tools, or for alternate energy harvesting, storage, or conversion, electrodes, sensors, etc. Conventional carbon fibers are produced from polyacrylonitrile (PAN) or petroleum-based pitch. Production of high performance carbon fibers from PAN is a high cost process, which limits the application of this material in its growing market. The growing carbon fiber market demands sustainable products at lower costs with various functionalities to meet requirements of different applications.

Raw material and processing costs are the two main elements of the high cost of PAN-based carbon fibers. Alternate raw materials and processing techniques have been studied for producing wide range of products which could be optimized for certain applications.

Lignin is a natural polymer available in the plants and is a coproduct of paper and cellulosic ethanol industries. It has been widely studied as a renewable low-cost precursor for producing carbon materials. Several spinning techniques have been studied for production of fibers from lignin such as dry-spinning, melt-spinning, and electrospinning.

Electrospinning of lignin is a versatile process which can be used to spin solutions of lignin, lignin – polymer blends, or lignin with particle fillers. Electrospun fibers can be collected as randomly oriented or aligned fiber mats with fiber diameters in the range of sub-micron to a few micro-meters.

In this study, electrospun fiber mats of lignin were produced by spinning an alkaline aqueous solution of lignin and poly(ethylene oxide) with the ratio of 95/5 wt%. For comparison of the properties, electrospun PAN fibers were also produced by spinning of solutions of PAN in N,N-dimethylformamide. Carbonization of the fibers was done in a two-step thermal treatment. The first step, thermostabilization, was done by heating the fibers in a tube furnace in presence of air from room temperature to 250 °C and kept at that temperature for two hours. The second step, carbonization, was the main subject of this research. The lignin and PAN fibers were carbonized at different combinations of conditions. The carbonization conditions were selected by applying factorial design of experiments. The studied variables were heating rate, carbonization temperature, and time. Morphology of the carbonized fibers was studied by scanning electron microscopy. Lignin fibers spun from aqueous solutions had about 25% reduction in their diameter after carbonization. In contrast, PAN fibers melted and created a network of fibers at high temperatures (1100 °C) and longer times of 10h. The characteristics of carbon structure of the fibers were studied by Raman spectroscopy. It showed that the R ratio, which is a measure of degree of ordered graphitic structure, for PAN is strongly dependent on the temperature. In comparison, for lignin fibers, the interaction between temperature and heating rate showed the highest influence. The same trend happened for graphitic crystalline size. Carbon structure of the fibers has effect on the final properties. Therefore, the knowledge obtained on the effects of carbonization conditions on the carbon structure is useful to change the structure and optimize the properties for specific applications.

Acknowledgement

The authors are thankful for the financial support from the Natural Sciences and Engineering Research Council (NSERC), Canada, for the Discovery grants individual (to Misra), NSERC NCE AUTO21, the Ontario Ministry of Economic Development and Innovation (MEDI) for the Ontario Research Fund (ORF) Research Excellence (RE) Round-4, and the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) - University of Guelph Bioeconomy Industrial uses research program. The authors are also thankful to Lignol Innovations Ltd. for providing the Organosolv Lignin samples