Lignin based carbon fiber production from raw materials having different lignin structure

In this study, raw materials, black pine (Pinus nigra) as coniferous wood, trembling poplar (Populus tremula) as a deciduous wood and wheat straw (Triticum aestivum) as an annual plant was obtained from Kastamonu province. Chemical components of the raw materials were examined and waste black liquors were obtained by kraft pulping by the studies in the literature. Besides, the kraft black solution was obtained from OYKA Paper Packaging Industry and Trade Inc.'s Zonguldak Caycuma factory (OBL) for comparison purposes. Moreover, the density of the black solutions, the amount of contained solid, the amount of inorganic material, the amount of organic matter, the pH and residual alkali content were determined. Isolation of lignin from black solutions was carried out according to the acidification method. Lignins isolated by acidification and commercially available Indulin AT lignin were compared with the content of Klason lignin, ash, volatile organic compounds, phenolic, carboxyl groups, inorganic substances. Furthermore, the lignins and Indulin AT were compared were results of GPC, FT-IR, UV-Vis spectroscopy and thermogravimetric analysis. According to the results, it was found that the isolated lignins except wheat straw were more pure in terms of ash content than Indulin AT lignin (<2.02%). Besides, it was found that the amounts of volatile organic componds of all isolated lignins were 17.86% to 62.43% higher than Indulin AT. Lignins have a high phenolic content (439,41-882.91 mgGAE/ l) and ICP-OES analysis showed that the highest element was sulfur (9862,00-24567,28 ppm) in all lignins. According to the results of TGA analysis, Indullin AT, OBL lignin, poplar, wheat straw, and black pine lignin were determined as the most resistant to thermal effects, respectively. In GPC analysis, the highest average molecular weight was found in black pine lignin with 6395 Da and the lowest average molecular weight was found in Indulin AT with 2176 Da according to the universal calibration method. In order to investigate the evaluation of kraft lignins in carbon nanofiber production, firstly, nanofiber production was carried out by the electrospun method from polyacrylonitrile (PAN) - lignin (50:50) and thermoplastic polyurethane (TPU) - lignin (87:13) solutions. Besides, fiber production was carried out from the fiber spinning polymers to compare the properties of the obtained lignin fibers. The obtained scanning electron microscopy images were examined by using the image analysis program. Additionally, the diameter average and diameter distributions of the obtained fibers were determined by statistical programs. According to the diameter measurements made after the first production, the fiber uniformity of all groups containing lignin was found to be lower than those produced using only polymers. The obtained fibers were subjected to thermal stabilization of PAN fibers at 250 ° C and TPU fibers at 220 ° C in the presence of air and the presence of argon as the inert medium for 2 hours. The samples were examined by SEM analysis and it was concluded that all fibers produced using TPU lost their own fiber structure after thermal stabilization. When first produced, the PAN-OBL and PAN-black pine lignin fibers had micro-dimensions, but after air stabilization these fibers were found to be nano-sized. Also, it was concluded that stabilization temperature and heating rate should be reduced while poplar lignin used with PAN. The samples applied to thermal stabilization were then subjected to carbonization for 1 hour at 1000 ° C in an inert medium. SEM images obtained from the samples were examined and it was observed that TPU samples were completely away from fiber structure and transformed into porous structures as activated carbon. With the except of wheat straw that is stable in the air environment and poplar lignin that is stable in the inert environment, All carbon nanofibers were produced from PAN samples. The elemental component analysis of the samples was carried out by the energy distribution spectrometry (SEM-EDS) method, which operates according to the X-ray reflection principle of the elements. According to SEM-EDS results, better carbon ratios were obtained from PAN groups stabilized in the air environment than stabilized in the inert environment. The highest carbonization content was found to be 96,60% by weight in black pine-PAN fibers carbonized after air stabilization. The electrical conductivity and capacitance values of the carbonized samples were determined. The highest capacitance value of 138,81 mF was determined from Indulin AT-PAN carbon fibers that were stabilized in an inert environment, besides the best self-conductivity value was found in black pine-PAN fibers stabilized in the air environment with 16,15 S/cm. According to electrical conductivity results, it has been determined that the obtained samples can be evaluated as electrodes in the electronic field and lithium-ion batteries. As a result, it was determined that isolated kraft lignins can be easily used in carbon nanofiber production and also obtained fibers have a high potential for use in advanced engineering products such as electronic, lithium-ion cell production and nanomedicine applications.