U.S. patent application number 15/860275 was filed with the patent office on 2018-05-24 for carbon fibers derived from poly-(caffeyl alcohol) (pcfa).
This patent application is currently assigned to University of North Texas. The applicant listed for this patent is University of North Texas. Invention is credited to Fang Chen, Nandika D'Souza, Richard Dixon, Mangesh Nar.
Application Number | 20180142380 15/860275 |
Document ID | / |
Family ID | 54769110 |
Filed Date | 2018-05-24 |
United States Patent
Application |
20180142380 |
Kind Code |
A1 |
Dixon; Richard ; et
al. |
May 24, 2018 |
CARBON FIBERS DERIVED FROM POLY-(CAFFEYL ALCOHOL) (PCFA)
Abstract
Poly-(caffeyl alcohol) (PCFA), also known as C-lignin, is a
promising new source of both carbon fibers and pure carbon. PCFA
can be used to produce carbon fibers by direct electrospinning,
without blending with another polymer to reduce breakage. Analyses
have shown that the carbon obtained from PCFA is superior to that
obtained from other lignins. The fibers formed from PCFA are
smoother, have a narrower diameter distribution, and show very low
defects. The PCFA can be obtained by extraction from plant seed
coats. Examples of these plants include the vanilla orchid, Vanilla
planifolia, and Jatropha curcas. The fibers may be formed through
electrospinning, although other methods for forming the fibers,
such as extrusion with a carrier polymer, could be used. The fibers
may then be carbonized to increase the carbon yield.
Inventors: |
Dixon; Richard; (Sulphur,
OK) ; D'Souza; Nandika; (Plano, TX) ; Chen;
Fang; (Denton, TX) ; Nar; Mangesh; (Denton,
TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
University of North Texas |
Denton |
TX |
US |
|
|
Assignee: |
University of North Texas
Denton
TX
|
Family ID: |
54769110 |
Appl. No.: |
15/860275 |
Filed: |
January 2, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14729369 |
Jun 3, 2015 |
9890480 |
|
|
15860275 |
|
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|
62008424 |
Jun 5, 2014 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
D01D 5/0023 20130101;
D01F 9/16 20130101; D01D 5/003 20130101; D01D 5/0046 20130101; D01F
9/17 20130101; D01D 5/0038 20130101; Y10T 428/2918 20150115 |
International
Class: |
D01F 9/17 20060101
D01F009/17; D01D 5/00 20060101 D01D005/00; D01F 9/16 20060101
D01F009/16 |
Claims
1. Carbon fibers, consisting of: 100 wt. % PCFA, wherein the carbon
fibers are produced by extracting poly-(caffeyl alcohol) (PCFA)
from a plant to produce extracted PCFA and electrospinning the
extracted PCFA to produce the carbon fibers.
2. The carbon fibers of claim 1, wherein the carbon fibers have a
narrower size distribution than carbon fibers comprised of 100 wt.
% Kraft lignin.
3. The carbon fibers of claim 1, wherein the carbon fibers have
diameters of about 10.5 .mu.m to about 14 .mu.m.
4. The carbon fibers of claim 1, wherein the carbon fibers are
further produced by carbonizing to produce carbonized PCFA carbon
fibers, and wherein the carbonized PCFA carbon fibers have higher
crystallinity than carbonized carbon fibers comprised of 100 wt. %
Kraft lignin.
5. The carbon fibers of claim 4, wherein the carbonized PCFA carbon
fibers have higher purity than carbonized carbon fibers comprised
of polyacrylonitrile (PAN).
Description
[0001] This application is a divisional application of and claims
priority to U.S. patent application Ser. No. 14/729,369, entitled
"Methods For Producing Carbon Fibers From Poly-(Caffeyl Alcohol),"
filed on Jun. 3, 2015, which claims priority to U.S. Provisional
Patent Application Ser. No. 62/008,424, entitled "Carbon Fibers
Derived From Poly-(Caffeyl Alcohol) (PCFA)," filed on Jun. 5, 2014,
and the entire content of both applications is hereby incorporated
by reference.
BACKGROUND
[0002] This disclosure pertains to plant-sourced carbon. In
particular, this disclosure relates to poly-(caffeyl alcohol)
("PCFA"), also named as C-lignin as a source for carbon.
[0003] Carbon fibers are a high volume high performance product in
applications ranging from carbon fiber reinforced epoxy for
aerospace and marine applications, electromagnetic interference
shielding, biomedical applications for regenerative medicine and
cancer treatment, energy storage devices and water filtration.
Recently, concerns about greenhouse gas emissions and climate
change have motivated a shift to lighter automobiles. To this end,
significant efforts are being focused on the development and
deployment of carbon fiber-reinforced composites. Modeling studies
have indicated that over 60% of the steel in a vehicle could be
replaced by carbon fiber-reinforced composite materials,
dramatically reducing its weight while maintaining the vehicle's
impact protection. Furthermore, for every 10% reduction in weight
of the vehicle, the fuel economy is estimated to increase by
6%.
[0004] Carbon fiber composites (CFCs) display several properties
that are very attractive in structural applications: high strength
and stiffness, low density, they are chemically inert and show high
electrical and thermal conductivity. However, methods for producing
these CFCs are less than ideal. Currently, carbon fiber is
manufactured predominantly from polyacrylonitrile (PAN) with a
small fraction originating in pitch. PAN based on the acrylonitrile
monomer has a high cost. Pitch raw materials are cheaper but the
processing involves cleanup leading to high final cost. Pitch from
petroleum is preferred over coal pitch from raw material clean up
perspectives, but needs vacuum cleaning to remove volatile matter.
To form carbon fibers, wetting of PAN prior to carbonization is
employed. Typical carbon yields for PAN-based and pitch-based
carbon fibers are about 50-60% and 70-80% respectively. A
pre-oxidation step to carbonization has been shown to result in
higher carbon yield, and additional graphitization with argon has
increased the carbon yield to 80% for PAN fibers.
[0005] Synthetic polymers such as polyacetylene, polyethylene, and
polybenzoxazole have also been investigated as a potential route
for obtaining carbon fibers. While the strength to weight ratio of
these polymers exceeds that of glass, the cost/weight ratio remains
prohibitive. Thus, fiberglass based composites remain the high
volume product. This raises further environmental concerns as the
carbon footprint for producing fiberglass is prohibitive. Because
of such concerns, development of a source of carbon fiber based on
plant material is being strongly promoted.
[0006] Kraft lignin, extracted from hardwoods, has been extensively
studied as a feedstock for biomaterials. To facilitate the melting
of the lignin, organic solvent based extraction, chemical treatment
or melt blending are employed. The value of lignin as a source for
carbon fibers obtained from melt and dry spinning of hardwood Kraft
lignin (HKL), softwood Kraft lignin (SKL) and alkali softwood Kraft
lignin has been shown. Hydrogenation with NaOH using Raney-Ni,
followed by steam explosion to isolate the lignin and then
modification to lower its softening point, thereby facilitating
melt spinning of the fibers, has been used. However, this method
was expensive and a cheaper alternative was attempted using
creosote for phenolysis. Although phenolysis improved the yield to
40%, tensile properties were low when compared to hydrogenation.
Acetic acid pulping from hardwood gave fusible lignin that could be
melt-spun. Lignin from softwood resulted in a high fraction of high
molecular weight infusible lignin, that must be separated from the
fusible fraction in order to facilitate melt spinning.
[0007] Chemo-enzymatic treatment (sulfonication) has been shown to
transform water insoluble Kraft and organosolv lignins to water
soluble material, and facilitates grafting of acrylic compounds
onto the lignin backbone. Esterification of lignins from sources
such as palm trunk, poplar, maize, barley, wheat, and rye with
succinate anhydride showed relatively lower substitution of
succinate, but gave thermal stability ranging from 100 to
600.degree. C., with the highest for lignin from rye.
[0008] Blending polymers with lignin enables fiber integrity
through improved melt strength. Poly(ethylene oxide) (PEO) has been
widely studied for imparting ability for spinning lignin into
fibers. Incorporation of 5% and 3% PEO in hardwood Kraft lignin
(HKL) improved spinning capability and tensile properties,
respectively. With an Alcell/PEO blend, strong hydrogen bonding
results in miscible blends aiding spinning of fibers, although
addition of PEO did not improve the mechanical properties of the
fiber. To overcome brittleness, lignin was blended with
polyethylene terephthalate (PET) and polypropylene (PP). Blends of
PET and PP with HKL gave fiber diameter ranges from 30 to 76 .mu.m,
and blends with 25% polymers yielded 60% carbon after carbonation;
however, this route did not improve the physical properties of the
fibers. Similarly, polyethylene glycol (PEG)-lignin was used for
single needle melt spinning to obtain 23 .mu.m diameter fibers at
170.degree. C. and PVA by researchers in the field.
[0009] The above examples clearly demonstrate that considerable
processing is necessary to obtain high carbon yields, good
spinnability and useful fiber properties from typical bulk lignin,
such as the Kraft lignin obtained as a by-product from the pulp and
paper industry.
SUMMARY
[0010] The present disclosure relates generally to carbon fibers
derived from poly-(caffeyl alcohol) (PCFA), also known as C-lignin,
and to methods for preparing the carbon fibers. The carbon fibers
derived from PCFA are 100% PCFA with no carrier polymer and
demonstrate properties superior to other commercially available
carbon fibers such as those derived from Kraft lignin.
[0011] Lignocellulose is a dominant constituent of plant dry
matter, consisting of a complex of cellulose and hemicellulose
embedded in lignin. Lignin is the second most abundant natural
polymer on earth, produced by oxidative polymerization of
p-hydroxycinnamyl alcohols (monolignols). Lignins are primarily
found in plant secondary cell walls, and are particularly abundant
in vascular tissues. The presence of this lignin reduces forage
digestibility and hinders agro-industrial processes for generating
pulp or biofuels from lignocellulosic plant biomass, and there has
therefore been considerable attention given to reducing lignin
content in plant feedstocks. In general, lignin polymers found in
stem tissues are composed of three units; p-hydroxyphenyl (H,
generally a minor unit), guaiacyl (G), and syringyl (S) units.
These are derived biosynthetically from p-coumaryl, coniferyl, and
sinapyl alcohols. These units are joined in the polymer through a
range of different linkage types, resulting in a branched polymer
that is also cross-linked to cell wall polysaccharides. Compared to
PAN and pitch precursors, lignin is cost effective and has an
aromatic structure that is carbon rich for higher carbon yield.
There is therefore considerable interest in determining whether
lignin can be developed as a cost-effective feedstock for
carbon-based applications, potentially as a byproduct of the
processing of lignocellulosic liquid biofuels.
[0012] It has been discovered that the seed coats of a variety of
plant species contain a previously unsuspected class of lignin-like
molecule made entirely from caffeyl alcohol units (essentially G
units lacking the methyl group on the 3-oxygen). This molecule is
termed C-lignin or poly-(caffeyl alcohol) (PCFA). The
ortho-dihydroxy substitution of the caffeyl alcohol monomer results
in polymerization to yield a linear homopolymer containing
benzodioxane rings. Without wanting to be bound by theory, such a
linear structure appears to enhance the ability to generate carbon
fibers by electrospinning.
[0013] Significantly, PCFA can be used to produce carbon fibers by
direct electrospinning, without blending with another polymer to
reduce breakage. In contrast, Kraft lignin is generally blended
with another polymer to increase the extensional flow strength and
allow long spools of uniform fiber to be produced without breakage.
This is an advantage for the use of a C-lignin precursor rather
than Kraft lignin. Analyses have shown that the carbon obtained
from PCFA is superior to that obtained from Kraft lignin. The
fibers formed from PCFA are smoother than those from Kraft lignin,
have a narrower diameter distribution, and show very low defects
compared to Kraft lignin. Carbon defects are associated with
inferior mechanical and thermal properties. Thus the carbon fibers
derived from PCFA appear to be far superior to the Kraft lignin
sourced carbon.
[0014] The carbon fibers derived from PCFA would be useful in
composites for everything from aircraft, cars, sports rackets, to
water purification devices, and could be developed as high value
co-products from lignocellulosic biofuels.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 shows ESEM images of electrospun PCFA fibers (A,B)
and Kraft fibers (C,D);
[0016] FIG. 2 shows histograms of the diameters of electrospun PCFA
fibers (A), and Kraft lignin fibers (B);
[0017] FIG. 3 shows (A) ESEM image of electrospun PCFA fibers and
(B, C and, D) surface variation analysis for the areas marked with
open yellow rectangle in (A);
[0018] FIG. 4 shows (A) ESEM image of electrospun Kraft lignin
fibers and (B, C and, D) surface variation analysis for the areas
marked with open yellow rectangle in (A);
[0019] FIG. 5 shows Zeta potentials for carbon from (A) PCFA powder
and (B) Kraft lignin; and
[0020] FIG. 6 shows Raman spectroscopy of (A) PCFA powder and
derived carbon, and (B) Kraft lignin powder and derived carbon.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0021] Generally, the present disclosure relates to carbon fibers
derived from poly-(caffeyl alcohol) or PCFA and the methods for
preparing these fibers.
[0022] In preferred embodiments, the carbon fibers are made up of
100% PCFA with no carrier polymer. Alternatively, the carbon fibers
may be made up of about 10% to about 90% PCFA in combination with a
polymer carrier PAN, polyesters, polyolefins, polyamides and other
thermoplastic and thermoset polymers can be used.
[0023] The PCFA is preferably obtained by plant extraction from any
plant that has PCFA in its seed coats. Examples of these plants
include the vanilla orchid, Vanilla planifolia, and Jatropha
curcas. Any suitable method for extraction can be used. The fibers
may be formed through electrospinning, although other methods for
forming the fibers, such as extrusion with a carrier polymer, could
be used. The fibers may then be carbonized to increase the carbon
yield.
[0024] In the present disclosure, PCFA was extracted from the
ground seed coats of Vanilla planifolia using an alkaline solvent
and the lignin was precipitated from solution. In parallel, Kraft
lignin was precipitated from the black liquor obtained from the
paper and pulping industry. 50% solutions of each sample in dioxane
were prepared and electrospun through a syringe needle to which a
voltage has been applied. Neither lignin sample was blended with
other polymers to facilitate electrospinning. The spun fibers were
then carbonized. This process resulted in similar carbon yields for
PCFA and Kraft lignin. However, the electrospinning process
produced more continuous fiber with a narrower size distribution in
the case of PCFA compared with Kraft lignin. Both lignins produced
fibers of higher percentage crystallinity (by Raman spectroscopy)
than PAN-based carbon fibers, with PCFA fiber showing the highest
crystallinity, consistent with its more linear molecule. The higher
purity of PCFA and Kraft fibers over PAN-based fibers is expected
to translate into higher mechanical stiffness, thermal and
electrical conductivity.
[0025] Generally, PCFA offers a linear molecular architecture that
helps enable the formation of fibers. The fiber formed from Kraft
lignin has high surface roughness compared to the smooth PCFA
carbon fibers. PCFA based carbon fiber also shows very low defects
compared to Kraft lignin. Carbon defects are associated with
inferior mechanical and thermal properties. Finally, higher ionic
conductivity of Kraft lignin points to remnant impurities and
complex sources of the originating liquid compared to that of
PCFA.
[0026] This disclosure pertains to the fabrication of PCFA-based
carbon fiber. As shown more fully in the examples below, Kraft
lignin has been used as a comparative basis for examining the
carbon fiber obtained from PCFA. Notable is that the PCFA fibers
were successfully electrospun directly from solutions without any
chemical treatment or addition of polymers to provide fiber
extensional flow strength to produce uniform fibers. As reported
previously, Kraft lignin in the unmodified state produced fibers
that were of high diameter (.about.50 .mu.M) and exhibited surface
roughness. In contrast, the PCFA-sourced carbon fibers were of low
diameter (.about.10 .mu.m) and smooth. Manufacture of Kraft lignin
based carbon has utilized co-axial electrospinning to enable melt
strength for long fiber spools to be formed, and the porosity has
been used for activated carbon. However, smooth PCFA-based carbon
fibers can be obtained by direct electrospinning with no fiber
breakage.
[0027] Carbonization at 900.degree. C. imparted more graphitic
properties to the PCFA carbon than to the Kraft lignin, as seen in
the Raman spectroscopy analysis described below, with G/D ratios of
1.92 vs 1.15 respectively. In this respect, the PCFA-derived carbon
compares very well to commercial carbon from PAN and approaches
that based on pitch. The carbon yield is around 86% for both
sources of carbon. Zeta potential shows good dispersion stability
in DI water for carbon from both fibers. On the basis of the
results of the analyses described below, PCFA appears to be a
promising new source of both carbon fibers and pure carbon.
Example 1. Extraction of PCFA
[0028] PCFA (C-lignin) was obtained from seed coats of Vanilla
planifolia. Vanilla seeds were ground to a powder using a
Freezer/Mill 6870 (SPEX Sample Prep, Metuchen, N.J.), then
extracted with chlorofom and methanol three times consecutively. To
isolate PCFA, the extracted seeds were mixed with 1% NaOH in a
liquid to solid ratio of 10. The mixture was then heated to
120.degree. C., and the temperature maintained at 120.degree. C.
for one hour in an autoclave. After cooling, the black liquid was
separated from the residue by filtration, and PCFA was precipited
from the liquid phase by adjusting the pH to 3.0 with concentrated
HCl. The precipited PCFA was separated by centrifugation, washed
with water and freeze dried.
Example 2. Extraction of Kraft Lignin
[0029] The kraft lignin extraction process was as follows. Black
liquor with pH 11.0 and total solids 88.9%, Klasson lignin 25.1%,
and ash 63.8% was received from Zellstoff Pols AG, Austria. The
black liquor was produced as by-product during sulphate pulping of
70% spruce, 25% pine and 5% larch. Kraft lignin (KL) was isolated
from black liquor by acid precipitation with 37% hydrochloric acid.
After lowering the pH to 2, the precipitated sample was filtered on
a Buchner funnel and washed with distilled water twice, to remove
unreacted compounds. The filtered sample was dialyzed against fresh
distilled water for 7 days, and subsequently was freeze dried.
Example 3. Comparative Chemical Analyses of Lignin Samples
[0030] The purity of Kraft lignin samples was determined from the
analyses for Klason lignin, acid-soluble lignin and ash according
to Tappi Standard procedures (T 13 m-54, T 222 om-02, and T15
os-58). The composition of lignin samples was determined by
elemental analysis for C, H, N and S contents by a
Universal-Elemental analyser Vario El III (Elementar, Germany). The
results of elemental analysis and ash were used to calculate lignin
C.sub.9 formulae. Average molecular weights (M.sub.n and M.sub.w),
and polydispersity PDI (M.sub.w/M.sub.n) were determined by gel
permeation chromatography (GPC) instrument equipped with L6000A
Merck-Hitachi pump, PPS sizing column (5 .mu.m, 8.times.50 mm),
three linear PPS gel columns (5 .mu.m, 8.times.300 mm) connected in
series, and a Viscotek differential refractometer/viscometer
(Malvern, UK). The columns were calibrated using a series of 12
narrow molecular weight polystyrene standards with molar mass
ranging from 680 to 1 600 000 g/mol (Polymer Standard Service). The
samples were dissolved in tetrahydrofurane at concentration 4 mg/ml
and were analysed at room temperature. THF was used as eluent at
flow rate of 1.0 ml/min and the injection volume was 100 .mu.l. The
results are shown below in Table 1.
TABLE-US-00001 TABLE 1 Kraft Lignin Sample Klason lignin (wt
%).sup.a 89.6 Total lignin content (wt %).sup.b 92.5 Ash (wt %) 0.2
Carbon (wt %) 64.1 Hydrogen (wt %) 5.6 Nitrogen (wt %) 0.1 Sulphur
(wt %) 2.4 Molecular weight 1749 Polydispersity 2.38
.sup.aEstimated by difference .sup.bKlason lignin with acid soluble
lignin part
Example 4. Electrospinning and Carbonization
[0031] A 50% solution of PCFA was prepared in 1,4 dioxane (boiling
point of 101.degree. C.). The powder was mixed at 50.degree. C. for
4 h, and then transferred to the syringe for electrospinning. The
same process was repeated for Kraft lignin. A 5 ml syringe
(National Scientific, Rockwood, Tenn., Model #57510-5) with an 18
gauge (1.27 mm) 1'' long stainless steel blunt needle with a Luer
polypropylene hub was used. The syringe with needle was placed on a
Razel syringe pump (Model #R99-FM, Razel Scientific Instruments,
St. Albans, Vt.). The rate of syringe pump was 0.763 with a flow
rate of 2.65 ml/h, the distance between the needle and the plate
was 20 cm and the voltage was 20 kV. The solution was pumped from
the syringe. The needle was then charged to the prescribed voltage
using a high voltage power supply (Model #ES30P-5W/DAM, Gamma High
Voltage Research Inc., Ormond Beach, Fla.). The collector plate was
set at the prescribed distance from the needle, covered with
non-stick aluminium foil, and grounded. As the syringe pump and the
high voltage power supply were switched on, the lignin solution
came out of the needle forming a Taylor cone that was attracted by
the electrostatic force towards the grounded collector plate.
[0032] The electrospun fibers from PCFA and Kraft lignin were
subjected to carbonization in a horizontal tube furnace. The
heating and cooling ramp rate was set at 5.degree. C./min. Fibers
were held at 900.degree. C. for 45 min under a flow of nitrogen of
0.5 standard cubic feet per hour (SCFH). The carbon obtained was
analyzed for carbon yield.
Example 5. Comparative Analyses
[0033] Environmental Scanning Electron Microscopy (ESEM):
[0034] A FEI Quanta Environmental Scanning Electron Microscope
(ESEM; FEI Company, Oregon, USA) was used to image the cross
section of the burnt PCFA and Kraft lignin fibers at an
accelerating voltage of 12.5 kV at 10 mm working distance. The
samples were sputter coated with gold-palladium to make them
conductive and make imaging possible.
[0035] Raman Spectroscopy:
[0036] A 532 nm intensity laser was used at 25% power with aperture
of 10 .mu.m slit and objective lens with 10.times. zoom to give a
spot size of 2.1 .mu.m. The scan was done from 750 to 2000 l/cm.
The exposure time was 15 sec. Background and sample exposure was
performed five times. Background was collected before every sample.
This background was subtracted from the Raman spectroscopy results
and a baseline correction was performed.
[0037] Zeta Potential:
[0038] A Delta NanoC particle analyzer from Beckman Coulter
(Pasadena, Calif.) was used to determine Zeta potential. The
dispersions of the PCFA powder and Kraft lignin were made in
deionized water at room temperature and dispersed using sonication
for 1 h.
[0039] Results:
[0040] Solutions of both PCFA and Kraft lignin are
electrospinnable. Continuous electrospun fibers were obtained under
conditions of 20 kV and 2.65 cc/h solution flow rate with a
distance of 20 cm to the stationary collector plate. The ESEM
images of PCFA (FIG. 1A, 1B) and Kraft lignin (FIG. 1C, 1D) suggest
that the fibers obtained are highly uniform with no beads.
Obtaining bead-free fibers depends on the conductivity of the
solution which elongates the Taylor cone formed at the tip of the
needle to give electrospun fibers. During electrospinning of both
PCFA and Kraft lignin a minimum voltage of 20 kV was essential to
overcome the surface tension of the Taylor cone. A 50% solution in
1,4 dioxane at 50.degree. C. gives enough entanglement of PCFA to
spin it into fibers.
[0041] The ESEM images were analyzed using ImageJ.RTM. software
(NIH). The images were corrected for the scale from pixels of the
original tiff image to the known distance on the image to calibrate
for scale. A total of 58 measurements of diameters were made and
the histogram was plotted for the most frequent occurrence of the
diameter range, as shown in FIG. 2. PCFA produced fine uniform
fibers and processed unceasingly compared to Kraft lignin which
could only electrospin for a short period of time. The diameters of
the electrospun fibers from PCFA and Kraft lignin were in the range
of 10.5 to 14 .mu.m and 30 to 40 .mu.m, respectively.
[0042] ESEM images of PCFA and Kraft lignin were also analyzed for
surface variation. As shown in FIGS. 3 and 4, the surface is
significantly smoother in the PCFA lignin compared to the Kraft
lignin sourced carbon fibers.
[0043] As shown in FIG. 5, the Zeta potentials for PCFA and Kraft
lignin carbon powders in deionized water are similar, around
-43.35.+-.0.48 mV and -42.05.+-.2.37 mV respectively. This suggests
that the stability of the carbon particles obtained from PCFA is
good enough to keep them in suspension for long durations. The Zeta
value indicates repulsion between the particles, thus stopping them
from attracting each other and flocculating. The low mobility and
conductivity values indicate that the ionic double layer is thick
due to low ionic strength. The mobility of the particles in the
suspension, 3.40e-004.+-.00 cm.sup.2/Vs, indicates that the
attraction of particles to the electrodes is very low. Table 2
below shows the suspension properties of carbon from PCFA and Kraft
lignin.
TABLE-US-00002 TABLE 2 Zeta potential Mobility Ionic conductivity
Sample (mV) (cm.sup.2/Vs) (mS/cm) PCFA powder -43.35 .+-. 0.48
3.40e-004 .+-. 00 0.028 .+-. 0.00042 Kraft lignin -42.05 .+-. 2.37
-3.1e-04 .+-. 2.0E-06 0.6457 .+-. 0.00051
[0044] Raman spectroscopy was performed to compare the purity of
carbon obtained from PCFA and Kraft lignin powder samples (FIG. 6).
The D and G bands give the defect-derived structures and graphite
derived structure of the carbon, respectively. The D band is due to
the breathing modes of sp.sup.2 atoms in the aromatic ring while
the G band results from sp.sup.2 site stretching of C.dbd.C bonds.
A high G/D ratio is symptomatic of higher crystalline structure.
Carbon from PAN has G/D ratios ranging from 0.57-0.67, while
pitch-based carbon shows higher crystal perfection with G/D ratios
ranging from 2.27 to 7.6. The G/D ratios of PCFA powder and PCFA
carbon are 2.68 and 1.92, respectively (see Table 3 below). It is
important to note that carbonization at 900.degree. C. has
increased the graphitic structure as seen by the intensity of the
G-band (FIG. 6A). There is a 133% increase in highly ordered
G-band. The G-band intensity can also be used to check the purity
of the samples. This is because, unlike with the D band, there is
no effect of chirality on the G-band. Thus the Raman spectra show
that high purity carbon is obtained from PCFA, comparable to PAN.
The highly ordered graphitic structure in PCFA-derived carbon is
correlated to higher mechanical stiffness, thermal and electrical
conductivity. Also, the unburnt Kraft lignin shows no presence of G
and D bands, whereas the carbonized material shows a distinct
presence of both bands (FIG. 6B). The G/D ratio is 1.15, indicating
the presence of ordered graphitic structure. However, PCFA shows a
higher carbon purity when produced with the carbonization method
described in Example 4 above.
[0045] Table 3 below shows a comparison of the Raman spectral
parameters of PCFA and Kraft lignin, before and after
carbonization.
TABLE-US-00003 TABLE 3 PCFA PCFA Kraft Kraft lignin powder carbon
lignin carbon G/D 2.68 1.92 -- 1.15 D/G 1.96 0.92 -- 1.84 G' -- --
2 0.57 M -- -- 7 0.3 Full Width Half Maximum (FWHM) of the related
peaks G 59 122 -- 85 D 116 112 -- 156 G' -- -- 58 30 M -- -- 31 40
FWHM(D) x 227 103 -- 287 (D/G)
* * * * *