recycling of carbon fibres from epoxy composites · fibre polymer composites was improved to about...
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Recycling of Carbon Fibres From Epoxy Composites
João Pedro dos Santos Carvalho
Department of Chemical Engineering, Instituto Superior Técnico
Abstract
The aim of this work was the recuperation of carbon fibres from an aviation composite. The acid
solvolysis with nitric acid was the method chosen. Various concentrations of nitric acid were utilised to
various composite weight/volume of solution ratio of composite. The samples were also exposed to
ultraviolet treatments to simulate atmospheric conditions.
After recuperating the carbon fibres, these were submitted to various characterization tests in order to
conclude which method is better to obtain carbon fibres similar to virgin carbon fibres. The carbon
fibres were submitted to Raman spectroscopy, thermogravimetry, X-ray diffraction, infrared
spectroscopy, mechanical tests and sweeping electronic microscopy.
Virgin fibre samples were submitted to some of the characterization tests and the results were
compared to the recuperated carbon fibres.
It was observed that ultraviolet rays degrade the resin in the composite and slightly damage the
carbon fibre. It was also observed that the fibres submitted to the combination nitric acid at 4M and a
ratio of composite weight/volume of solution equal to 4 obtained better results in the characterization
tests.
Keywords: Carbon fibres, acid solvolysis, nitric acid, composite, ultraviolet.
1. Introduction CFRP is lightweight and easier to
manipulate into shapes than other materials,
including various metals, and it lasts longer
than any other material. That is actually a great
problem. Carbon fibre, which at its most basic
form is carbon graphite, will last virtually
forever. The material is typically not
biodegradable or photo-degradable. The
problem is made worse by the fact that unlike
plastics, aluminium and many other materials,
CFRPs will not degrade, and it is extremely
difficult to recycle. There are, however,
industries that have methods of recycling
CFRPs, but the results are still unsatisfactory.
Yang et al. (2012) verified that using
poly(ethylene glycol) (PEG)/NaOH is highly
efficient at recycling epoxy resins. They
conclude that the main reaction mechanism is
a ester hydrolysis accompanied by a
transetherification process and the most
plausible mechanism is as follows:
Fig. 1 – Plausible reaction mechanism for
solubilisation of epoxy resin using PEG-
200/NaOH
The exact solubilisation products were
investigated using ESI-MS (electrospray
ionization-mass spectrometry) that mainly
include:
Fig. 2 – Solubilisation product obtained by
PEG-200/NaOH reaction
Yang et al. also stated that the
solubilisation rate in this process is better than
the solubilisation rate obtained by Gersifi et al.
(2003).[11]
Gersifi et al. (2003) stated that the
association of titanium (IV) n-butoxide with
diethyleneglycol proved effective in solubilising
epoxy resins. The depolymerisation of the
model matrix and the composite waste are
pushed to the monomer stage and are mainly
constituted by esterdiols, tetraalcohols and
excess of glycol. It was also studied the
solvolysis with monoethanolamine and it was
observed that the reaction is more effective but
its products are solid. Characterization of the
depolymerisation products by NMR (nuclear
magnetic resonance) and MALDI-TOF (matrix-
assisted laser desorption/ionization - time of
flight mass spectrometer) has confirmed the
reaction mechanism of transesterification,
similar to those observed by Yang et al.
(2012), and the presence of other alcoholysis
side reactions.[1]
Palmer et al. (2009) and Pickering et al.
(2006) used the mechanical recycling
approach, which involves breaking down the
composite by shredding, crushing, milling or
other similar mechanical processes and
segregating the resulting scrap pieces, by
sieving, into powdered products (rich in resin)
and fibrous products (rich in fibres). These
recycled composites are usually re-
incorporated in new composites, as filler or
reinforcement, and used in construction
industry, as fillers for asphalt or as mineral
sources for cement. However, this method
reduces significantly the mechanical properties
of the fibres and limits the possibilities for re-
manufacturing.[7, 8]
Meyer et al. (2009) used pyrolysis as a
recycling method. During pyrolysis, the
composite is heated up to 450ºC to 700ºC in
an inert atmosphere. While the fibres are inert
and are eventually recovered, the resin is
volatilized into lower weight molecules..[5, 6]
Pinero-Hernanz et al. (2008) tried various
supercritical fluids to dissolve the resin and
obtained interesting results. However this
method is very expensive to use in industry,
because of the high temperature and the high
pressure that is needed to achieve the
supercritical stage of a fluid.[9, 10]
Kumar et al. (2002) observed extensive
erosion of the epoxy resin when CFRPs are
exposed to ultraviolet radiation and/or
condensation. While the longitudinal fibre-
dominated properties are not affected for the
exposure durations investigated, it was noted
that extensive matrix erosion would ultimately
limit effective load transfer to the reinforcing
fibres and lead to the deterioration of
mechanical properties even along the fibre
dominated material direction.[2]
Lee et al. (2011) proposed and adapted a
circulating flow reactor to a recycling process
for carbon fibre from carbon fibre reinforced
epoxy composite. Lee at al. proposed a
recycling system that was composed of
hexahedral circulating flow reactor made of
quartz, teflon supporter, acid resistance pump
and auxiliaries. They concluded that the
circulating flow reactor most effectively
recycled carbon fibre at 12M of concentration
of nitric acid aqueous solution, 90ºC of
decomposition temperature, 1.8L of nitric acid
aqueous solution per 100g composite and
1.0cm/sec of linear flow rate of circulating
solution. They observed that the liquid phased
decomposition product was a mixture of
nitrated compounds and the tensile strength
loss of the recycled single carbon fibre was
2.91%.[3]
Feraboli et al. (2011) bathed the
composites in boiling sulfuric acid to separate
the fibres from the resin. They observed that
the recycled reinforcement consisted of long
fibres arranged in a random, entangled mat.
They concluded that the recycled panels show
trends and traits similar to those of advanced
carbon fibre and that with further development
efforts, it is possible to realign the recycled
long fibres and re-establish their original
performance.[12]
Shi et al. (2012) recycled glass and carbon
fibres using superheated steam. These fibres
were then surface treated for reuse as fibre-
reinforced polymer composite. They observed
that most residual resin impurities were
removed by the surface treatment and that the
treatment did no adverse effect on bending
strength. The mechanical properties of the
treated composites were determined and
compared with those of recycled reinforced
polymers. The bending strengths of recycled
reinforced polymers were very low, compared
to that of virgin glass fibre-reinforced polymer
and that of virgin carbon fibre-reinforced
polymer. The bending strength of treated glass
fibre polymer composites was improved to
about 90% of that of the virgin glass fibre one,
and the bending strength of the treated carbon
fibre polymer composites was improved to
about 80% of that of the virgin carbon fibre
one.[13]
Akonda et al. (2011) used chopped
recycled carbon fibres from Recycled Carbon
Fibre Ltd., UK, to produce yarns of comingled
carbon fibre and polypropylene blended with
matrix polypropylene staple fibres using a
modified carding and wrap spinning process.
They observed with microscopic analysis that
more than 90% of the recycled carbon fibres
were aligned along the yarn axis and
thermoplastic composite test specimens
fabricated from the wrap-spun yarns had 15–
27.7% recycled carbon fibre volume content.
They also observed that similar to the yarn,
greater than 90% of the recycled carbon fibres
comprising each composite sample made,
showed a parallel alignment with the axis of
the test specimens. The mechanical tests
showed that the average values obtained for
tensile, and flexural strengths were 160 MPa
and 154 MPa, respectively for composite
specimens containing 27.7% recycled carbon
fibres by volume. Akonda et al. concluded that
with such mechanical properties, thermoplastic
composites made from recycled carbon fibres
could be used as low cost materials for many
non-structural applications.[14]
McNally et al. (2007) used reclaimed
carbon fibres obtained from the CARBON-
CLEAN® technology, a thermochemical
process that recovers 95% of the CF, to
manufacture a composite with polyethylene.
They observed that both the Young’s modulus
and ultimate tensile stress increased with
increasing carbon fibre loading, but the
percentage stress at break was unchanged up
to 5 wt % loading, then decreased with further
successive addition of carbon fibres.
McNally et al. concluded that carbon fibres
reclaimed from carbon fibre filled epoxy
prepregs can be readily melt blended with
polyethylene, the carbon fibre was highly
dispersed in the matrix at all loadings and that
the Young’s modulus and tensile strength of
the composites increased by 180 and 27.5%,
respectively, with increasing carbon fibre
loading, suggesting that some degree of
interfacial adhesion is present between the
carbon fibre and polyethylene. They also
concluded that good interfacial adhesion and
wetting between polyethylene and the recycled
carbon fibres is possible, in part, due to the
presence of polar functional groups along the
length of the carbon fibre.[15]
Dannenhauer et al. (2003) proposed a
process for recycling composite materials that
comprise fibres and a matrix by exposing the
composite material to electromagnetic waves
in the form of microwaves. This method is
assisted by suitable solvent, and this solvent
has to be polar. The fibres and the solvent are
put in a TEFLON® container and put inside a
microwave oven with a frequency used
between 300 MHz and 300 GHz. They
observed that the resin was completely
removed from the fibres.[16]
Adam et al. (2014) proposed a method of
extracting and recycling carbon fibres with
furan-2-carbaldehyde. This method consists in
bathing the composite in the solvent and a
swelling agent at a temperature between 50ºC
and 90ºC during about 1 hour to 24 hours. It
was observed that the matrix was removed
and that can be reutilized, and that the fibres
were reusable as well.[17]
2. Experimental Method
The method chosen in this work was the
solvolysis approach[4]
. In this case, different
concentrations of nitric acid were used in
different weight to volume ratios and with
different UV light treatments.[2]
This treatment was performed in a POL-
EKO Apartura UVB oven. Each cycle is
composed by three steps:
Heating to 30ºC and stabilization at that
temperature for 30 minutes;
Heating to 60ºC and stabilization at that
temperature and at 60% humidity for 2
hours and 30 minutes;
Cooling to 25ºC and returning to 0%
humidity and stabilizing at that
temperature for 30 minutes.
The method is carried out in a glass vessel
with a magnetic agitator, at 95ºC for 12 hours
with a 250ml of a nitric acid solution. The nitric
acid solutions were made with a Panreac nitric
acid at 65% (w/w).
Various experiments were carried out to
optimize the method. The variables that were
altered during the optimization process are:
The concentration of nitric acid solution;
The mass of composite to volume of acid
ratio;
The number of cycles of the UV
treatment.
The following table describes the specific
conditions of each experiment.
Table 1 – Conditions of each experiment
[HNO3]
(M)
α (g of
composite/100mL
of solution)
UVB
cycles
Experiment 1 4 4 0
Experiment 2 4 2 0
Experiment 3 6 2 0
Experiment 4 4 2 120
Experiment 5 4 2 249
Experiment 6 2 2 309
Experiment 7 8 6 0
During the experiments, some changes
were observed. At 10 minutes into the reaction,
the liquid solution started showing a light
yellow colour. At 30 minutes into to the
reaction, the solution is bright yellow shows
signs that the viscosity rises. At approximately
1h30 min. the first loose fibres are starting to
show in the liquid, and at approximately 4
hours most of the fibres appear loose from the
matrix.
3. Equipment
The X-Ray diffraction was performed by
using a Philips PW3020, at room temperature,
using Cu Kα radiation (λ= 1.541874 Å)
generated at 40 kV and 30 mA, in the range
(2θ) 10o to 35o, with a step of 3 o/min. with an
aluminium support. The SEM images were
obtained by a FEG microscope. A Instron 5566
was used to do the mechanical tests on the
fibres. A Labram hr evolution Raman
spectrometer was used to do the Raman
spectra. A HATR-FTIR equipment was used to
do the infrared spectra using reflectance
mode (HATR). A Netzsch STA 409 PC
equipment was used to do the TGA spectra.
4. Results
XRD spectra is used to test the structure of
CF’s as shown in Fig. 3. The detailed data are
listed in Table 3.
It is seen that a strong peak ((002) plane)
exists at approximately 2θ = 25o. The interlayer
spacing (d002) and apparent crystallite
thickness (Lc) derived from (002) reflection are
also used as an experimental parameter to
assess the structural properties of the CF’s
Layers of the stacked crystallite are used to
measure the degree of graphitization of CF
determined by Lc/d002.
The value of d002 was calculated using
Bragg’s law, and the crystallite size Lc was
calculated using Scherrer’s formula:
θ
θ
Where θ is the scattering angle, is the
wavelength of the X-rays used (1.541874 Å),
and is the full width at half maximum
intensity (FWHM). The form factor K is 0.9.[18,
20]
Fig. 3 – XRD of carbon fibres with different treatments
Table 2 – XRD results of CFs treated under different reaction conditions
Sample β (o) 2θ(o) Lc (nm) d002 (nm) Lc/d002
Virgin fibres 4.83 26.5 1.7 0.34 5.00
V.F. Pre-treatment 5.41 25.6 1.5 0.35 4.28
C_epoxi 4.13 25.4 2.0 0.35 5.71
C_epoxi_4M_2alfa 5.92 25.9 1.5 0.34 4.41
C_epoxi_4M_UV249_2alfa 5.39 25.7 1.5 0.35 4.28
C_epoxi_4M_4alfa 5.04 26.2 1.6 0.34 4.70
C_epoxi_6M_2alfa 5.56 25.7 1.5 0.35 4.28
Raman spectrogram combines a
prominent surface selectivity with an
exceptional sensitivity to the degree of
structural disorder. Carbon mainly shows two
peaks in the first order (1000–2000cm-1
). One
near 1580cm-1
is corresponding to an ideal
graphitic lattice called G band while the other
near 1360cm-1
is the characteristic peak of sp3
0
500
1000
1500
2000
2500
3000
3500
10 15 20 25 30 35
Inte
nsi
ty
2θ(o)
C_epoxi
C_epoxi_4M
C_Epoxi_4M_UV249
C_epoxi_6M
C_epoxi_4M_4alfa
Virgin Fiber
V. F. Pretreatment
state of C called D band which is due to the
existence of structural disorder.
The D to G band integrated intensity ratio
(ID/IG) is a parameter to quantify the degree of
disorder. Usually, the smaller value of ID/IG
indicates the higher degree of graphitization
of the CF’s.[18, 19, 20, 21]
Fig.4 – Raman spectra of the carbon fibres
Table 3 – Raman results of CFs treated under different reaction conditions
Sample Raman Shift (cm-1
) Area (x10-5
)
ID/IG
FC2M 1347 6.8
2.08 1582 3.3
FC4M 1350 5.1
1.33 1580 3.8
FC4M 120 UV 1348 6.4
1.30 1576 4.9
FC4M 4 alpha 1359 5.8
1.18 1583 4.9
FC6M 1347 9.0
1.35 1571 6.6
FC8M 1358 7.3
1.32 1576 5.5
Virgin Fibre 1345 5.5
1.32 1569 4.1
V. F. Pre-Treatment 1342 7.1
1.20 1574 5.9
Due to limitations of the machine, it was
only possible to test fibres which were 5
centimetres or more long.
Firstly the carbon fibres were glued to a
card specimen holder. Then these specimens
were placed in the machine and underwent a
strain test. The results are exemplified in the
image below (Fig. 5).
After all the specimens were tested, it
estimated the number of fibres in all the
specimen holders so that it was possible to
calculate the Young’s modulus and the
ultimate tensile strength of a single carbon
fibre.
800 1000 1200 1400 1600 1800 2000
Inte
nsi
ty [
a.u
.]
Wavenumbers [1/cm]
CF 2M
CF 4M UV 120
CF 4M
CF 4M 4alfa
CF 6M
CF 8M
Virgin CF
V. F. Pre-treatment
Fig.5- Mechanical test result of a specimen of
carbon fibre
Table 4 – Mechanical properties of the CF
Sample Tensile strength (Mpa) Young's Modulus
(Gpa)
4M 3176 150
4M 4 α 4569 161
6M 4308 221
The ultimate tensile strength corresponded
with the maximum load value and the Young’s
Modulus corresponded with the slope in the
Tensile vs Axial Extension chart.
The values of the ultimate tensile strength
and the Young’s modulus are displayed in the
table 4.
The morphology and geometry are shown
in the SEM figures below:
The diameter measured is approximately
5.5 µm and is assumed that this diameter is
the same among all of the recovered fibres.
The infrared spectra are shown in the
figure 9.
The carbon fibre samples were also
characterized by thermogravimetry under
air, to evaluate the thermal stability after the
recuperation process .The fibres were heated
from 30oC to 1100
oC at 30oC/min using
alumina crucibles. The TGA spectra is shown
in the figure 10 and the fibre degradation
temperatures are shown in the table 5.
Fig.6 – SEM image of Carbon fibre composite
Fig.7 - SEM image of Carbon fibre treated with
a 4M solution of HNO3
Fig.8- SEM image of Carbon fibre with the
diameter measured
0
0,5
1
1,5
2
2,5
3
3,5
0 0,5 1 1,5
Load
(N
)
Extension (mm)
CF 4M test 4
Fig.9- Infrared adsorption spectra of the carbon fibres (virgin and recuperated)
Fig.10- Thermogravimetric analysis spectra of the recuperated carbon fibres
Table 5 – Degradation temperatures of the
carbon fibres
Sample Degradation Temperature (oC)
Virgin CF 767
CF 4M 4alfa 650
CF 4M 120UV A 704
CF 4M 120UV B 754
CF 8M A 638
CF 8M B 654
5. Discusion
The FTIR spectra shows the organic
residue in the fibres, as well as evidence of
CO2 adsorbed in the fibres. Generally, the
recovered carbon fibres show adsorption
bands around 2900 cm-1
and 1700 cm-1
which
are derived from carboxylic acid and carbonyl
functionality respectively. These groups are
formed due to the degradation of the epoxy
resin during the recovery process.[15]
The adsorption band present in all the
recuperated fibres at around 2350 cm-1
corresponds to the CO2 adsorbed. Generally,
the longer the UV treatment, the value of the
CO2 adsorbed in the fibres is higher. This
adsorption band can also correspond to the C-
N triple bond resulting from the nitric acid
attack on the epoxy resin[9]
.
Some fibres show an adsorption band at
around 850 cm-1
, which represents evidence
of epoxy groups[9]
.
The TGA analysis shows that the fibres
treated with UV lose more mass than the fibres
which are not treated with UV, however, they
show more thermal stability than the other
recuperated fibres. This may be because,
during the UV treatment, the graphite
crystallises thus increasing the resistance to
the oxidation process.
0
0,5
1
1,5
2
800 1300 1800 2300 2800
Ab
sorb
ance
Wavenumbers [1/cm]
CF 4M 4alfa CF 6M CF 8M CF 8M Poly CF 2M 309 UV CF 4M
50
100
0 100 200 300 400 500 600 700 800 900 1000 1100
Weig
ht (%
)
Temperature (oC)
CF 4M 4alfa Composite CF 4M 120UV A CF 4M 120UV B CF 8M A CF 8M B Virgin CF
All the samples analyzed in the Raman
show a D to G band integrated intensity ratio
similar to the virgin carbon fibres, except the
samples recovered with the two treatments of
2M concentration of nitric acid and with 309
cycles of UV treatment, which shows a higher
ration, indicating a lower degree of
graphitization. The sample recovered with the
treatment of 4M concentration of nitric acid and
an alpha of 4 shows the lowest integrated ratio,
which indicates a higher degree of
graphitization. The Raman results of this fibre
show that it has less damage and less residue
to the surface[22].
Comparing the Raman of the virgin fibres
and the virgin fibres with a thermal pre-
treatment, it is shown that the fibres with the
treatment show a higher degree of
graphitization, thus the treatment is shown to
increase the strength of the carbon fibre. The
recovered fibre which was treated with a 4M
solution of nitric acid and an alpha of 4 has a
similar integrated intensity ratio in comparison
to the virgin fibre with the thermal pre-
treatment, this tells us that the degree of
graphitization is similar to the virgin fibre with
the thermal pre-treatment.
XRD spectra were also used to test the
structure of CFs. The detailed data are listed in
Table 2. It is seen that a strong peak ((002)
plane) exists at a 2θ = 25o. It is found that the
layers of the stacked crystallite of the
recovered CFs are comparable with the virgin
ones. It reveals that the recovered CFs retains
high degree of graphitization of the virgin CFs.
These results indicate that the structure of CFs
is damaged slightly during the recycling
process. Combining the SEM, Raman and
XRD results, it is believed that the structure of
CFs is not damaged dramatically, and this can
be testified by the tensile strength[22]
.
The mechanical tests show that all the
samples are in the gamma of the tabulated
values for virgin fibres and the fibres treated
with 4M of nitric acid and a α = 4 have a higher
ultimate tensile strength. This result backs up
the conclusions of the XRD and the Raman,
however the sample treated with the 6M of
nitric acid shows a higher Young’s modulus.
In the SEM images the results back up the
previous conclusions; the more UV cycles
used, the more resin is removed. But the SEM
images also show that the more cycles used,
the more damage the fibres suffer. It also
shows that, the more concentrated the
solution, more resin is removed and more
damage appears on the fibres.
Without the UV treatment, the fibres
treated with the 4M solution show less damage
on the surface than those treated with the 6M
and 8M solutions, and the loss of resin is
almost the same.
With the UV treatment, the fibres treated
with 309 cycles of UV light, even though they
were treated with a 2M solution, show a lot
more damage than those with less cycles and
a 4M solution.
6. Conclusion
In conclusion, it is possible to recuperate
carbon fibres from an epoxy resin with nitric
acid and that the best concentrations of nitric
acid are between 4M and 6M, the best α is
between 2 and 4 and that the UVB light
treatment is helpful in removing the epoxy
resin, but too much exposure can damage the
carbon fibres.
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