Optimization and Characterization of Electrospun Untreated and Bio-cleaned Lignin-based Carbon Nanofibers

• Author / Creator

  Jiawei Chen

• Carbon fibers are reinforcing materials extensively used in composite applications. Since their conventional petroleum-based precursors are high-priced and unsustainable, demand on alternative green precursor has spawned substantial research works on lignin.
  This study reports on production and characterization of carbon nanofibers based on untreated lignin (KLA, KLB) and lignin purified with a bio-cleaning process (Bio-KLA, Bio-KLB). Small amounts of PEO were used for improving spinnability of lignin. The production parameters optimized for generating random fibers were electric field, lignin/PEO ratio, and molecular weight of PEO. The electrospinning parameters optimized for aligned fibers were electric field, flow rate, and rotating speed of collector. Spinnability test showed the transformation from uniform fibers to ribbon-like fibers as the total solid concentration increased. At higher total solid concentration, less PEO fraction was required to attain continuous spinning. Increase in lignin fraction embrittled the fiber mats. The optimization was conducted in order to obtain fibers with small diameter and good mechanical properties. Optimal condition for generating random KLA fibers occurred at electric field of 50 kV/m, lignin/PEO ratio of 95/5, and 1000 kDa PEO. Defects were detected when optimal condition was not met. Random KLA and Bio-KLA fibers were thermostabilized and carbonized at this optimal condition. Bio-cleaning improved the properties of random KLA carbon nanofibers by decreasing fiber diameter to 70%, increasing tensile strength by 2.1 times and elastic modulus by 2.7 times. The mechanical properties of random KLA carbon nanofibers achieved at optimal condition were as follows: average fiber diameter of 663.17±64.51 nm, tensile strength of 5.52±4.05 MPa, and elastic modulus of 886.29±471.47 MPa.
  Applying optimized solution condition (22 wt% total solid concentration, 95/5 lignin/PEO ratio) for random fibers, electric field of 80 kV/m, flow rate of 440 nl/s, and rotating speed of 2000 rpm were found to be the optimal condition for generating aligned KLA fibers. Some aligned fibers exhibited strong adhesion to aluminum foil, so release agent was used to assist with removal. Release agent posed a risk of weakening the mechanical properties of the fabricated fibers, but it preserved the integrity of the fiber mats for further characterizations. Aligning fibers increased the tensile strength and elastic modulus of KLA fibers by 16.7 times and 10.6 times respectively, compared to random fibers with release agent. Aligned KLA carbon nanofibers possessed average fiber diameter of 697.07±96.41 nm, tensile strength of 23.65±7.70 MPa and elastic modulus of 3960.98±1155.67 MPa.
  KLB produced gel-like solution with high viscosity, which was by itself not spinnable. Bio-cleaning converted KLB to spinnable substance. Random Bio-KLB carbon nanofibers showed average fiber diameter of 278.95±49.89 nm, tensile strength of 16.72±5.21 MPa, and elastic modulus of 1532.87±439.63 MPa when using optimal condition for random KLA fibers. Bio-KLB was incapable of producing testable fibers at optimal condition for aligned KLA fibers.
  In summary, this work demonstrates the feasibility of using bio-cleaning as a purification method to yield lignin-based carbon fibers. Bio-cleaning and aligning fibers can extensively enhance the mechanical properties of the lignin-based carbon fibers while adopting a greener manufacturing approach.

• Subjects / Keywords

  • lignin
  • carbon nanofibers
  • electrospinning

• Graduation date

  Fall 2020

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-1q38-9796

• License

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