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Research Article
Mechanical Characterization of Carbon Fibres Recycled by Steam
Thermolysis: A Statistical Approach
M. Boulanghien ,
1
,
2
,
3
M. R’Mili,
4
G. Bernhart,
1
F. Berthet,
1
and Y. Soudais
2
1
Institut Cl´
ement Ader (ICA), Universit´
ede Toulouse, CNRS, Mines Albi, UPS, INSA, ISAE-SUPAERO, Campus Jarlard,
81013 Albi CT Cedex 09, France
2
Universit´
ede Toulouse, Mines Albi, UMR CNRS 5302, Centre RAPSODEE, Campus Jarlard, F-81013 Albi cedex 09, France
3
Alpha Recyclage Composites, 4 rue Jules V´
edrines, 31400 Toulouse, France
4
INSA-Lyon, MATEIS, UMR CNRS 5510, Universit´
ede Lyon, 7 Avenue Jean Capelle, 69621 Villeurbanne, France
Correspondence should be addressed to M. Boulanghien; mboulang@mines-albi.fr
Received 16 March 2018; Accepted 23 April 2018; Published 20 May 2018
Academic Editor: Akihiko Kimura
Copyright ©2018 M. Boulanghien et al. is is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is
properly cited.
e recent development of technologies for recycling carbon fibre reinforced plastics (CFRPs) leads to the need to evaluate the
mechanical response of recycled carbon fibres. As these fibres are likely to be degraded during the recycling treatment, it is very
important to determine their tensile residual properties so as to evaluate their ability as reinforcement for new composite
materials. Carbon fibres reclaimed by a steam-thermal treatment applied to degrade the epoxy resin matrix of a CFRP are here
analysed. Two conditions were chosen so as to reach two degradation efficiency levels of the steam thermolysis. Several carbon
fibre samples were selected for mechanical testing carried out either on single filaments using single fibre tensile tests or on fibre
tows using bundle tensile tests. It is shown that the single fibre tensile test leads to a wide variability of statistical parameters
derived from the analysis. Bundle tensile tests results were able to indicate that fibre strength of recycled carbon fibre is similar to
corresponding as-received carbon fibres thanks to a statistically relevant database. Wide number of tested filaments enabled
indeed to obtain low scatters.
1. Introduction
Carbon fibre reinforced plastics (CFRPs) have been widely
used these last years in many industrial, sportive, and
transport applications, especially for their low weight and
high strength. e global carbon fibre market is expected to
reach high annual growth rates until the next few years.
Although the current global demand for carbon fibre, 82,400
tons per year, is lower than expected in last year’s market
reports [1, 2], it is still expected to grow at a minimal annual
rate of 9.0%. Global demand in carbon fibres is expected to
reach 116,000 tons per year in 2021 for the less optimistic
scenario [3] whereas other projections estimate a 150,200
tons demand [4, 5]. In addition, the carbon fibre reinforced
composites market obviously shows very similar growth
trends. While in 2013 the global demand for this kind of
material was 72,000 tons, recent reports expect this market
to reach a 191,000 tons demand by 2022 [6]. e high growth
perspectives of wind turbines and aerospace industries can
mainly explain the intensification in using CFRP as their
recent introduction in the automotive industry. is dra-
matic increase in using carbon fibre means that the quantity
of generated waste will also rise significantly, either as an off-
cut or as an end-of-life composite product. us, it appears
to be critical to develop suitable composite recycling tech-
nologies that could offer interesting environmental and
economic perspectives. If the environmental and social
responsibilities are the first arguments for such development
efforts, market economics is still a key factor. Considering
that the carbon fibre market’s potential is clearly affected by
the high price of carbon fibre, although its production ca-
pacity is nowadays growing, there is a huge opportunity for
Hindawi
Advances in Materials Science and Engineering
Volume 2018, Article ID 8630232, 10 pages
https://doi.org/10.1155/2018/8630232
future or existing recycled carbon fibre producers and
processers to answer new needs.
Although landfilling is currently the main option to
manage CFRP wastes, the high added value of carbon fibre
associated with a restrictive European legislation [7, 8] has
driven researchers and engineers to look for new recycling
technologies, especially as life cycle analysis already showed
that the environmental benefit is much higher for a recycling
scenario than for a classical incineration or landfilling [9–11].
ese last years, the main studied approach has been to
degrade the organic matrix to leave clean the carbon fibres,
these ones being valorised as reinforcement in second-
generation composite products. Various technologies fo-
cused much effort in this way: solvolysis [12], pyrolysis [13],
and steam thermolysis [14].
Solvolysis is a chemical process based on the organic
matrix depolymerisation by means of a solvent. Most of the
time, near- or supercritical conditions are required to obtain
the best results and avoid the use of aggressive chemical
solvents that make the treatment more complex. Methanol
[15], propanol [16–18], water [19, 20], or even a mixture of
water and ethanol [21] in supercritical conditions were
successfully used: the removal of an epoxy matrix can reach
100% without loss of tensile strength of reclaimed carbon
fibres. Although more investigation efforts have been made
in these methods, there is still no example of an industrial
scale launch of this technology applied to the CFRP recy-
cling: supercritical reactors are expensive as they have to be
designed for high temperatures, high pressure, and a cor-
rosive environment.
Pyrolysis is based on the organic matrix thermal deg-
radation. It has been the most studied thermal process
[22–24], and some variations can be found as the microwave
heating pyrolysis [25, 26]. Depending on the matrix nature,
and the considered variation, the efficiency of such a treat-
ment is variable: from 80% to 99% of eliminated resin.
Reclaimed carbon fibre tensile strength can be degraded due
to the presence of char on the fibre surface that needs to be
eliminated by an air posttreatment. However, in spite of
lower results than what can be obtained in solvolysis, py-
rolysis is a cost-efficient technology well suited to the rel-
atively undeveloped composites recycling market. ese
research efforts have even been commercially applied by
European companies such as ELG Carbon Fibre (United
Kingdom), Karborek (Italia), Reciclalia (Spain), or CFK
Valley Stade Recycling (Germany) and American ones such
as Adherent Technologies Inc or Carbon Conversions.
Finally, steam thermolysis is a thermochemical process
using superheated steam at environmental pressure for
degrading organic materials. It is a cost-efficient technology
as no energy-consuming posttreatment of reclaimed fibres is
needed, nor high pressure environment requiring discon-
tinuous working flow and expensive reactors. It has been
applied to the material recovery of circuit boards [27], to the
degradation of polyimide [28], or to the production of oil
from biomass [29]. Only few studies focused on the steam-
thermolysis process applied to the recovery of carbon fibre
from CFRP wastes [10, 30–33]. Steam thermolysis enables to
efficiently degrade the organic matrix of the CFRP waste,
which makes this technology a serious alternative. e aim
here is to evaluate the efficiency of this technology by
proposing a true mechanical characterization of the
reclaimed carbon fibres considering two techniques: single
fibre tensile test (SFTT) and bundle tensile test (BTT). Using
the widely used SFTT technique, some inherent variability
sources of the tensile strength determination can appear as
the specimen selection, the damage of the fibres during the
sampling operation, and the difficulty in getting a perfect
alignment of the fibre with the tensile machine. Hence, the
bundle tensile test can become an alternative for the tensile
strength determination of recycled carbon fibres, as it has
been successfully used to study virgin glass, ceramic, and
carbon fibres.
2. Theoretical Background
2.1. Bundle Model. e theoretical model of dry bundle of
fibres considers a discrete set of Nparallel fibres with sta-
tistically distributed strength. When the bundle is loaded,
fibres’ mechanical behaviour is linear elastic until their
failure at the applied stress σi,i�1,..., N. When a fibre
breaks in the bundle, the supplementary load that was
carried by the broken fibre is equally distributed. Two
distribution cases can be differentiated. e global load
sharing (GLS) considers that the supplementary load is
equally distributed among the survival fibres whereas the
local load sharing (LLS) considers that the supplementary
load is equally distributed among the neighbouring fibres.
e first case is here considered but needs to fit assumptions,
called Coleman’s conditions: fibre length must be constant
within the bundle, stress-strain relationship follows Hooke’s
law until failure, the released load at a fibre break is uni-
formly distributed among the surviving fibres, and no ex-
ternal phenomena should lead to a premature fracture of
fibres. As a consequence, any friction phenomena between
fibres within the bundle must be avoided as it would lead to
a catastrophic fracture of the whole bundle. Specific cares are
taken to avoid this effect.
2.2. Statistical Distribution of Fibre Strength. Fracture of
carbon fibres is likely to be caused by flaws within the
gauge length. Flaws are randomly distributed and show
a high heterogeneity in size, location, and severity. en,
a wide variation in failure load is expected and the ultimate
tensile strengths measured on specimens have a statistical
distribution. Weibull analysis is a well-known method
typically used for fracture statistics for brittle materials.
For a single gauge length and uniform uniaxial tensile
stresses, the Weibull equation of failure probability is
given by:
P�1−exp −V
V0
ε
ε0
m
,(1)
where Vis the stressed volume and V0a reference volume,
ε0�σ0/Ef,σ0being the scale factor, Efthe Young modulus
of a fibre, and mthe Weibull modulus. As a common ap-
proach, a Weibull diagram is usually constructed by using
2Advances in Materials Science and Engineering
empirical estimators of failure probability. en, the sta-
tistical parameters are obtained by fitting (1) to the Weibull
plot.
However, the validity of the normal distribution to
describe the distribution of strengths has already been
shown [34] and the statistical parameters that were derived
from are considered to provide a better fit to the data.
Besides, it was demonstrated that the statistical parameters
derived from a Weibull distribution showed a wide vari-
ability due to the construction of Weibull plots using an
estimator and the sample size generally too small that does
not enable to take into account the natural variability of
material properties. erefore, a statistically relevant data-
base and normal distribution are used for the analysis of
failure data. Equations of probability density function f(ε)
and normal distribution PN(E≤ε)are as follows:
f(ε) � 1
S���
2π
√exp −(ε−μ)2
2S2
,
PN(E≤ε) � ε
0
f(ε)dε,
(2)
with εthe strain, μthe mean of strain, and Sits standard
deviation.
2.3. Bundle Behaviour. Assuming that the applied load is
uniformly distributed among the surviving fibres in the tow
and that fibres have a linear load-strain relationship up to
breakage, the force-strain relation during a tensile test is
given by [35]:
F(ε) � N0·Af·Ef·ε· [1−P(ε)],(3)
where N0is the number of initially loaded fibres, Afis the
cross-sectional area of each of the fibres, Efis their Young’s
modulus, εis the applied strain, and P(ε)is the probability of
failure of a fibre at a strain ε, given by (2).
3. Materials and Methods
3.1. Composite Manufacturing. Composite samples were
made by liquid resin infusion. A low-viscosity bicomponent
system was used: a Sicomin SR1710 Infusion epoxy resin
mixed with a Sicomin SD8822 hardener. A twenty hours at
room temperature plus sixteen hours at 60°C polymerisation
cycle were applied before removing the system from the
mould. Details of procedure can be found in a work of Balea
et al. [36]. Carbon reinforcement was a carbon twill 2 ×2
(Hexcel 46285 U1200) made from AS4C carbon fibres.
Sixteen 400 ×400mm plies were stacked so as to obtain an
approximately 800 grams plate, for a 4 mm final thickness.
e average fibre mass fraction was 66% corresponding to
a fibre volume fraction of 55.5%. ese plates were cut by the
mean of a circular saw in order to get 50 ×120 mm samples
able to be used in the steam-thermolysis reactor.
3.2. Recycling Carbon Fibres. e recycling was conducted in
a bench-scale reactor as shown in Figure 1. Previous in-house
produced composite samples are treated by steam thermolysis
so as to reclaim carbon fibres. e thermochemical process
uses superheated steam at atmospheric pressure in order to
degrade the organic matrix of the composite.
A removable crucible was made from a stainless-steel
fabric (own design, 1000 mL). is crucible was coupled with
a thermogravimetric analyser and placed within the heating
zone. e experimental reactor is provided with an easy
opening chamber located on the top of the apparatus. Once
the experimental parameters reached the desired level, the
chamber that contains scrap composites samples (100 g) is
opened so as to let them fall into the reactor. After epoxy
resin was decomposed, and once the system cooled down,
recycled carbon fibres were collected from the reactor. No
cleaning of the surface is required before their use. ree
categories of products are actually collected: a solid fraction
that is constituted of recycled carbon fibres, a permanent
gaseous fraction principally constituted of methane and
carbon monoxides, and a last condensable gaseous fraction
that is constituted of pyridines, benzene, and phenols [31].
A unit made up of a steam generator and a nitrogen
input manages atmosphere control. e experimental device
is designed to operate in a wide range of conditions: tem-
perature from 100 to 1000°C, steam flow rate from 0 to
1000 g·h
−1
, and nitrogen flow rate from 0 to 20 L·min
−1
.
Experimental conditions and reclaimed samples are de-
scribed in Table 1. Experiments were carried out under
atmospheric pressure for two hours at two temperatures:
400°C and 500°C. In both cases, the nitrogen flow rate was set
to 10.8 L·min
−1
whereas the steam flow rate was 90 g·h
−1
.
Reclaimed carbon fibres from these treatments are, re-
spectively, named RF400 and RF500.
3.3. Fibre Morphology. Yields of eliminated resin were
measured by dissolution of remaining resin with hot sul-
phuric acid according to the French standard NF EN 2564.
Environmental scanning electron microscopy (ESEM)
was used to observe surface texture and morphology of the
fibres as well as visual signs of residual resin impurities. Fibre
bundles of each sample were randomly selected and
mounted on an adhesive carbon layer stuck onto an alu-
minium stub. As carbon fibre is conductive, no other specific
preparation was needed. e acceleration voltage was 20 kV.
Diameters of the fibres were also measured using image
analysis with ImageJ software. For each sample, an average
diameter was determined by measuring a population of 200
fibres from an image database obtained during the corre-
sponding ESEM analysis.
3.4. Mechanical Properties
3.4.1. Single Fibre Tensile Test. e most common technique,
the single fibre tensile test (SFTT), measures the strength of
individual fibres. By measuring many fibres, a wide pop-
ulation can be formed and used for stress analysis. is test
was employed to determine the tensile strength of the three
fibre types of the study. Method is based on international
standards ISO 11566 [37]. A single filament is bonded to
Advances in Materials Science and Engineering 3
a paper window with cyanoacrylate Loctite 409. en, the
specimen is inserted into a tensile rig equipped with a 5 N
load cell. e carbon fibre has to be carefully aligned with the
tensile testing machine axis. Each side of the paper window
was cut before testing. e gauge length was 25 mm. e
crosshead speed was set to 0.1mm/min. Carbon fibre
specimens were loaded at room temperature until failure,
and the force displacement curve was recorded. At least 40
filaments were tested for each fibre type, that is, VF, RF400,
and RF500.
3.4.2. Bundle Tensile Test. Mechanical tests were also carried
out on fibre bundles using bundle tensile tests (BTTs) so as to
quantify the tensile strength of the recycled carbon fibres. It
is based on the random and individual fibre failure within
the bundle. erefore, statistics laws are used for analysis.
is statistical data approach enables to take into consid-
eration a wide single filament population.
One of the difficulties is the measurement of a reliable
bundle strain. An extensometer is placed on heat shrink
tubes previously threaded on each tip of the bundle, as it is
shown in Figure 2, to define the gauge length. Each tip is
impregnated with Araldite 2015 resin and then poly-
merised at 70°C for one hour. Impregnated tips are then
inserted in metallic tubes and filled again with Araldite
2015 resin and polymerised at 70°C for one hour. Metallic
tubes enable a regular clamping by tensile grips. During
any of these preparation steps, a specific care must be
taken to avoid any handling of the fibre bundle within the
gauge length. Before loading, the sample is lubricated by
petroleum wetting, avoiding premature rupture due to
friction phenomena between fibres within the bundle.
is meticulous experimental procedure is also described
in [38, 39].
e tensile tests were performed using a pneumatic
testing machine with a 2 kN cell. ey were carried out at
Table 1: Samples of the study and associated steam-thermolysis experimental conditions.
Samples Treatment temperature (°C) Nitrogen flow rate (L/min) Steam flow rate (g/h) Treatment time (h)
VF Virgin fibre (reference) — — —
RF400 400 10.8 90 2
RF500 500 10.8 90 2
Removable crucible
ermogravimetric analyser
Composite samples
Easy opening chamber Output Gas
Steam
Nitrogen
Heating zone
Temperature measurement
Figure 1: A schematic diagram for the recycling process.
4Advances in Materials Science and Engineering
room temperature under constant displacement rate of
0.06 mm/min on specimens prepared according to the
previous procedure with a gauge length of 60 mm. Carbon
fibre bundles were loaded until failure, and the load dis-
placement curve was recorded. For each fibre type, about
3000 filaments were tested in each tow. For RF500 fibre, 3
tows were tested so as to make sure measurements are
repeatable.
3.4.3. Methods of Failure Data Analysis. For single fibre
analysis, the means of ultimate strengths are known by
collecting individual data. Both normal and Weibull dis-
tributions are used. Weibull plots are constructed using an
empirical distribution function Pj� (j−0.5)/N with Nthe
sample size and jthe specimen number.
For bundle analysis, the mean of fibre tensile strength
and its standard deviation are obtained by fitting an ana-
lytical curve based on (4) to the experimental data. Firstly,
the load-strain curve of the bundle is determined by the
tensile test as described in 3.4.2. en, the initial slope of the
linear part of the analytical curve is fitted to the experimental
one. Equation (3) can also be written as:
F(ε) � N0·Af·Ef·ε· [1−P(ε)] � R0ε· [1−P(ε)],(4)
where R0is the initial slope of the (F−ε) curve. Finally, by
fitting the nonlinear parts of experimental and analytical
curves, the mean of strains to failure μand its standard
deviation Sare determined. Assuming Young’s modulus of
each type of fibre is constant, the mean of ultimate tensile
strength of each type of fibre and its standard deviation can
be determined.
4. Results
4.1. Efficiency of Steam-ermal Treatments. e tempera-
ture is an important parameter on the degradation kinetic
and thus on the efficiency of the treatment. Measurements of
yields of eliminated resin are shown in Table 2. A 400°C
thermolysis did not enable the elimination of all the resin of
the composite (yield of eliminated resin reached 95% in
mass) whereas the 500°C treatment was more effective and
enabled to degrade all the epoxy resin (yield of eliminated
resin is higher than 99% in mass).
Figure 3 shows an ESEM image of the virgin fibre VF
and recycled carbon fibre RF500. Examination of images of
several fibres from different batches clearly showed no
visible alteration of the surface topography due to steam
thermolysis. Similar regular and clean surfaces are observed,
indicating the efficiency of the treatment that removed the
most part of the resin of the composite material. Recycled
RF400 fibres are shown in Figure 4. A few small particles can
be seen and are attributed to resin residues that stuck on the
surface. e 400°C steam-thermal treatment left little
quantities of residual resin on a smooth and regular surface
(5% by mass of residual resin). e particles have a size
ranging from 2 to 20 micrometres, avoiding individual fibres
to be properly separated. ese observations obviously show
the importance of temperature on the degradation kinetic.
e mean diameters were calculated as 7.1, 6.9, and 6.9 μm,
respectively, for VF, RF400, and RF500 fibres (Table 1). is is
in good agreement with the value of 6.9 μm provided by the
manufacturer [40]. It may be inferred from the similarity of the
mean and standard deviation values of the fibres, with visual
evidence from the ESEM, that there was no alternation to the
fibre morphology.
4.2. Mechanical Properties
4.2.1. Single Fibre Mechanical Analysis. Two statistical pa-
rameters are deduced from the analysis: the mean of strength
μand its standard deviation S. From the experimental data,
the 95% confidence interval of mean value is also established
as it is often used as an indicator of the precision of an
estimate derived from an analysis. For a sample size N�40,
μthe sample mean, and Sthe standard deviation, the 95%
confidence interval of mean value (Ic) is given by:
Ic�μ−2.02 S
��
N
√;μ+2.02 S
��
N
√
.(5)
Statistical parameters of normal distribution of strength
deduced from SFTT are reported in Table 3. e average
tensile strength of RF500 fibre is slightly different from that
of the corresponding virgin fibre VF. A 4% decrease was
observed. However, the result shows a high degree of var-
iability with a standard deviation of about 540 MPa for
a tensile strength of 3610 MPa. Looking at the frame given by
these 95% confidence intervals, it appears to be difficult to
obtain reliable results. Indeed, there is no statistically rep-
resentative difference between the two samples. us, it
could be premature to affirm that the tensile strength loss is
really significant or not, although it could be negligible
regarding the low decrease of only 4%. Nevertheless, it can
be stated that reclaimed RF400 fibre showed substantial
strength degradation relatively to the virgin fibre. Even
taking into account the high degree of variability of mea-
surements, tensile strength loss of RF400 fibre is likely to be
Table 2: Studied samples and associated steam-thermolysis ex-
perimental conditions.
Fibres Yield of eliminated
resin (%)
Mean diameter (standard
deviation) (μm)
VF — 7.1 (0.7)
RF400 95 6.9 (0.7)
RF500 >99 6.9 (0.7)
Fibre bundle
Extensometer
Tensile grip
Figure 2: Bundle tensile test configuration.
Advances in Materials Science and Engineering 5
significant. is could be explained by the presence of re-
sidual resin on the fibre surface that could act as stress
concentrators leading to a premature failure of the fibre.
Figure 5 shows mean stress-displacement curves obtained
from single fibre testing and confidence interval on the mean
value of tensile strength and displacement at failure. As it can
be seen that average value of failure strain is lower than that
of virgin and RF500 fibres, it confirms that the single fibre
fails before reaching its maximum stress level.
Weibull diagrams derived from this analysis are also
presented in Figure 6, as they are a usual approach for
describing failure behaviour of brittle materials. ey are
compared to log-log graphs of normal distributions of stress
to failure for each sample. A good agreement between both
distributions is obtained for RF400 fibre. However, a clear
discrepancy can be noticed at the low failure probabilities for
VF and RF500 fibres. e RF500 Weibull plot suggests the
presence of two domains reflecting two distinct failure
modes for this fibre whereas it does not seem to be the case in
Figure 7 showing the probability density function of this
fibre and the associated experimental points. Indeed, at the
lower stress values, experimental data do not clearly show
two distinct populations. Many other reasons can be ad-
vanced to explain discrepancies on Weibull plots and high
scatters observed on SFTT results: the use of an empirical
estimator, the selection and damage of the fibres during the
operation of sampling, or the low sample size leading to
a low representativity in the case of brittle materials [35, 41].
A wide distribution in flaw size is inevitable considering the
selection of test specimens. While variability cannot be
avoided until a relevant database is used for failure analysis,
Table 3: Statistical parameters of normal distributions obtained
from single fibre tensile tests analysis and related 95% confidence
intervals.
Fibre
samples
Mean of tensile
strength (MPa)
Standard
deviation
(MPa)
95% confidence
interval (MPa)
VF 3776 547 146
RF400 3272 672 179
RF500 3610 540 144
4000
3000
2000
1000
0
Applied stress (MPa)
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Displacement (mm)
VF
RF400
RF500
Figure 5: Mean displacement-applied stress curves obtained from
SFFT and 95% confidence intervals.
(a) (b)
Figure 3: ESEM images of VF (a) and RF500 (b) fibres (×5000).
(a) (b)
Figure 4: ESEM images of RF400 fibre (a: ×1000; b: ×5000).
6Advances in Materials Science and Engineering
there is no means to evaluate the validity of this selection and
so to validate the full strength retention of recycled carbon
fibres.
4.2.2. Bundle Mechanical Analysis. Figure 8 shows typical
load-strain curves obtained from bundle mechanical anal-
ysis. It is easy to see that a good agreement is obtained
between experiment and model: experimental curve and
normal distribution-based curve were well fitted. e load
decrease beyond maximum fits well with that obtained
experimentally. Maximum load also depends on the number
of filaments in each tested tow and is consequently not
always the same for a same sample. Exact number of fila-
ments is determined from the initial slope of the load-strain
curve. Statistical parameters of normal distribution of
strength extracted from analysis of bundle tensile tests are
listed in Table 4.
e RF400 sample shows the lowest mean strength of
3657 MPa whereas the VF sample and RF500 sample show
a quite similar mean strength of about 3860 MPa. Variability
of the results first seems to be as high as that obtained for
single filament tensile tests. However, as the tested pop-
ulation is very wide (Table 4), confidence intervals are lower
than those obtained by single fibre tensile tests analysis. is
enables to get more confidence on the precision of the es-
timate. us, no significant difference can be noticed be-
tween RF500 fibres and VF fibres, indicating that steam
3
2
1
0
–1
–2
–3
–4
–5
Ln[–Ln(1 – P)]
7.6 7.8 8.0 8.2 8.4 8.6
Ln[σ (MPa)]
RF400 normal distribution
RF400 Weibull distribution
(a)
3
2
1
0
–1
–2
–3
–4
–5
Ln[–Ln(1 – P)]
7.6 7.8 8.0 8.2 8.4 8.6
Ln[σ (MPa)]
RF500 normal distribution
RF500 Weibull distribution
VF normal distribution
VF Weibull distribution
(b)
Figure 6: Comparison of Weibull plot and normal distribution of stress to failure of RF400, RF500, and VF fibres (log-log plots).
2
4
6
8
10
Failure events
2000 3000 4000 5000 6000
Tensile strength (MPa)
Experimental points
RF500 probability density function
Figure 7: Normal probability density function of RF500 fibre and
frequency histogram of failure events.
100
200
300
400
Load (N)
0.0 0.5 1.0 1.5 2.0 2.5
Strain (%)
VF-analytical
VF-experimental
RF400-analytical
RF400-experimental
RF500-analytical
RF500-experimental
Figure 8: Typical load-strain curves obtained from bundle me-
chanical analysis and their fitted analytical curve.
Advances in Materials Science and Engineering 7
thermolysis enables to retain tensile strength of the
reclaimed carbon fibre RF500. It shows that steam-thermal
process has only little effect on carbon fibres’ mechanical
properties, although the recycling was performed at 500°C.
On the contrary, a decrease of almost 200 MPa affected
RF400 fibres. Resin nodules on the surface of RF400 fibres
could be a contribution to the increase of friction between
filaments during the tensile test. Friction in BTT leads to
a premature failure of the neighbouring fibres in the tow [42]
contributing to a steep load decrease beyond the highest
measured load. However, the curve seems to be smooth and
does not show any signs of fibre friction especially as an-
alytical curve fits very well with experimental data. Indeed,
analytical data are based on bundle theory that considers
that fibres are independent. As in single fibre tensile tests, the
RF400 tensile strength decrease rather suggests that resin
nodules could act as stress concentrators that lead to pre-
mature failure of single filaments in the tow.
5. Discussion
Figure 9 shows that tensile strength values obtained from
bundle tensile tests are in good agreement with those ob-
tained by SFTT although a slight difference can be noticed.
However, when taking into account the larger gauge length
of tows (60 mm instead of 25 mm for single fibres), tensile
strengths should be much lower than those obtained by
single fibre testing. Indeed, carbon fibre tensile strength is
dependent on its length [43]. More generally, the geometry
of carbon fibre plays an important role in its strength
[44, 45]. e higher is its length, the larger is the number of
flaws and thus the probability to find a severe flaw that leads
to fracture of the fibre. Just as the fibre diameter that is
related to the fibre volume that increases the probability to
find a severe flaw. at is why higher gauge lengths should
lead to lower strengths. For these reasons, experimental data
must be statistically analysed. Taking into account a statis-
tically significant sample size, bundle tensile test enables to
overcome uncertainties that usually affect single fibre tensile
tests analysis as the specimen selection, the damage of fibres
during sampling, or the sample size. is is why it is rea-
sonable to consider that differences observed between
bundle and single filament testing results confirm that
variability in single fibre testing is high and inevitable and
that results that are derived from could have likely been
higher or lower if experiments were repeated. On this point,
repeatability of the bundle tensile test was investigated on
RF500 fibre. Table 5 shows that only very slight difference
can be seen between average tensile strengths, lower than
1%. Most of all, the 95% confidence interval is quite the same
from one experiment to another. It only changes a little on
account of the change in the number of filaments in the tow
that directly has a consequence on this interval value. is
test is a repeatable way to generate large databases in
a reasonable amount of time in order to take into account the
heterogeneity of carbon fibres that naturally leads to high
scatter in tensile strength results [45] if only a small pop-
ulation is considered. In this study, bundle tensile test en-
abled to characterize mechanical properties of recycled
carbon fibres with a good precision. However, the BTT needs
very meticulous preparation and advanced statistics to be
implemented. At the contrary, SFTT only needs an easy-to-
follow procedure and data can be readily analysed. Also,
geometry of most of CFRP recycling bench-scale reactors
does not enable to reclaim recycled carbon fibre lengths
higher than 50 mm, which makes it difficult to determine
their tensile properties by BTT.
6. Conclusion
Steam-thermal process was used in a bench-scale reactor to
recycle carbon fibre from epoxy resin/carbon fibre com-
posites. Properties of the recycled carbon fibres were
characterized using ESEM, single fibre tensile test, and
bundle tensile test. Carbon fibres were properly separated
from polymer matrix during the treatment, showing that
a steam-thermal treatment is efficient and enables to reach
high resin elimination levels.
4500
4000
3500
3000
2500
Mean strength (MPa)
VF VFRF400 RF400RF500 RF500
Single bre
tensile test
Bundle tensile
test
Figure 9: Mean strengths and their 95% confidence intervals
obtained from single fibre tensile tests and bundle tensile tests.
Table 5: Statistical parameters of normal distributions obtained
from three RF500 bundle tensile tests analysis and related 95%
confidence intervals.
Sample
number
Number of
filaments
tested
Mean of
tensile
strength
(MPa)
Standard
deviation
(MPa)
95%
confidence
interval
(MPa)
1 2940 3852 591 18
2 2615 3849 598 19
3 2850 3864 644 19
Table 4: Statistical parameters of normal distributions obtained
from bundle tensile tests analysis and related 95% confidence
intervals.
Fibre
samples
Number of
filaments
tested
Mean of
tensile
strength
(MPa)
Standard
deviation
(MPa)
95%
confidence
interval
(MPa)
VF 3240 3864 573 20
RF400 3390 3657 437 15
RF500 2940 3852 591 19
8Advances in Materials Science and Engineering
Two techniques were used for mechanical character-
ization of recycled carbon fibres. Single fibre tensile test did
not allow to validate the full strength retention of recycled
carbon fibres due to unavoidable high variability of the
results. Bundle tensile tests were able to show that a 500°C
steam-thermal treatment enables to leave clean carbon fibres
with no degradation of tensile properties. us, advantages
of bundle tensile tests were highlighted: no selection of
specimen and a relevant database that enabled to get reliable
results.
erefore, steam thermolysis not only degrades the
whole part of matrix resin of the composite so as to leave
perfectly clean carbon fibres but also enables to recover
fibres with full tensile strength retention. Valorisation of
these fibres could be possible. Properties of composites made
from recycled carbon fibres should be measured so as to
reveal the viability of such a process to produce recycled
carbon fibres from epoxy-based composite materials. Recent
works considered different ways to reintroduce them in
structural components although potential applications are
critical to identify [46–48]. e recycling of CFRP is ac-
quiring a considerable importance due to legislative context,
and the need to find sustainable solutions for waste pro-
cessing. Steam-thermal process also demonstrated its abil-
ities in this field.
Data Availability
Analysed and generated datasets underlying the findings of the
current study are available from the corresponding author on
request.
Conflicts of Interest
e authors declare that there are no conflicts of interest re-
garding the publication of this paper.
Acknowledgments
e work presented in this paper was funded by French
company Alpha Recyclage Composites and French association
ANRT (Association Nationale Recherche Technologie). e
authors gratefully acknowledge their support.
References
[1] IHS Markit, “IHS chemical carbon fibre,” in Chemical Eco-
nomics Handbook, IHS Markit, London, UK, 2016.
[2] Smithers Apex Market Intelligence, e Future of Carbon
Fiber to 2017: Global Market Forecasts, Smithers Apex,
Leatherhead, UK, 2012.
[3] T. Kraus and M. K¨uhnel, e Global CFRP Market, Com-
posites market report 2015: market developments, trends,
outlook and challenges, 2015.
[4] S. Das, J. Warren, and D. West, Global Carbon Fiber Composites
Supply Chain Competitiveness Analysis, CEMAC (Clean Energy
Manufacturing Analysis Center) Technical report, 2016.
[5] C. Red, “Global markets for carbon fiber composites: adap-
tations to high growth and market maturity, composites
world,” in Proceedings of the Carbon Fiber 2015 Conference,
Knoxville, TN, USA, December 2015.
[6] M. K¨uhnel and T. Kraus, “e global cfrp market 2016,” in
Proceedings of the Experience Composites 2016 Conference,
Augsburg, Germany, September 2016.
[7] European Parliament, Council Directive 2000/53/EC (End-of-
Life Vehicles), Official Journal of the European Union, L269,
September 2000.
[8] European Parliament, Council Directive 2010/5/UE (Industrial
emissions), Official Journal of the European Union, L334,
December 2010.
[9] F. Meng, J. McKechnie, T. A. Turner, and S. J. Pickering,
“Energy and environmental assessment and reuse of fluidized
bed recycled carbon fibres,” Composites Part A: Applied
Science and Manufacturing, vol. 100, pp. 206–214, 2017.
[10] A. O. Nunes, L. R. Viana, P.-M. Guineheuc et al., “Life cycle
assessment of a steam thermolysis process to recover carbon
fibers from carbon fiber-reinforced polymer waste,” In-
ternational Journal of Life Cycle Assessment, vol. 22, no. 11,
pp. 1–14, 2017.
[11] R. A. Witik, R. Teusher, V. Michaud, C. Ludwig, and
J.-A. Manson, “Carbon fibre reinforced composite waste: an
environmental assessment of recycling, energy recovery and
landfilling,” Composites Part A: Applied Science and
Manufacturing, vol. 49, pp. 89–99, 2013.
[12] F. Cansell, C. Aymonier, C. Morin, and A. Loppinet-Serani,
“Near and supercritical solvolysis of carbon fibre reinforced
polymers (CFRPs) for recycling carbon fibers as a valuable
resource: state of the art,” Journal of Supercritical Fluids,
vol. 66, pp. 232–240, 2012.
[13] S. Pimenta and S. T. Pinho, “Recycling carbon fibre reinforced
polymers for structural applications: technology review and
market outlook,” Waste Management, vol. 31, no. 2, pp. 378–
392, 2011.
[14] G. Oliveux, L. O. Dandy, and G. A. Leeke, “Current status of
recycling of fibre reinforced polymers: review of technologies,
reuse and resulting properties,” Progress in Materials Science,
vol. 72, pp. 61–99, 2015.
[15] R. Pinero-Hernanz, J. Garcia-Serna, C. Dodds et al.,
“Chemical recycling of carbon fibre composites using alcohols
under subcritical and supercritical conditions,” Journal of
Supercritical Fluids, vol. 46, no. 1, pp. 83–92, 2008.
[16] G. Jiang, S. J. Pickering, E. H. Lester et al., “Characterization of
carbon fibres recycled from carbon fibre/epoxy resin com-
posites using supercritical n-propanol,” Composites Science
and Technology, vol. 69, no. 2, pp. 192–198, 2009.
[17] J. R. Hyde, E. Lester, S. Kingman, S. Pickering, and
K. H. Wong, “Supercritical propanol, a possible route to
composite carbon recovery: a viability study,” Composites Part
A: Applied Science and Manufacturing, vol. 37, no. 11,
pp. 2171–2175, 2011.
[18] H. Yan, C.-X. Lu, D. Jing et al., “Recycling of carbon fibers in
epoxy resin composites using supercritical 1-propanol,” New
Carbon Materials, vol. 31, no. 1, pp. 46–54, 2016.
[19] R. Pinero-Hernanz, C. Dodds, J. Hyde et al., “Chemical
recycling of carbon fibre reinforced composites in nearcritical
and supercritical water,” Composites Part A: Applied Science
and Manufacturing, vol. 39, no. 3, pp. 454–461, 2008.
[20] I. Okajima, K. Yamada, T. Sugeta, and T. Sako, “De-
composition of epoxy resin and recycling of CFRP with
sub- and supercritical water,” Kagaku Kogaku Ronbunshu,
vol. 28, no. 5, pp. 553–558, 2002.
[21] L. Henry, A. Schneller, J. Doerfler et al., “Semi-continuous flow
recycling method for carbon fibre reinforced thermoset poly-
mers by near- and supercritical solvolysis,” Polymer Degradation
and Stability, vol. 133, pp. 264–273, 2016.
Advances in Materials Science and Engineering 9
[22] A. Torres, I. de Marco, B. M. Caballero et al., “Recycling by
pyrolysis of thermoset composites: characteristics of the
liquid and gaseous fuels obtained,” Fuel, vol. 79, no. 8,
pp. 897–902, 2000.
[23] J. Yang, J. Liu, W. Liu, J. Wang, and T. Tang, “Recycling of
carbon fibre reinforced epoxy resin composites under various
oxygen concentrations in nitrogen–oxygen atmosphere,”
Journal of Analytical and Applied Pyrolysis, vol. 112,
pp. 253–261, 2015.
[24] M. A. Nahil and P. T. Williams, “Recycling of carbon fibre
reinforced polymeric waste for the production of activated
carbon fibres,” Journal of Analytical and Applied Pyrolysis,
vol. 91, no. 1, pp. 67–75, 2011.
[25] E. Lester, S. Kingman, K. H. Wong et al., “Microwave heating
as a means for carbon fibre recovery from polymer com-
posites: a technical feasibility study,” Master Research Bulletin,
vol. 39, no. 10, pp. 1549–56, 2004.
[26] D. Akesson, Z. Foltynowicz, J. Christ´een, and M. Skrifvars,
“Microwave pyrolysis as a method of recycling glass fibre from
used blades of wind turbines,” Journal of Reinforced Plastics
and Composites, vol. 31, no. 17, pp. 1136–42, 2012.
[27] P. Evangelopoulos, E. Kantarelis, and W. Yang, “Experimental
investigation of pyrolysis of printed circuit boards for energy
and materials recovery under nitrogen and steam atmo-
sphere,” Energy Procedia, vol. 105, pp. 986–991, 2017.
[28] S. Kumagai, T. Hosaka, T. Kameda, and T. Yoshioka, “Py-
rolysis and hydrolysis behaviors during steam pyrolysis of
polyimide,” Journal of Analytical and Applied Pyrolysis,
vol. 120, pp. 75–81, 2016.
[29] E. P. Onal, B. Bureau Uzun, and A. E. P ¨ut¨un, “Steam pyrolysis
of an industrial waste for bio-oil production,” Fuel Processing
Technology, vol. 92, no. 5, pp. 879–885, 2011.
[30] S. Y. Ye, A. Bounaceur, Y. Soudais, and R. Barna, “Parameter
optimization of the steam thermolysis: a process to recover
carbon fibers from polymer-matrix composites,” Waste and
Biomass Valorization, vol. 4, no. 1, pp. 73–86, 2013.
[31] S. Y. Ye, Valorisation de d´
echets composites `
a matrices poly-
m´
eriques renforc´
ees de fibres de carbone par un proc´
ed´
e de
vapo-thermolyse, Ph.D. thesis, University of Toulouse, Tou-
louse, France, 2012.
[32] K.-W. Kim, H.-M. Lee, J.-H. An et al., “Recycling and
characterization of carbon fibers from carbon fiber reinforced
epoxy matrix composites by a novel super-heated-steam
method,” Journal of Environmental Management, vol. 203,
pp. 872–879, 2017.
[33] J. Shi, J. Kato, L. Bao, and K. Kemmochi, “e mechanical
property of recycled fiber reinforced polymer composites by
superheated steam,” Applied Mechanics and Materials, vol. 339,
pp. 687–690, 2013.
[34] M. R’Mili, N. Godin, and J. Lamon, “Flaw strength distri-
butions and statistical parameters for ceramic fibers: the
normal distribution,” Physical Review E, vol. 85, no. 5,
pp. 1106–1112, 2012.
[35] M. R’Mili, V. Massardier, P. Merle et al., “e effect of thermal
exposure on the strength distribution of B
4
C coated carbon
fibers,” Carbon, vol. 37, no. 1, pp. 129–145, 1999.
[36] L. Balea, G. Dusserre, and G. Bernhart, “Mechanical behav-
iour of plain-knit reinforced injected composites: effect of
inlay yarns and fibre type,” Composites Part B: Engineering,
vol. 56, pp. 20–49, 2014.
[37] ASTM D 3379-75, Standard Test Method for Tensile
Strength and Young’s Modulus for High-Modulus Filament
Materials, ASTM International, West Conshohocken, PA,
USA, 1989.
[38] M. R’Mili, T. Bouchaour, and P. Merle, “Estimation of
Weibull parameters from loose-bundle tests,” Composites
Science and Technology, vol. 56, no. 7, pp. 831–834, 1996.
[39] M. R’Mili and M. Murat, “Caract´erisation des fibres par
am´
elioration de l’essai sur m`
eches avec mesure directe de la
deformation,” Comptes Rendus de l’Acad´emie des Sciences-
Series IIB-Mechanics-Physics-Chemistry-Astronomy, vol. 324,
pp. 355–364, 1997.
[40] Hexcel, HexTow AS4C carbon fiber, Technical datasheet, 2010.
[41] J. L. omason, “On the application of Weibull analysis to
experimentally determined single fibre strength distributions,”
Composites Science and Technology, vol. 80, pp. 77–74, 2013.
[42] M. R’Mili and J. Lamon, “Investigation of subcritical crack
growth using load relaxation tests on fiber bundles,” Acta
Materiala, vol. 59, no. 7, pp. 2850–2857, 2011.
[43] G. G. Tibbetts and C. P. Beetz, “Mechanical properties of
vapour-grown carbon fibers,” Journal of Physics D: Applied
Physics, vol. 20, no. 3, pp. 292–297, 1987.
[44] T. Tagawa and T. Miyata, “Size effect on tensile strength of
carbon fibers,” Materials Science and Engineering A, vol. 238,
no. 2, pp. 336–342, 1997.
[45] M. Huson, J. S. Church, A. Kafi et al., “Heterogeneity of
carbon fibre,” Carbon, vol. 68, pp. 240–249, 2014.
[46] S. Pimenta and S. T. Pinho, “e effect of recycling on the
mechanical response of carbon fibres and their composites,”
Composites Structures, vol. 94, no. 12, pp. 3669–3684, 2012.
[47] K. Stoeffler, S. Andielic, N. Legros, J. Roberge, and S. Schougaard,
“Polyphenylene sulfide (PPS) composites reinforced with
recycled carbon fiber,” Composites Science and Technology,
vol. 84, no. 29, pp. 65–71, 2013.
[48] D. W. N. Feng, X. Wang, and D. Wu, “Surface modification of
recycled carbon fibre and its reinforcement effect on nylon 6
composites: mechanical properties, morphology and crys-
tallization behaviors,” Current Applied Physics, vol. 13, no. 9,
pp. 2038–2050, 2013.
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