Carbon Fibers Derived from FractionatedâSolvated Lignin Precursors

Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Carbon Fibers Derived from Fractionated−Solvated Lignin Precursors for Enhanced Mechanical Performance Jing Jin,∥ Junhuan Ding,∥ Adam Klett, Mark C. Thies,* and Amod A. Ogale*

ACS Sustainable Chem. Eng. Downloaded from pubs.acs.org by UNIV OF NEW ENGLAND on 10/06/18. For personal use only.

Department of Chemical and Biomolecular Engineering, and Center for Advanced Engineering Fibers and Films, 127 Earle Hall, Clemson University, Clemson, South Carolina 29634-0910, United States ABSTRACT: The sustainable solvent system acetic acid + water was used to simultaneously fractionate, solvate, and clean a softwood Kraft lignin for conversion to carbon fibers. By exploiting the novel liquid− liquid equilibrium (LLE) phase behavior exhibited by this pseudoternary system, three fractionated−solvated lignin precursors (FSLPs) of increasing molecular weight (7200, 13 800, and 28 600) were obtained via the continuous Aqueous Lignin Purification using Hot Acids (ALPHA) process. It is noteworthy that all three FSLPs, isolated as the lignin-rich liquid phase, had very low metals/ash content (230 ppm of Na and 0.07 wt % ash), in contrast to that of the bulk feed lignin (1400 ppm of Na and 0.60 wt % ash). Lignin fibers were successfully spun from the FSLPs by dry-spinning. Subsequently, the lignin fibers could be rapidly stabilized and carbonized at 1000 °C to produce carbon fibers with equivalent diameters less than 7 μm. Carbon fibers obtained from the highest molecular weight FSLP possessed an average tensile strength and modulus of 1.39 and 98 GPa, respectively, representing the highest mechanical properties ever obtained for carbon fibers derived from low-cost, chemically unmodified lignin. KEYWORDS: Sustainable precursor, Acetic acid, Fractionation, Purification, Molecular weight, Dry-spinning, Graphitization, Tensile strength

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INTRODUCTION Carbon fibers are among the highest-strength reinforcements in a wide range of commercial composite materials.1,2 Among various precursors used to obtain commercial carbon fibers, the most dominant precursor today (with 95% of the market) is synthetic polyacrylonitrile (PAN), which is quite expensive at the quality required for making high-strength carbon fibers and thus contributes up to 50% of the overall manufacturing cost.3 Furthermore, from an environmental standpoint, the manufacturing process for PAN involves the generation of toxic byproducts, especially hydrogen cyanide.4 The other organic precursors being used today, carbonaceous pitch and rayon, either are too expensive or produce carbon fibers with lower strength.5,6 Among bioderived sustainable feedstocks, lignin is of increasing interest as a precursor for carbon fibers. Not only is it unique among biopolymers in having aromaticity7 that facilitates carbon layer formation, but also it is the second most abundant organic compound on the planet. Thus, lignin-based carbon fibers could be quite inexpensive if precursor-grade lignin could be recovered and purified economically. Like PAN, most native lignins do not melt: they simply start decomposing when heated above 200−250 °C. Therefore, a wide range of modification techniques have been applied to lignin so that it can be spun into precursor fibers (and then converted into carbon fibers).8 The most common approach © XXXX American Chemical Society

has been melt-spinning, either by using specialty grades of lignin types that can be melted or by chemically modifying the lignin to achieve the desired processability. Uraki et al.9 were one of the first groups to accomplish this by melt-spinning a hardwood lignin recovered from Organosolv pulping without any chemical modification, yielding carbon fibers with tensile strength and modulus of 0.35 and 39 GPa, respectively. Baker and Rials10 melt-spun a “modified technical lignin”, but details about the lignin or its processing were not presented. Although most early melt-spinning work focused on the use of hardwood lignins (mainly Organosolv but also Kraft) because of their better spinnability. Hardwood lignins contain more syringyl (S) units and show thermoplasticity with a meltprocessable temperature.11 Softwood lignins contain a larger amount of guaiacyl (G) unit and tend to form dibenzodioxocin linkages.12 Thus, hardwood lignin is more likely to form a linear structure than softwood lignin due to the excess of S units.11 Norberg et al.13 showed that softwood lignins do have the advantage of much higher stabilization rates. Softwood Kraft lignins recovered from black liquor via the LignoBoost process13,14 were melt-spun and converted to carbon fiber, but tensile strength and modulus (TS/TM) of no more than 0.5 Received: June 8, 2018 Revised: September 7, 2018 Published: September 21, 2018 A

DOI: 10.1021/acssuschemeng.8b02697 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering and 30 GPa14,15 were obtained. In addition, the unusually large diameters of the resultant fibers (30−90 μm vs the desired 10 μm or less) indicated difficulties with the melt-spinning step. More recently, lignins derived from alternative biomasses have also been evaluated. For example, a corn-stover lignin fractionated with methanol and then acetylated yielded carbon fiber with strength and modulus of 0.45 and 62 GPa, respectively.16 However, stabilization rates were slow, and the final fibers were large (∼40 μm). Blends of tulip poplar/ switchgrass lignins (50/50) extracted from biomass via Organosolv fractionation were melt-spun and converted to carbon fiber (TS/TM = 0.52/38 GPa), with the stabilization rates being improved by the addition of switchgrass lignin.17 In related work,18 a phenolic bio-oil recovered from hardwood pyrolysis was catalytically polymerized to produce a meltspinnable precursor that led to better properties for ligninbased fibers (TS/TM = 0.85/85 GPa). As seen above, there are significant challenges to generating lignin-based carbon fibers via melt-spinning with the strength and modulus desired for even moderately high-performance composites (e.g., values of at least 2 and 200 GPa, respectively, as displayed by inexpensive PAN-based carbon fiber).19 In particular, the Tg of the lignin must be low enough so that it can be spun without solvents at temperatures below decomposition, but Tg must also be high enough for the lignin to undergo rapid stabilization. To get around this conundrum, Zhang and Ogale20,21 partially acetylated a softwood Kraft lignin so that it would dissolve in acetone but also still have an acceptable rate of stabilization (total acetylation resulted in a lignin that could not be stabilized). The lignin−acetone solution could then be dry-spun (that is, the volatile solvent evaporates, leaving behind lignin fibers), eliminating the need for a low-Tg lignin. The resultant carbon fibers were among the strongest (TS/TM = 1.04/52 GPa) reported in the literature from lignin, albeit with some chemical modification. Several studies have been done on mixing other polymers with lignin to improve the properties of resulting carbon fibers. Several studies have investigated the addition of PAN,22−24 but the improvement of CF tensile strength is primarily contributed by PAN, with little contribution from lignin. Unfortunately, even when present as a blend, PAN still poses the same environmental concerns, viz., generation of HCN during oxidative stabilization. Thus, no synergistic effects are found in the lignin/PAN blends, with the carbon fiber properties resulting primarily from the PAN component. In most of the previous work (cited above) on melt-spun lignins, solvent fractionation has been used to isolate a lower molecular weight (and thus lower-Tg) portion of the lignin suitable for melt-spinning, followed by washing to remove contaminants (e.g., inorganics) from the lignin. In a recently developed process known as Aqueous Lignin Purification using Hot Acids (ALPHA), Thies and co-workers25−27 used the liquid−liquid equilibrium of lignin with sustainable acetic acid−water solutions to isolate cleaned lignin fractions with different molecular weights (MW). In this study, a novel approach is reported whereby the higher-MW fractions of lignin, isolated via ALPHA, were directly dry-spun to obtain lignin (precursor) fibers. This was feasible because the higher-MW portion is simultaneously purified and solvated, whereas the impurities and lower MW portion of the lignin are preferentially extracted into the solvent phase. The primary objectives of the study were (i) to

use the continuous-flow version of the ALPHA process to isolate higher-MW, purified, and solvated fractions of lignin and (ii) to investigate the role of (higher) molecular weights on the dry-spinning of precursor fibers and properties of the resulting carbon fibers.

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EXPERIMENTAL SECTION

Materials. The lignin-recovery process known as SLRP,28,29 which is in an advanced state of development, was used to isolate a softwood Kraft lignin from a black liquor having a solids content of 42 wt %. SLRP produces a low-ash Kraft lignin in a manner conceptually similar to that of the commercial LignoBoost and LignoForce processes and with similar lignin yields; however, it is a continuous process and thus has lower capital and operating costs. The Kraft lignin used as the starting material of the ALPHA process is recovered from a black liquor that has a Kappa number of 25. The lignin content was determined to be 99% via a simplified version of the Klason method.30 The water content of the lignin, as determined by Karl− Fischer titration,31 ranged from 30% to 45%, depending on ambient conditions in the lab. Mixtures of glacial acetic acid (ACS grade, 99.7% purity, VWR cat. no. MKV193-45) and distilled, deionized (DD) water with a resistivity >18.2 MΩ cm (Millipore Milli-Q Academic water purification system) were used as the solvent system for the ALPHA process. HPLC-grade (99.7+%) N,N-dimethylformamide (VWR cat. no. AA22915-K7) with the additive lithium bromide (VWR cat. no. 35705-14) was used as the mobile phase for GPC analysis. Lignin Fractionation and Purification via ALPHA. A simplified schematic of the continuous-flow ALPHA process is shown in Figure 1. Additional details of the experimental apparatus can be found

Figure 1. Continuous-flow version of the Aqueous Lignin Purification using Hot Acids (ALPHA) process. Adapted from ref 26. Copyright 2016 American Chemical Society. elsewhere.26 For a typical experimental run, a lignin−water slurry consisting of ∼30−45% lignin in water is charged to the hopper and continuously delivered with the progressive cavity pump. The acetic acid−water solution of interest is delivered through the heated static mixer so that the slurry and solution are intimately mixed at 90 ± 5 °C. Within ∼30 s,26 the mixture of lignin, acetic acid, and water attains liquid−liquid equilibrium (LLE) and splits into two liquid phases. As has been previously shown,26 these very short residence B


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ACS Sustainable Chemistry & Engineering times prevent condensation reactions from occurring between the acetic acid and the lignin. The solvent-rich phase contains most of the metal salts/impurities and the lower MW lignin, whereas the phase rich in lignin (solute) contains the purified, high MW lignin solvated with ∼50% solvent, hence the term “fractionated−solvated lignin precursor” (FSLP). Metals content of the dried lignin polymer phase was determined via inductively coupled−plasma atomic emission spectroscopy (ICPAES). Analyses were carried out by the Agricultural Service Laboratory at Clemson University using a Spectro Analytical Instruments spectrometer, model ARCOS. To prepare a typical sample for ICP-AES analysis, 0.1 g of a dried lignin sample was digested in 5 mL of concentrated nitric acid at ambient temperature for 30 min. After digestion, the sample was dried at 200 °C for 1 h. The above dried samples were diluted in 10 mL of 1.6 M nitric acid and, after cooling, in another 50 mL of deionized water. The resulting liquids were then transferred to the ICP tube for analysis and detection. The FSLP can be directly fed to a fiber-spinning device, but here, it was dried for chemical analysis in a vacuum oven at ambient temperature and 29 in Hg vacuum for at least 24 h. Because it is highly hygroscopic, the dried lignin was then stored in a desiccator before use. Ash content was measured by weighing 3−4 g of dry lignin in a crucible and placing the crucible in a muffle furnace at 550 ± 3 °C for 20 ± 4 h. After cooling, the crucible was removed from the furnace and the residue (ash) mass was measured, for three duplicates. The lignin was analyzed for metals content by inductively coupled plasma−atomic emission spectroscopy (ICP-AES). GPC Analysis of FSLPs. The molecular weight of the lignin fractions isolated in the polymer phase was determined by GPC (Alliance GPCV 2000). Two columns were used in series: a Waters Styragel HT5 column (10 μm, 4.6 mm × 300 mm) followed by an Agilent PolarGel-L column (8 μm, 7.5 mm × 300 mm). The mobile phase consisted of 0.05 M lithium bromide in N,N-dimethylformamide (DMF) at a flow rate of 1 mL/min. Samples of the polymer phase recovered and dried as described above were dissolved in the mobile phase at a concentration of 1 mg/mL and filtered using a 0.2 μm nylon-membrane syringe filter (VWR, part no. 28145−487). Poly(ethylene glycol) (PEG) calibration standards were used for MW determination and were detected by refractive index using a Waters differential refractometer, while lignin samples were detected by UV− vis with a Waters 2487 detector at 280 nm. PEG standards were used instead of those from polystyrene, on the basis of previous work indicating that polystyrene standards gave unreasonable molecular weights for low MW lignin fractions. Conversion of FSLPs into Carbon Fibers. For the purpose of dry-spinning, a solution was prepared from the dried polymer phase (i.e., the FSLP) by mixing it with equal weight of 85/15 (w/w) acetic acid−water (AcOH/H2O) solution in a Paar reactor at 40−45 °C for 30 min. This solution was then fed to a custom-designed spinning unit (AJA Inc., Greenville, SC) that consisted of a steel barrel−plunger assembly. A sintered metal filter (5AL3/100/20 W/1100, Purolator Inc.) was placed above the spinneret, which consisted of 12 holes that were 50 μm in diameter and 250 μm long. The barrel−spinneret assembly was held at various elevated temperatures from 30 to 80 °C (during different experiments), and the solution was extruded at a constant flow rate. Elevated temperatures enabled evaporation of the solvent from the extrudate as the “dry” lignin fibers were taken up on a roll at speeds ranging from 20 to 30 m/min. Next, these precursor fibers were thermo-oxidatively stabilized. Lignin fiber tows (2−3 cm long and weighing 10−30 mg) were cut and mounted within graphite end-tabs using fast-cure epoxy. Fiber tows were inserted and hung in a preheated air convection oven (Thermolyne 9000). Weights were loaded at the bottom of fiber tows, and the temperature was raised from 165 to 250 °C at a rate of 20 °C/min and then held there for 1 h at 250 °C. This heating rate enabled the fiber tows to be poststretched up to 400% before the oxidative cross-linking of lignin occurred. The stabilized tows were mounted on graphite tabs to conduct carbonization under constantlength conditions (i.e., to prevent fiber shrinkage). Carbonization was

performed in a Thermolyne 21100 tubular furnace by heating from room temperature to 1000 °C at a rate of 7 °C/min under a nitrogen flow of 0.1 L/min. Characterization of Fibers. Morphological analysis of the precursor and carbon fibers was conducted by scanning electron microscopy (SEM) with a Hitachi 4800 SEM unit. The as-spun fibers were coated with platinum using a sputter coater, whereas carbon fibers were analyzed without coating (as carbon fibers are electrically conducting). The cross-sectional area of single fibers was measured using SEM micrographs and Quartz PCI software. Single filaments were mounted on paper tabs following ASTM test method D-337975. Accurate values for cross-sectional area, as measured by SEM, were used to calculate tensile modulus and strength of carbon fibers. A compliance correction method was used in conjunction with three different gage lengths (10, 25, and 35 mm) to accurately measure the tensile modulus. Raman spectra of the carbon fibers were obtained using a 785 μm laser in a Raman microscope system (Renishaw, West Dundee, IL). Carbon fibers were mounted on a glass slide and fixed with scotch tape at both ends. Raman spectra were obtained using an objective lens of 50× magnification at 25 mW laser power with an exposure time of 10 s. WiRE 3.4 software was used to analyze the spectra with Gaussian−Lorentzian curve-fitting. Wide-angle X-ray diffraction (WAXD) analysis was conducted on bundles of fibers using a Bruker D8 Venture Dual Source diffractometer with Cu Kα radiation (λ = 0.15406 nm). The unit was equipped with an IμS microfocus source, a Photon 100 CMOS detector, and Apex3 software to generate integrated azimuthal (2θ) profiles. Samples were glued with fast-cure acrylate glue to form fiber bundles and were then mounted on a sample holder. One sample from each group was sprinkled with NIST-grade silicon standard powder for accurate location of the 2θ position of various planes. The scan time was 120 s per run. The baseline correction and peak fitting were processed on WiRE 3.4 software. The d-spacing of (0 0 2) planes was calculated using Bragg’s law.

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RESULTS AND DISCUSSION Lignin Molecular Weight and Purity. The effect of ALPHA processing conditions on the MW and purity of FSLPs was investigated. Recent measurements of the ternary phase behavior for the lignin−acetic acid−water system31 were essential in guiding the selection of appropriate operating conditions for ALPHA. A conceptualization of the type of ternary phase diagram that has been identified in ongoing work27 with lignin and aqueous, one-phase solvent systems is shown in Figure 2. For lignin with the acetic acid−water system, the liquid−liquid equilibrium (LLE) region of interest

Figure 2. Conceptualization of ternary phase behavior identified for lignin with aqueous, one-phase solvent systems. C


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ACS Sustainable Chemistry & Engineering exists at solvent compositions ranging from about 15/85 AcOH/H2O (at higher water concentrations, the lignin is a solid) to about 70/30 AcOH/H2O (at higher AcOH concentrations, the system becomes a single liquid phase above the liquid−liquid critical point). Note from the diagram that the lignin-rich phase is in general highly solvated (e.g., for the AcOH/H2O system, it contains 50−70 wt % solvent). The mass distribution of lignin between the lignin- and solvent-rich phases that was obtained in this work when using the continuous-flow apparatus at 90 °C for generating FSLPs is given as Figure 3.

Figure 4. Molecular weight distributions of the lignin fractions (i.e., FSLPs) isolated via the ALPHA process: Feed SLRP (−), Med MW (- -), Higher MW (− •), and Highest MW (− −).

AcOH/H2O ratio in the feed solvent enabled the isolation of FSLPs of increasing MW in the lignin-rich phase for conversion to carbon fibers. Furthermore, with ALPHA, we were able to reduce metals (and thus ash) impurities to levels significantly lower than ever reported for the purification of lignins. (The ash content of these FSLP lignins is almost 2 orders of magnitude cleaner than today’s commercially available softwood Kraft lignins and is also cleaner than today’s best experimental organosolv lignins.) Finally, because we were able to reduce metals in the lignin-rich phase to essentially the same low levels (e.g., ∼200 ppm of Na) for all fractions, the effect of impurities as a variable was eliminated, allowing us to focus exclusively on the effect of molecular weight (MW) differences. To our knowledge, neither the effect of lignin MW nor the effect of lignin impurities levels on carbon fibers derived from lignin had heretofore been investigated. Previous studies have reported the effect of MW on 50/50 blends of lignin with PAN32 and on repolymerized pyrolyzed lignin18 but not on neat lignins. Furthermore, in neither of these studies was the effect of metals/ash content investigated. Note that the polydispersity index (PDI) of these fractions (Table 1) increases with the molecular weight; at this time, we have no good explanation of this phenomenon, which is the subject of current investigation. Dry-Spinning and Carbonization. During dry-spinning of a solution, the solvent must evaporate out of the extruded filament before the filament gets wound on a roll; otherwise, the moist filaments will stick together and prevent successful production of fibers. Thus, initial dry-spinning experiments were performed using the higher-MW FSLPs at elevated temperatures ranging up to 80 °C. Representative SEM micrographs of lignin fibers produced at 70 and 80 °C are displayed in Figure 5. Although solutions could be extruded out of the spinneret, rapid evaporation of the solvent resulted in a very dry extrudate that could not be continuously drawn down (stretched). Note the significant extent of crenulation on the fiber surface, resulting from rapid out-diffusion of solvent that leaves a skin on the outside while the core is still wet. As the core dries out, the skin collapses and results in the wavy/ crenulated surface. The existence of doubly convex crenulation as well as occlusions is evident in Figure 5. The sharp crevices or occlusions (defects created where the two convex crenulations meet) lead to stress concentration when such fibers are subjected to tensile forces.

Figure 3. Equilibrium mass distribution of lignin into the lignin-rich (LR) phase for continuous-flow apparatus runs at 90 °C.

Our hypothesis was that higher-MW fractions of the lignin would lead to improved properties in the final carbon fibers; thus, ALPHA was operated so as to isolate increasingly higherMW FSLPs in the solvated and cleaned lignin-rich phase. For the first ALPHA run, the bulk feed lignin was processed with a 30/70 AcOH/H2O solution at 90 °C. As seen in Figure 3, at these conditions, only 10% of the lignin by weight was extracted into the relatively weak solvent phase, so 90% of the lignin separated out to form the lignin-rich phase (i.e., the FSLP) for subsequent conversion into carbon fibers. Note that this fraction is labeled in Figure 3 as being of medium MW, because it is only slightly higher in MW than the bulk feed lignin (defined as being of average or medium molecular weight). With this FSLP, the idea was to produce a lignin that was much cleaner than the feed but of similar molecular weight. Next, the feed lignin was processed with a 56/44 AcOH/ H2O solution; as seen in Figure 3, now 50% of the lignin was extracted into the solvent phase, and 50% of the lignin was separated out to form the lignin-rich phase. Thus, this fraction is labeled as having a higher MW than the feed lignin. Finally, the feed lignin was contacted with a strong 67/33 AcOH/H2O solution; now, 90% of the feed lignin was extracted into the solvent phase, so that only 10% of the lignin formed the ligninrich phase. This fraction is labeled (see Figure 3) as having the highest MW of the three lignin-rich phases (i.e., FSLPs) that were isolated by ALPHA for conversion to carbon fibers. GPC chromatograms for the feed and for the medium-, higher-, and highest-MW fractions are given in Figure 4 and indicate that ALPHA was successful in producing FSLPs of increasing MW. The results of this ALPHA processing of a softwood Kraft lignin are summarized in Table 1. As expected, increasing D


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Table 1. Molecular Weight and Purity of Fractionated−Solvated Lignin Precursors (FSLPs) Obtained via ALPHAa lignin type

yield [wt %]

AcOH/H2O ratio

feed lignin medium-MW FSLP higher-MW FSLP highest-MW FSLP

100 90 50 10

N/A 30/70 56/44 67/33

num. avg. MW [Da] 5270 7200 13 800 28 600

± ± ± ±

70 100 150 300

polydispersity index (PDI) 6.60 3.82 4.44 5.15

± ± ± ±

0.01 0.03 0.03 0.02

ash content [wt %] 0.6 0.059 0.069 0.088

± ± ± ±

0.3 0.003 0.005 0.005

Na content [ppm] 1400 250 210 220

± ± ± ±

50 80 50 70

a

Molecular weights were measured using size exclusion chromatography (i.e., GPC) in conjunction with PEG standards.

bone cross section observed for precursor fibers is also retained in carbon fibers (the shape was also retained in the stabilized fibers). The resulting carbon fibers displayed a void-free microstructure with no fusion of fibers. If any surface fusion occurs on carbonized fibers, the fused regions serve as structural defects and deteriorate carbon fiber strength. The equivalent diameters of the resulting carbon fibers ranged from 6 to 7 μm, which is significantly smaller than those previously reported for carbon fibers spun from chemically unmodified softwood lignin.14,15 The fine diameters obtained in this study are similar to those obtained for PAN-based commercial carbon fibers.35 The lateral surface area of the CFs obtained from lignin fibers spun at 80 °C is 26% higher than that of equivalent circular fibers (i.e., possessing equal cross-sectional area). For CFs obtained from lignin fibers spun at 30−40 °C, the relatively smoother surface found in precursor fiber was retained in the carbonized state and the surface area is still 19% larger than that of equivalent circular fibers. The enhanced interfacial area increases fiber/matrix interfacial bonding in composite applications and can help improve the strength of resulting composites. It is also worth noting that T300 grade PAN-based carbon fibers are also not circular. Their kidneybean shape is a consequence of fast out-diffusion of solvent during wet-spinning. Slower out-diffusion rates lead to higher strength, circular fibers (such as T800 and IM7), but the slower process contributes to their higher cost. The same phenomenon can occur during dry-spinning; i.e, a slower outdiffusion process can result in circular fibers, which would only increase the strength beyond that already achieved. The structure of carbon within the lignin-based carbon fibers was studied using Raman spectroscopy and wide-angle X-ray diffraction (WAXD). As displayed in Figure 8, the Raman spectra for carbon fibers displayed the typical D- and G-bands. Note that the D-band at ∼1320 cm−1, associated with structural disorder, is much stronger than the G-band at ∼1585 cm−1, which is attributed to the graphitic structure. The ratio of areas for D- and G-peaks (ID/IG) for carbon fibers derived from the medium-MW FSLP (similar in MW to the bulk feed lignin) was 5.4. The ID/IG area ratio of lignin-derived carbon fibers has been reported in prior literatures studies36,37 to be 2−5, which is consistent with the values obtained in this

Figure 5. SEM micrographs of lignin fibers dry-spun from the higherMW fractionated−solvated lignin precursor (FSLP) at (a) 70 °C and (b) 80 °C.

Within the range of conditions investigated in this study, 32−40 °C was found to be the best temperature window where the spinning solution could flow easily and be drawn down. In addition, this temperature range is below the flash point of acetic acid−water mixture, so there are no significant safety concerns with the overall process. SEM micrographs of lignin fibers obtained from the three FSLPs are shown in Figure 6. The resulting noncircular fibers were in the shape of dog bones. Note that a slower rate of solvent out-diffusion (as compared to that at 70−80 °C) leads to the exterior fiber surface being smoother with no crenulations. This phenomenon has also been observed in our earlier study during dry-spinning of partially acetylated lignin/acetone solutions,33 as well as others reported in the literature for wet-spun PAN fibers.34 The average equivalent diameters of lignin fibers derived from the medium, higher, and highest-MW fractions (FSLPs), displayed in Table 1, were 21 ± 1.0, 18 ± 0.7, and 17 ± 0.6 μm, respectively. Note that fibers spun from the medium-MW FLSP exhibited larger diameters than those spun from the higher and highest-MW FSLPs, because they could not be stretched as much during the drawdown step. The lignin fibers were stabilized at 250 °C for a duration of only 1 h. Because the source lignin was a Kraft softwood lignin, which is known to undergo rapid cross-linking reactions,13 the lignin fibers had a high degree of reactivity that resulted in rapid thermal−oxidative stabilization. Next, the stabilized lignin fibers were successfully carbonized at 1000 °C. The cross sections of carbon fibers obtained from the three FSLPs are shown in the SEM images of Figure 7. Note that the dog-

Figure 6. SEM micrographs of lignin fibers dry-spun from the (a) medium-, (b) higher-, and (c) highest-MW FSLPs in the low-temperature regime (30−40 °C). E


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Figure 7. SEM micrographs of carbon fibers derived from the (a) medium-, (b) higher-, and (c) highest-MW FSLPs.

Figure 8. Raman spectra of carbon fibers derived from medium-, higher-, and highest-MW FSLPs. Intensity values on the y-axis scale are in arbitrary units.

Figure 9. Integrated azimuthal (2θ) profiles from WAXD of carbon fibers derived from medium-, higher-, and highest-MW FSLPs. Intensity values on the y-axis scale are in arbitrary units.

Figure 9 displays wide-angle X-ray diffractograms for carbon fibers produced from the medium-, higher-, and highest-MW FSLPs. NIST-grade silicon powder was added to one sample of each type as a calibration standard to provide an accurate location of the 2θ peak position; the (1 1 1) peak for silicon must appear at 28.4°. The integrated 2θ profile for the carbon fibers produced from the medium-, higher-, and highest-MW FSLPs displayed (0 0 2) peaks at 23.0°, 23.8°, and 24.6°, respectively. The d002 spacings between layer planes, calculated using Bragg’s law, were 0.386, 0.374, and 0.365 nm, respectively. A d002 spacing value of 0.387 nm, reported by

study. The current value is similar to the ratio observed for PAN-derived fibers that contain turbostratic carbon but much higher compared to 0.5−1.0 for carbon fibers derived from mesophase pitch, which are significantly more graphitic.38 Thus, a low level of graphitic development is observed in lignin-based carbon fibers as inferred from a large ID/IG ratio. However, the ratio decreased to 4.4 for carbon fibers made from the higher-MW FSLP and further to 3.6 for the highestMW FSLP. Clearly, an increase in the MW of lignin fractions led to better carbon-layer formation in the resulting fibers. F


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Table 2. Tensile Properties of Carbon Fibers Obtained from Medium, Higher, and Highest Molecular Weights of FSLPs As Compared with Literature-Based Results precursor lignin medium MW higher MW highest MW Ace-SKL dry-spun20 repolymerized pyrolytic lignin melt-spun18

tensile strength (GPa) 1.05 1.03 1.39 1.04 0.86

± ± ± ± ±

modulus (GPa)

0.14 0.09 0.23 0.10 0.16

74 87 98 52 85

Baker et al.39 for melt-spun Organosolv lignin fibers carbonized also at 1000 °C, is consistent with that reported here for carbon fibers produced from medium-MW FSLPs. Overall, these interlayer spacing values are significantly higher than that for pure graphite (0.335 nm), indicating that these ligninderived carbon fibers possess a low degree of graphitic crystallinity. However, the d002 spacing of the lignin-based carbon fibers consistently decreased with increasing MW of the FSLPs. These WAXD results, in conjunction with Raman, confirm that a better packed carbon-layer structure was formed as the MW of the FSLPs increased. The tensile properties of carbon fibers derived from the three grades of FSLPs are displayed in Table 2. Also included are properties of other lignin-based carbon fibers reported in the literature, including those made from hardwood, softwood, and pyrolytic lignin via dry-spinning and melt-spinning. The carbon fibers obtained from the medium and higher-MW FSLPs presented similar tensile strengths, slightly above 1.0 GPa. In contrast, carbon fibers derived from lignin fibers produced at 70−80 °C with undesired sharp crevices/defects possessed a tensile strength of only 0.6 GPa. While comparable to many of the prior carbon fibers reported in literature studies (derived from melt-spun lignin), these fibers were not as good as those obtained from 30 to 40 °C dry-spun lignin fibers. Carbon fibers produced in the current study displayed increasing moduli values as the MW of the FSLPs increased resulting from a refinement of carbon structure. Enhanced graphitic content (better carbon layer formation) is known to enhance lattice-controlled properties such as modulus.1,2 Thus, carbon fibers produced from the medium- and higher-MW FSLPs (obtained at 90 and 50 wt % yield from the original lignin) displayed tensile moduli of 74 ± 4 and 87 ± 7 GPa, respectively, whereas the highest-MW FSLP (10 wt % yield) displayed a tensile modulus of 98 ± 5 GPa. The highest tensile strength of 1.39 ± 0.23 GPa was also measured for carbon fibers produced from the highest-MW FSLP. These strength and modulus values of 1.39 and 98 GPa, respectively, represent the best-quality lignin-based carbon fibers produced to date. We would like to clarify that, although the highest properties of carbon fibers were obtained from the “highest 10% MW” fraction obtained at 10 wt % yield, the remaining 90 wt % of feed lignin is not useless or wasted. Rather, in an integrated biomanufacturing enterprise, lower MW fractions of that lignin (the remaining 90 wt %) could be used to produce prepolymers or resins for bioplastics (i.e., not for carbon fibers). Further, we note that, although the strength is almost 40% higher than the highest value reported in the literature and the modulus has approached 100 GPa, further improvement of these properties is needed if such fibers are to compete with low-cost PAN-derived CFs and low-cost glass fibers. In future studies, we plan to change the source and the method of pretreatment of feed lignin to generate superior FSLPs that, in

± ± ± ± ±

4 6 5 2 37

strain (%)

equivalent diameter (μm)

± ± ± ± ±

6.2 ± 0.3 5.7 ± 0.4 5.6 ± 0.2 5.9 ± 0.2 29−50

1.4 1.2 1.4 2.0 1.01

0.1 0.2 0.2 0.2 0.3

turn, can help improve the yield/MW of FSLPs and properties of the resulting carbon fibers.

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CONCLUSIONS

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AUTHOR INFORMATION

By taking advantage of the liquid−liquid equilibrium that forms between lignin and acetic acid−water solutions at elevated temperatures, very clean lignin fractions of higher molecular weight can be isolated within the solvated, ligninrich phase, as the lower MW lignin and impurities are extracted into the coexisting solvent phase. Compared to the different types of lignins that have been investigated for making carbon fibers, FSLPs possess promising characteristics, including high purity, low ash content, high MW, good spinnability, and good thermal reactivity for rapid stabilization. These FSLPs were continuously produced via the ALPHA process and were then directly processed into precursor fibers by dry-spinning; it is noteworthy that both of these processes are scalable manufacturing routes. The resultant carbon fibers from the highest-MW FSLP, which comprised 10% of the feed lignin, possessed the highest average tensile strength of 1.39 ± 0.23 GPa, almost 40% stronger than any lignin-based carbon fiber reported to date. The results of this study indicate that both the purity and molecular weight of the lignin precursor can play an important role in improving the performance of ligninbased carbon fibers.

Corresponding Authors

*E-mail: [email protected]. Phone: 864-656-5483. *E-mail: [email protected]. Phone: 864-656-5424. ORCID

Adam Klett: 0000-0002-3365-644X Mark C. Thies: 0000-0002-2833-485X Amod A. Ogale: 0000-0003-0804-1862 Author Contributions ∥

J.J. and J.D. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation under Grant CMMI-1462804. The project also made use of CAEFF facilities originally supported by NSF ERC Grant EEC 9731680. The authors acknowledge Lignin Enterprises for providing the SLRP lignin and Graham Tindall for assisting with the continuous ALPHA runs.

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REFERENCES

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