A low carbon yield
is a major limitation for the use of cellulose-based
filaments as carbon fiber precursors. The present study aims to investigate
the use of an abundant biopolymer chitosan as a natural charring agent
particularly on enhancing the carbon yield of the cellulose-derived
carbon fiber. The ionic liquid 1,5-diazabicyclo[4.3.0]non-5-enium
acetate ([DBNH]OAc) was used for direct dissolution of cellulose and
chitosan and to spin cellulose–chitosan composite fibers through
a dry-jet wet spinning process (Ioncell). The homogenous distribution
and tight packing of cellulose and chitosan revealed by X-ray scattering
experiments enable a synergistic interaction between the two polymers
during the pyrolysis reaction, resulting in a substantial increase
of the carbon yield and preservation of mechanical properties of cellulose
fiber compared to other cobiopolymers such as lignin and xylan.
One main challenge
to utilize cellulose-based fibers as the precursor
for carbon fibers is their inherently low carbon yield. This study
aims to evaluate the use of keratin in chicken feathers, a byproduct
of the poultry industry generated in large quantities, as a natural
charring agent to improve the yield of cellulose-derived carbon fibers.
Keratin–cellulose composite fibers are prepared through direct
dissolution of the pulp and feather keratin in the ionic liquid 1,5-diazabicyclo[4.3.0]non-5-enium
acetate ([DBNH]OAc) and subsequent dry jet wet spinning (so-called
Ioncell process). Thermogravimetric analysis reveals that there is
an increase in the carbon yield by ∼53 wt % with 30 wt % keratin
incorporation. This increase is comparable to the one observed for
lignin–cellulose composite fibers, in which lignin acts as
a carbon booster due to its higher carbon content. Keratin, however,
reduces the mechanical properties of cellulose precursor fibers to
a lesser extent than lignin. Keratin introduces nitrogen and induces
the formation of pores in the precursor fibers and the resulting carbon
fibers. Carbon materials derived from the keratin–cellulose
composite fiber show potential for applications where nitrogen doping
and pores or voids in the carbon are desirable, for example, for low-cost
bio-based carbons for energy harvest or storage.
Cellulose can be dissolved with another biopolymer in a protic ionic liquid and spun into a bicomponent hybrid cellulose fiber using the Ioncell® technology. Inside the hybrid fibers, the biopolymers are mixed at the nanoscale, and the second biopolymer provides the produced hybrid fiber new functional properties that can be fine-tuned by controlling its share in the fiber. In the present work, we present a fast and quantitative thermoanalytical method for the compositional analysis of man-made hybrid cellulose fibers by using thermogravimetric analysis (TGA) in combination with chemometrics. First, we incorporated 0–46 wt.% of lignin or chitosan in the hybrid fibers. Then, we analyzed their thermal decomposition behavior in a TGA device following a simple, one-hour thermal treatment protocol. With an analogy to spectroscopy, we show that the derivative thermogram can be used as a predictor in a multivariate regression model for determining the share of lignin or chitosan in the cellulose hybrid fibers. The method generated cross validation errors in the range 1.5–2.1 wt.% for lignin and chitosan. In addition, we discuss how the multivariate regression outperforms more common modeling methods such as those based on thermogram deconvolution or on linear superposition of reference thermograms. Moreover, we highlight the versatility of this thermoanalytical method—which could be applied to a wide range of composite materials, provided that their components can be thermally resolved—and illustrate it with an additional example on the measurement of polyester content in cellulose and polyester fiber blends. The method could predict the polyester content in the cellulose-polyester fiber blends with a cross validation error of 1.94 wt.% in the range of 0–100 wt.%. Finally, we give a list of recommendations on good experimental and modeling practices for the readers who want to extend the application of this thermoanalytical method to other composite materials.
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