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Biochar as an Effective Filler of Carbon Fiber Reinforced Bio-Epoxy Composites

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The goal of this work was to investigate the effect of the biochar additive (2.5; 5; 10 wt.%) on the properties of carbon fiber-reinforced bio-epoxy composites. The morphology of the composites was monitored by scanning electron microscopy (SEM), and the thermomechanical properties by dynamic mechanical thermal analysis (DMTA). Additionally, mechanical properties such as impact strength, flexural strength andtensile strength, as well as the thermal stability and degradation kinetics of these composites were evaluated. It was found that the introduction of biochar into the epoxy matrix improved the mechanical and thermal properties of carbon fiber-reinforced composites.
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Article
Biochar as an Eective Filler of Carbon Fiber
Reinforced Bio-Epoxy Composites
Danuta Matykiewicz
Institute of Materials Technology, Faculty of Mechanical Engineering, Poznan University of Technology,
Piotrowo 3, 61-138 Pozna´n, Poland; danuta.matykiewicz@put.poznan.pl
Received: 23 May 2020; Accepted: 17 June 2020; Published: 22 June 2020


Abstract:
The goal of this work was to investigate the eect of the biochar additive (2.5; 5; 10 wt.%) on
the properties of carbon fiber-reinforced bio-epoxy composites. The morphology of the composites
was monitored by scanning electron microscopy (SEM), and the thermomechanical properties by
dynamic mechanical thermal analysis (DMTA). Additionally, mechanical properties such as impact
strength, flexural strength andtensile strength, as well as the thermal stability and degradation kinetics
of these composites were evaluated. It was found that the introduction of biochar into the epoxy
matrix improved the mechanical and thermal properties of carbon fiber-reinforced composites.
Keywords: composites; biochar; epoxy; carbon fiber
1. Introduction
The increase in the consumption of raw materials obtained from non-renewable resources is
aecting the growing popularity of natural-origin materials. In addition, the circular economy, widely
implemented by the European Union, requires that the design of polymer materials should result
in a minimum impact on the natural environment [
1
]. Among the natural additives for polymers,
plant, animal and mineral origin can be distinguished. They may occur in the form of powder,
microparticles or nanoparticles, or long and short fibers [
2
4
]. The use of such modifiers as
biochar
[57]
, carbon nanotubes [
8
10
], basalt powder [
11
14
], lignin [
15
,
16
], and ground waste
from the agricultural [
17
19
] and food industries [
20
22
] in polymer composites has been the subject
of many scientific papers in recent years. A significant change in the properties of the polymer
matrix is obtained by using additives of various shapes in the form of particles and fibers. Therefore,
fiber-reinforced epoxy composites undergo modification with various bio-fillers in order to form
hybrid materials [
23
]. Dinesh et al. [
24
] described the eects of wood dust, such as Rosewood and
Padauk, on the properties of jute fiber/epoxy composites. The introduction of this filler has improved
the mechanical and thermal properties of epoxy materials. In turn, Azadirachta indica seed powder,
Camellia sinensis powder and their combinations have been used to modify epoxy resin reinforced with
jute fabrics [
25
]. Epoxy composites with kenaf fiber and modified with various nanofillers—nano palm
oil, empty bunch of fruit filler, montmorillonite, and organically modified montmorillonite—have also
been studied [
26
]. The eect of Pongamia pinnata seed cake waste on the dry sliding wear behavior of
basalt fabric-reinforced epoxy composites was examined by Mohan et al. [
27
]. Moreover, micro rice
husk powder and rice husk ash powder have also been applied to improve the flexural strength and
the fatigue life of the carbon fabric/epoxy composites [28].
Biocarbon is a porous solid obtained through the thermal decomposition of biomass. It is chemically
stable under ambient conditions [
29
]. Biochar properties depend mainly on the raw material and on the
pyrolysis temperature used in the production process [
30
]. The thermal stability of the biochar results
from the high pyrolysis temperature, which usually exceeds 350
C. In addition, it can have functional
groups on its surfaces, which facilitate its connection with the polymer matrix [
31
].
Bartoli et al.
[
32
]
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studied the eect of the addition of biochar with dierent morphologies on the properties of epoxy
composites. In another study [
30
], biochar was used to modify the electrical characteristics of epoxy
composites. The application of pure and high-temperature annealed biochar to enhance the mechanical
behaviors of epoxy materials was presented by Giorcelli et al. [
33
]. The introduction of biochar obtained
from dierent feedstock (rice husk, oil seed rape, softwoods, perennial rhizomatous cane, wheat straw),
pyrolyzed under the same conditions in order to improve the ductility of the epoxy resin, has also been
analyzed [
34
]. Additionally, biocarbons from Chinese Poplar and pine cones have been successfully
applied to improve the elastic behaviors of epoxy materials [
35
]. Savi et al. [
36
] showed that the tensile
properties of epoxy composites modified with 4 wt.% multi wall carbon nanotubes and 20 wt.% maple
wood biochar were similar. All of these reports have demonstrated the high application potential of
biocarbon as an additive to epoxy materials.
Polymer composites reinforced with carbon fiber show good chemical resistance and mechanical
properties at a low density [
37
]. Therefore, the introduction of a bio additive that can both reduce their
price and improve their properties is a desirable product solution. To the best of the author’s knowledge,
the use of biocarbon to modify the properties of carbon fiber-reinforced epoxy composites has not
yet been reported in the literature. Hence, the goal of this work was to investigate the impact of the
biochar additive on the properties of carbon fiber-reinforced bio-epoxy composites. The present paper
presents the results of a study into the structure, dynamic mechanical thermal behavior, mechanical
properties such as flexural strength, impact strength and tensile strength, as well as thermal stability
and degradation kinetics of these composites.
2. Materials and Methods
2.1. Materials
The structural composites were made using: high biobased content epoxy resin (37%) SuperSap
ONE (Entropy resin), hardener SuperSap ONS (Entropy resin), and plain carbon fabric SPREAD,
areal weight 160 g/m
2
(ECC). The mixing ratio of resin and hardener recommended by the producer
was 100:47 (by weight) or 2:1 (by volume). Biochar (BC) (Fluid SA, S˛edzisz
ó
w) from biomass
(mainly willows) was used in the pyrolysis process at 650
C, and then crushed in a ball mill for 24 h,
using steel balls as a filler. The biochar production process is repeatable and always takes place in
repetitive and controlled conditions, and its properties can be closely related to the type of biomass.
2.2. Preparation of Composites
In the first stage, biochar was introduced into the epoxy resin in amounts of 2.5, 5, and 10 weight
percent, using a high shear mixer (10 min/1000 rpm). Next, the compositions were combined with the
hardener (5 min/500 rpm). The mixture was then degassed for 15 min under a pressure of
0.8 bar
and used for laminating the carbon fabric layers. Using a hand lay-up technique, the composites
containing six layers of carbon fabric were made, and then cured for 7 days at 23
C and for 3 h at
80
C. The samples were described as 0 BC; 2.5 BC; 5 BC; and 10 BC, in reference to their biocarbon
(BC) content. The composite production scheme is presented in Figure 1.
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2.3.2. Dynamic Mechanical Thermal Analysis (DMTA)
Dynamic mechanical thermal analysis was used to determine the thermomechanical properties
of the composites, such as: the storage modulus (G’), the glass transition temperature (Tg) and the
damping factor (tan δ). Measurements were made in the torsion mode (Anton Paar MCR 301
apparatus) with a frequency of 1 Hz, in the temperature range from 25 to 150 °C, with a heating rate
of 2 °C/min. As a point for determining the value of the glass transition temperature (Tg), the
maximum tan δ value was selected.
Figure 1. Preparation of epoxy/carbon fiber composites modified with biocarbon.
2.3.3. Flexural Test
A three-point bending test (Zwick Roell Z010 testing machine) was conducted for 80 mm × 10
mm × 2.0 mm samples with DIN EN ISO 14125. The measuring speed was 1 mm/min, and the load
cell was 10 kN. The span length corresponded to 16 times the sample thickness.
2.3.4. Charpy Impact Strength
The impact strength of the unnotched samples was tested by the Charpy method (ISO 179) at
room temperature. In addition, the peak load as the maximum force (Fmax) was determined during
the test. A Zwick/Roell HIT 25P impact tester with a 5 J hammer was used.
2.3.5. Tensile Test
The tensile test of the composites was carried out using an INSTRON 4481 universal testing
machine according to ISO 527-4, at 25 °C, with a test speed of 1 mm/min and a load cell of 50 kN.
2.3.6. Thermogravimetry (TGA)
The thermal properties of the composites were determined by thermogravimetry (TGA) in an
atmosphere of nitrogen and air, in a temperature range from 30 to 900 °C and a heating rate of 10
°C/min (Netzsch TG 209 F1 apparatus). The samples with a mass of 10 mg were investigated in
ceramic vessels. The following values were designated: the temperature at which the weight loss was
10% (T10%), the residual mass at 900 °C (W%) and maximum thermal degradation temperatures from
derivative thermogravimetric (DTG) diagrams. Moreover, according to the Kissinger method, TGA
measurements were conducted in a nitrogen atmosphere at a heating rate of 5, 10, 15, 20 °C/min.
Figure 1. Preparation of epoxy/carbon fiber composites modified with biocarbon.
2.3. Methods
2.3.1. Scanning Electron Microscopy (SEM)
Scanning electron microscopy (SEM) was used to assess the morphology of the composites and
biocarbon. The structure of biocarbon was monitored at the magnification of 6000 and 20,000, while the
fracture surfaces of the composites were studied at the enlargement of 1000, and then recorded digitally
by a scanning electron microscope Zeiss Evo 40 (Oberkochen, Germany) with an electron accelerating
voltage of 12 kV. Before testing, all of the samples were sprayed with a layer of gold.
2.3.2. Dynamic Mechanical Thermal Analysis (DMTA)
Dynamic mechanical thermal analysis was used to determine the thermomechanical properties
of the composites, such as: the storage modulus (G’), the glass transition temperature (T
g
) and the
damping factor (tan
δ
). Measurements were made in the torsion mode (Anton Paar MCR 301 apparatus)
with a frequency of 1 Hz, in the temperature range from 25 to 150
C, with a heating rate of 2
C/min.
As a point for determining the value of the glass transition temperature (T
g
), the maximum tan
δ
value
was selected.
2.3.3. Flexural Test
A three-point bending test (Zwick Roell Z010 testing machine) was conducted for 80 mm
×
10 mm
×
2.0 mm samples with DIN EN ISO 14125. The measuring speed was 1 mm/min, and the load cell
was 10 kN. The span length corresponded to 16 times the sample thickness.
2.3.4. Charpy Impact Strength
The impact strength of the unnotched samples was tested by the Charpy method (ISO 179) at
room temperature. In addition, the peak load as the maximum force (F
max
) was determined during the
test. A Zwick/Roell HIT 25P impact tester with a 5 J hammer was used.
2.3.5. Tensile Test
The tensile test of the composites was carried out using an INSTRON 4481 universal testing
machine according to ISO 527-4, at 25 C, with a test speed of 1 mm/min and a load cell of 50 kN.
2.3.6. Thermogravimetry (TGA)
The thermal properties of the composites were determined by thermogravimetry (TGA) in an
atmosphere of nitrogen and air, in a temperature range from 30 to 900
C and a heating rate of 10
C/min
(Netzsch TG 209 F1 apparatus). The samples with a mass of 10 mg were investigated in ceramic vessels.
The following values were designated: the temperature at which the weight loss was 10% (T10%),
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the residual mass at 900
C (W%) and maximum thermal degradation temperatures from derivative
thermogravimetric (DTG) diagrams. Moreover, according to the Kissinger method, TGA measurements
were conducted in a nitrogen atmosphere at a heating rate of 5, 10, 15, 20 C/min.
3. Results
3.1. Composites Structure
The morphology of biocarbon particles depends on the biomass from which it was produced and
the method of its treatment. Biochar formed from organic biomass by pyrolysis can form a skeletal
structure with macropores, mesopores or micropores. Particles of the ground biocarbon with an
average diameter of 4–8
µ
m, obtained from deciduous trees, were characterized by a smooth and clean
surface (Figure 2). Basalt fiber coverage by the epoxy matrix was observed for all tested samples. In the
case of the biochar-modified composites, no filler agglomeration was observed, which may indicate
a good connection between all the components (Figure 3). This is important because, as reported
by [
38
], the mechanical interlocking of biochar in a polymer chain may result in improved mechanical
properties of the composites.
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3. Results
3.1. Composites Structure
The morphology of biocarbon particles depends on the biomass from which it was produced
and the method of its treatment. Biochar formed from organic biomass by pyrolysis can form a
skeletal structure with macropores, mesopores or micropores. Particles of the ground biocarbon with
an average diameter of 4–8 µm, obtained from deciduous trees, were characterized by a smooth and
clean surface (Figure 2). Basalt fiber coverage by the epoxy matrix was observed for all tested samples.
In the case of the biochar-modified composites, no filler agglomeration was observed, which may
indicate a good connection between all the components (Figure 3). This is important because, as
reported by [38], the mechanical interlocking of biochar in a polymer chain may result in improved
mechanical properties of the composites.
Figure 2. Structure of used biocarbon (a) magnification 6000×, (b) magnification 20000×.
Figure 3. Structure of the composites (a) 0 BC, (b) 2.5 BC, (c) 5 BC, (d) 10 BC at a magnification of
1000×.
Figure 2. Structure of used biocarbon (a) magnification 6000×, (b) magnification 20000×.
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3. Results
3.1. Composites Structure
The morphology of biocarbon particles depends on the biomass from which it was produced
and the method of its treatment. Biochar formed from organic biomass by pyrolysis can form a
skeletal structure with macropores, mesopores or micropores. Particles of the ground biocarbon with
an average diameter of 4–8 µm, obtained from deciduous trees, were characterized by a smooth and
clean surface (Figure 2). Basalt fiber coverage by the epoxy matrix was observed for all tested samples.
In the case of the biochar-modified composites, no filler agglomeration was observed, which may
indicate a good connection between all the components (Figure 3). This is important because, as
reported by [38], the mechanical interlocking of biochar in a polymer chain may result in improved
mechanical properties of the composites.
Figure 2. Structure of used biocarbon (a) magnification 6000×, (b) magnification 20000×.
Figure 3. Structure of the composites (a) 0 BC, (b) 2.5 BC, (c) 5 BC, (d) 10 BC at a magnification of
1000×.
Figure 3.
Structure of the composites (
a
) 0 BC, (
b
) 2.5 BC, (
c
) 5 BC, (
d
) 10 BC at a magnification of 1000
×
.
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3.2. Thermomechanical Properties
DMTA analysis was performed to identify the eect of BC introduction into the epoxy matrix on
the viscoelastic properties of the composites. Changes in the value of the storage modulus and damping
factor are associated with the interaction between the polymer matrix, fiber and fillers (Table 1). Figure 4
presents the dependence of the storage modulus and damping factor on the temperature. The storage
modulus values corresponding to the material stiness increased with the increasing BC content [
39
].
The G’ value began to decrease at a temperature above 50
C and when it reached the glass transition
temperature, it dropped significantly. The damping factor (tan
δ
) describes the relationship between
the elastic and viscous phases in polymeric materials; the high tan delta value is due to the high degree
of energy dissipation (non-elastic deformation), while the low tan delta value shows that the material
is more elastic [
40
]. For all tested materials, tan delta values and glass transition temperatures were
similar and averaged 0.50 and 73
C, respectively. These results are in good agreement with the study
by Temmink et al. [
41
]. It should be emphasized that the addition of biochar improves the stiness
of the material without reducing its glass transition temperature, which is important because of the
functional properties that this composite material should have.
Table 1.
DMTA analysis data: storage modulus (G’), glass transition temperature (T
g
) and damping
factor (Tan δ).
Sample G’ at 30 C (MPa) G’ at 100 C (MPa) Tg(C) Tan δ
0 BC 2150 ±35 75.1 ±3.5 73 ±2 0.51 ±0.2
2.5 BC 2720 ±40 82.4 ±2.5 73 ±2 0.49 ±0.3
5 BC 2810 ±45 93.3 ±3.0 71 ±2 0.55 ±0.1
10 BC 2990 ±35 129.0 ±3.0 73 ±2 0.45 ±0.2
Figure 4.
Graph of dependence of storage modulus (G’) and damping factor (tan
δ
) values on
temperature obtained by the DMTA.
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3.3. Flexural Properties
In order to emphasize the impact of the addition of biochar on the flexural behavior of epoxy
materials, flexural strength and modulus depending on the filler content are presented in Figure 5,
and the data are also presented collectively in Table 2. The flexural strength of the composite depends
mainly on the dispersion of the particles, and the wetting and infiltration of the polymer in the
particles [
42
]. The flexural strength value increased with increasing biochar addition from 275 to
323 MPa. This may have been due to the presence of rigid biochar with a high surface area in the
epoxy matrix. The value of the flexural modulus slightly decreased, which may have been a result of a
lower amount of epoxy in the composites, in comparison with the reference sample. The increase in
inorganic filler content caused an increase in the flexural modulus of the composites [43,44].
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3.3. Flexural Properties
In order to emphasize the impact of the addition of biochar on the flexural behavior of epoxy
materials, flexural strength and modulus depending on the filler content are presented in Figure 5,
and the data are also presented collectively in Table 2. The flexural strength of the composite depends
mainly on the dispersion of the particles, and the wetting and infiltration of the polymer in the
particles [42]. The flexural strength value increased with increasing biochar addition from 275 to 323
MPa. This may have been due to the presence of rigid biochar with a high surface area in the epoxy
matrix. The value of the flexural modulus slightly decreased, which may have been a result of a lower
amount of epoxy in the composites, in comparison with the reference sample. The increase in
inorganic filler content caused an increase in the flexural modulus of the composites [43,44].
Figure 5. Flexural strength and modulus of the composites.
Table 2. Flexural and impact values of the tested materials.
Sample Flexural Strength
(MPa)
Flexural Modulus
(GPa)
Impact Strength
(kJ/m2)
F max
(N)
0 BC 275 ± 2 7.6 ± 0.2 55.3 ± 0.8 183.9 ± 5.8
2.5 BC 280 ± 1 6.2 ± 0.15 65.5 ± 2.5 221.3 ± 4.1
5 BC 305 ± 2 6.3 ± 0.15 69.4 ± 0.7 210.2 ± 6.4
10 BC 323 ± 1 6.8 ± 0.2 72.7 ± 2.1 210.5 ± 9.1
3.4. Charpy Impact Strength
The impact strength and maximum force (peak load) values of the tested composites are
collected in Table 2. During the impact test of the fiber-reinforced composites, f energy is dissipated
by a combination of several phenomena, such as fiber pulling, fiber cracking, and matrix
deformations and crack [45]. The increase in the biocarbon content in the epoxy matrix resulted in an
increase in the impact strength of the composites. In addition, after the introduction of biocarbon into
the epoxy, the laminates showed an increase in their peak load (Fmax) from 183 to 221 N. This may
indicate good adhesion between all the components of the biocomposites [46]. The typical load–time
plots of the examined materials are presented in Figure 6. Based on the shape of the curves, it can be
assumed that the destruction mechanism was similar in all cases, and that the addition of the powder
filler did not cause voids that could act like notches in the structure of the composite.
Figure 5. Flexural strength and modulus of the composites.
Table 2. Flexural and impact values of the tested materials.
Sample Flexural Strength
(MPa)
Flexural Modulus
(GPa)
Impact Strength
(kJ/m2)Fmax (N)
0 BC 275 ±2 7.6 ±0.2 55.3 ±0.8 183.9 ±5.8
2.5 BC 280 ±1 6.2 ±0.15 65.5 ±2.5 221.3 ±4.1
5 BC 305 ±2 6.3 ±0.15 69.4 ±0.7 210.2 ±6.4
10 BC 323 ±1 6.8 ±0.2 72.7 ±2.1 210.5 ±9.1
3.4. Charpy Impact Strength
The impact strength and maximum force (peak load) values of the tested composites are collected
in Table 2. During the impact test of the fiber-reinforced composites, fenergy is dissipated by a
combination of several phenomena, such as fiber pulling, fiber cracking, and matrix deformations
and crack [
45
]. The increase in the biocarbon content in the epoxy matrix resulted in an increase in
the impact strength of the composites. In addition, after the introduction of biocarbon into the epoxy,
the laminates showed an increase in their peak load (F
max
) from 183 to 221 N. This may indicate good
adhesion between all the components of the biocomposites [
46
]. The typical load–time plots of the
examined materials are presented in Figure 6. Based on the shape of the curves, it can be assumed that
the destruction mechanism was similar in all cases, and that the addition of the powder filler did not
cause voids that could act like notches in the structure of the composite.
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Figure 6. The load–time curves of the investigated composites.
3.5. Tensile Behaviours
In order to obtain more complete mechanical properties of the composites, a tensile test was
carried out, and tensile strength, modulus of elasticity and elongation at break were determined
(Table 3). The tensile strength value of fiber-reinforced polymer materials usually differs from their
bending strength value. This is mainly due to the properties of the reinforcing fibers used, their
structure, and weave. The tensile strength of the composites with the largest amount of biochar was
similar to the reference sample. Moreover, for all of the modified composites, the tensile modulus
value was lower than that of the unmodified composite. Meanwhile, for the 10 BC sample, this value
was near to the value for 0 BC. The elongation at break was at the same level for all tested materials.
Table 3. Tensile test values of the tested materials.
Sample Tensile Strength
(MPa)
Tensile Modulus
(GPa)
Elongation at Break
(%)
0 BC 375 ± 5 16.6 ± 0.2 2.5 ± 0.2
2.5 BC 305 ± 4 13.5 ± 0.3 2.6 ± 0.1
5 BC 355 ± 2 14.3 ± 0.2 2.5 ± 0.2
10 BC 380 ± 3 15.5 ± 0.2 2.5 ± 0.3
3.6. Thermal Stability
The thermal stability of biochar and the composites was assessed by the thermogravimetric
method in both an inert and oxidizing atmosphere. Characteristic thermogravimetric (TG) and DTG
curves are shown in Figures 7–9, and data are collected in Tables 4 and 5. Biochar was thermally
stable in a nitrogen atmosphere up to a temperature of 530 °C, at which 10% weight loss was observed
and the residual mass was 83%. In the air atmosphere, a one-stage biocarbon decomposition was
observed, and T10%value and residual mass were 396 °C and 4.9%, respectively.
Figure 6. The load–time curves of the investigated composites.
3.5. Tensile Behaviours
In order to obtain more complete mechanical properties of the composites, a tensile test was
carried out, and tensile strength, modulus of elasticity and elongation at break were determined
(Table 3). The tensile strength value of fiber-reinforced polymer materials usually diers from their
bending strength value. This is mainly due to the properties of the reinforcing fibers used, their
structure, and weave. The tensile strength of the composites with the largest amount of biochar was
similar to the reference sample. Moreover, for all of the modified composites, the tensile modulus
value was lower than that of the unmodified composite. Meanwhile, for the 10 BC sample, this value
was near to the value for 0 BC. The elongation at break was at the same level for all tested materials.
Table 3. Tensile test values of the tested materials.
Sample Tensile Strength (MPa) Tensile Modulus (GPa) Elongation at Break (%)
0 BC 375 ±5 16.6 ±0.2 2.5 ±0.2
2.5 BC 305 ±4 13.5 ±0.3 2.6 ±0.1
5 BC 355 ±2 14.3 ±0.2 2.5 ±0.2
10 BC 380 ±3 15.5 ±0.2 2.5 ±0.3
3.6. Thermal Stability
The thermal stability of biochar and the composites was assessed by the thermogravimetric
method in both an inert and oxidizing atmosphere. Characteristic thermogravimetric (TG) and DTG
curves are shown in Figures 79, and data are collected in Tables 4and 5. Biochar was thermally stable
in a nitrogen atmosphere up to a temperature of 530
C, at which 10% weight loss was observed and
the residual mass was 83%. In the air atmosphere, a one-stage biocarbon decomposition was observed,
and T10%value and residual mass were 396 C and 4.9%, respectively.
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Figure 7. Thermogravimetric (TG) and derivative thermogravimetric (DTG) curves biocarbon under
nitrogen and air atmosphere.
In the case of the composites tested in a nitrogen atmosphere, the addition of biochar did not
significantly affect their thermal stability. A one-stage degradation process was observed for all the
tested samples, T10% was, on average, 330 °C and the residual mass was 43–45% (Table 3). Another
effect was observed for samples tested in the air atmosphere; the thermal stability of the composites
increased along with the increase in biochar content in the epoxy matrix. The temperature at which
a 10% weight loss of the examined materials was recorded increased from 302 to 325 °C, and the
temperature from the DTG peak increased from 345 to 353 °C (Table 4). For the samples examined in
the air atmosphere, a three-stage degradation process was observed. The first peak on the DTG curve
occurred in the temperature range of 345–353 °C, corresponding to resin degradation, and the
addition of biochar shifted this temperature to a higher value. The major peak associated with filler
oxidation at 508 °C was observed at the DTG biochar curve (Figure 7). Moreover, DTG peaks between
500 and 600 °C and above 650 °C were associated with the oxidation and degradation of biochar and
carbon fibers.
Figure 7.
Thermogravimetric (TG) and derivative thermogravimetric (DTG) curves biocarbon under
nitrogen and air atmosphere.
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Figure 8. The diagrams of TG and DTG for analyzed materials obtained in a nitrogen atmosphere.
In addition, in order to assess the effect of the addition of biochar on the thermal properties of
the composites, TGA measurements were conducted at different heating rates, and the activation
energy of degradation was determined based on the Kissinger method. This method takes into
account the maximum temperatures (Tm) determined from DTG curves obtained at different heating
rates (β) [47]. In this method, the peak of the DTG curve corresponds to the temperature at which the
reaction rate has its maximum value. The activation energy, according to the Kissinger method, is
described in the Equation (1):
/

1/
=−
(1)
where: Ea—activation energy; R—gas constant; β—heating rate; Tm—temperature of DTG peak.
In this method, the activation energy can be calculated from a plot of ln(β/Tm2) versus 1/Tm (Figure
10). The values of Ea were assessed from the values of the slope coefficient of a straight line, described
by Equation (2).
=−x (2)
The Kissinger method was employed to calculate the activation energy in nitrogen. The values
of the activation energy of the thermal degradation process, determined by the Kissinger method,
and Tm obtained from DTG curves at different heating rates, are collected in Table 6. It was observed
that as the heating temperature increased, the DTG value of the peak shifted to higher temperatures.
This effect is caused by shortening the time until the sample reaches the same temperature, and the
material does not manage to completely decompose, therefore resulting in the Tm values being higher
[48]. For the composites with the highest biochar content, a significant increase in the value was
observed from 136 to 151 kJ/mol. It can be concluded that during the thermal degradation, the biochar
additive may retard the thermal decomposition of the carbon fiber-reinforced composites.
Figure 8. The diagrams of TG and DTG for analyzed materials obtained in a nitrogen atmosphere.
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Figure 9. The diagrams of TG and DTG for analyzed materials obtained in air atmosphere.
Table 4. Results of thermogravimetry (TGA) analysis for composites and biochar examined in
nitrogen atmosphere.
Name T10% (°C) Residual Mass (%)
DTG Peak
Temperature
(°C)
Max
Degradation Rate
(%/min)
BC 528.9 82.9 - -
0 BC 329.5 45.57 355.6 6.22
2.5 BC 331.1 42.00 359.1 6.87
5 BC 329.0 42.89 360.2 7.04
10 BC 329.5 44.18 358.8 6.82
Table 5. Results of TGA analysis for composites and biochar examined in air atmosphere.
Name T10% (°C) Residual Mass (%)
DTG Peak
Temperature
(°C)
Max
Degradation Rate
(%/min)
BC 396.4 4.92 507.9 8.2
0 BC 302.6 1.0 345.1 4.98
2.5 BC 317.2 0.9 346.5 6.07
5 BC 318.3 0.5 350.2 5.68
10 BC 324.9 0.1 353.4 5.19
Figure 9. The diagrams of TG and DTG for analyzed materials obtained in air atmosphere.
Table 4.
Results of thermogravimetry (TGA) analysis for composites and biochar examined in
nitrogen atmosphere.
Name T10% (C) Residual Mass (%) DTG Peak
Temperature (C)
Max Degradation
Rate (%/min)
BC 528.9 82.9 - -
0 BC 329.5 45.57 355.6 6.22
2.5 BC 331.1 42.00 359.1 6.87
5 BC 329.0 42.89 360.2 7.04
10 BC 329.5 44.18 358.8 6.82
Table 5. Results of TGA analysis for composites and biochar examined in air atmosphere.
Name T10% (C) Residual Mass (%) DTG Peak
Temperature (C)
Max Degradation
Rate (%/min)
BC 396.4 4.92 507.9 8.2
0 BC 302.6 1.0 345.1 4.98
2.5 BC 317.2 0.9 346.5 6.07
5 BC 318.3 0.5 350.2 5.68
10 BC 324.9 0.1 353.4 5.19
In the case of the composites tested in a nitrogen atmosphere, the addition of biochar did not
significantly aect their thermal stability. A one-stage degradation process was observed for all the
tested samples, T10% was, on average, 330
C and the residual mass was 43–45% (Table 3). Another
eect was observed for samples tested in the air atmosphere; the thermal stability of the composites
increased along with the increase in biochar content in the epoxy matrix. The temperature at which
a 10% weight loss of the examined materials was recorded increased from 302 to 325
C, and the
temperature from the DTG peak increased from 345 to 353
C (Table 4). For the samples examined
in the air atmosphere, a three-stage degradation process was observed. The first peak on the DTG
curve occurred in the temperature range of 345–353
C, corresponding to resin degradation, and the
addition of biochar shifted this temperature to a higher value. The major peak associated with filler
Processes 2020,8, 724 10 of 13
oxidation at 508
C was observed at the DTG biochar curve (Figure 7). Moreover, DTG peaks between
500 and 600
C and above 650
C were associated with the oxidation and degradation of biochar and
carbon fibers.
In addition, in order to assess the eect of the addition of biochar on the thermal properties of the
composites, TGA measurements were conducted at dierent heating rates, and the activation energy
of degradation was determined based on the Kissinger method. This method takes into account the
maximum temperatures (T
m
) determined from DTG curves obtained at dierent heating rates (
β
) [
47
].
In this method, the peak of the DTG curve corresponds to the temperature at which the reaction rate
has its maximum value. The activation energy, according to the Kissinger method, is described in the
Equation (1):
dhlnβ/T2
mi
d1/T2
m
=
Ea
R(1)
where: Ea—activation energy; R—gas constant; β—heating rate; Tm—temperature of DTG peak.
In this method, the activation energy can be calculated from a plot of ln(
β
/T
m2
) versus 1/T
m
(Figure 10). The values of E
a
were assessed from the values of the slope coecient of a straight line,
described by Equation (2).
Ea=slopexR(2)
Processes 2020, 8, x FOR PEER REVIEW 11 of 14
Figure 10. Kissinger plots of investigated composite in nitrogen.
Table 6. Kissinger data of investigated composite in nitrogen.
Name Tp at
5 °C/min (°C) Tp at 10 °C/min (°C) Tp at 15 °C/min (°C) Tp at
20 °C/min (°C)
Ea
(kJ/mol)
0 BC 353.0 ± 2.0 355.6 ± 1.8 375.8 ± 2.1 375.3 ± 2.0 136.3 ± 2.2
2.5 BC 359.2 ± 1.8 361.1 ± 2.0 379.5 ± 1.7 385.2 ± 2.0 133.9 ± 3.0
5 BC 360.2 ± 2.5 361.5 ± 2.2 382.4 ± 1.9 385.7 ± 2.0 137.5 ± 1.9
10 BC 356.1 ± 2.1 359.6 ± 1.9 370.8 ± 2.1 382.0 ± 2.0 151.2 ± 2.0
4. Conclusions
In this work, biochar was successfully used as a low-cost filler in carbon fiber-reinforced epoxy
composites. Due to its specific properties, the introduction of biochar to the epoxy matrix has
contributed to the improvement of the thermomechanical properties of materials, such as storage
modulus, and mechanical properties, such as flexural and impact strength. The composites
containing the highest amount of filler, i.e., 10 wt.%, showed the most favorable properties. Moreover,
thanks to the thermogravimetric method, it was confirmed that the introduction of biochar to the
epoxy matrix improves the thermal stability of the composites and delays the process of their thermal
degradation in the air atmosphere. In addition, the Kissinger method determined the activation
energy of the thermal degradation process. The highest value was recorded for the composites
containing 10 wt.% biochar. To sum up, the paper found a beneficial effect of using biochar in epoxy
composites reinforced with carbon fibers, which confirms the usefulness of this carbon derivative as
a modifier in polymeric materials.
Funding: This research was funded by the Ministry of Science & Higher Education in Poland under Project No
0613/SBAD/4630.
Conflicts of Interest: The authors declare no conflict of interest.
References
1. Czarnecka-Komorowska, D.; Wiszumirska, K. Sustainability design of plastic packaging for the Circular
Economy. Polimery 2020, 65, 8–17.
2. Hamad, Q.A.; Abed, M.S. Investigation of thyme and pumpkin nanopowders reinforced epoxy matrix
composites. Journal of Mechanical Engineering Research and Developments 2019, 42, 153–157.
Figure 10. Kissinger plots of investigated composite in nitrogen.
The Kissinger method was employed to calculate the activation energy in nitrogen. The values
of the activation energy
Ea
of the thermal degradation process, determined by the Kissinger method,
and T
m
obtained from DTG curves at dierent heating rates, are collected in Table 6. It was observed
that as the heating temperature increased, the DTG value of the peak shifted to higher temperatures.
This eect is caused by shortening the time until the sample reaches the same temperature, and the
material does not manage to completely decompose, therefore resulting in the T
m
values being
higher [
48
]. For the composites with the highest biochar content, a significant increase in the
Ea
value
was observed from 136 to 151 kJ/mol. It can be concluded that during the thermal degradation,
the biochar additive may retard the thermal decomposition of the carbon fiber-reinforced composites.
Processes 2020,8, 724 11 of 13
Table 6. Kissinger data of investigated composite in nitrogen.
Name Tpat 5 C/min (C) Tp at 10
C/min (C)
Tp at 15
C/min (C)
Tp at 20
C/min (C) Ea (kJ/mol)
0 BC 353.0 ±2.0 355.6 ±1.8 375.8 ±2.1 375.3 ±2.0 136.3 ±2.2
2.5 BC 359.2 ±1.8 361.1 ±2.0 379.5 ±1.7 385.2 ±2.0 133.9 ±3.0
5 BC 360.2 ±2.5 361.5 ±2.2 382.4 ±1.9 385.7 ±2.0 137.5 ±1.9
10 BC 356.1 ±2.1 359.6 ±1.9 370.8 ±2.1 382.0 ±2.0 151.2 ±2.0
4. Conclusions
In this work, biochar was successfully used as a low-cost filler in carbon fiber-reinforced epoxy
composites. Due to its specific properties, the introduction of biochar to the epoxy matrix has
contributed to the improvement of the thermomechanical properties of materials, such as storage
modulus, and mechanical properties, such as flexural and impact strength. The composites containing
the highest amount of filler, i.e., 10 wt.%, showed the most favorable properties. Moreover, thanks
to the thermogravimetric method, it was confirmed that the introduction of biochar to the epoxy
matrix improves the thermal stability of the composites and delays the process of their thermal
degradation in the air atmosphere. In addition, the Kissinger method determined the activation energy
of the thermal degradation process. The highest value was recorded for the composites containing
10 wt.% biochar. To sum up, the paper found a beneficial eect of using biochar in epoxy composites
reinforced with carbon fibers, which confirms the usefulness of this carbon derivative as a modifier in
polymeric materials.
Funding:
This research was funded by the Ministry of Science & Higher Education in Poland under Project
No 0613/SBAD/4630.
Conflicts of Interest: The authors declare no conflict of interest.
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2020 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).
... Second, using the biochar as a reinforcement for a thermoset epoxy, investigating its effect on the thermal and electrical properties of epoxy. While, there is a good amount of work regarding preparation of biochar from date seeds, a little amount of works, as far as the authors know, focus on preparation of biochar and employing it to produce a conductive composite (e.g., the work of Giorcelli et al. [18] and Matykiewicz [19]). ...
... Biochar pyrolysis process is sensitive to many parameters, particularly, residence time, temperature, and heat rate. All these parameters are fixed by selecting the optimum conditions reported in Mahdi et al. [19] and are listed in Table 1. The conditions might not be perfect for our experiments however, it , perhaps, is a good guide for a successful pyrolysis operation. ...
... and, then, a pH meter is used. It is important to stress here that the properties listed in Table 1 are in a good agreement with those reported by Mahdi et al. [19]. ...
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The current research is about exploring the effect of biochar on electrical, thermal and mechanical characteristics of thermoset epoxy, aiming to produce a conductive biocomposite. Biochar is a natural material produced from pyrolysis of biomasses and here date seeds are used for this purpose. The pyrolysis has been achieved by exposing the seeds to 350 o C for 2 hrs in approximately a free-oxygen environment. The produced biochar, then, is characterized and used as a reinforcement for a thermoset epoxy (quick mast 105 from DCP company). The biochar is added in different volume fractions to the epoxy during the polymerization reaction (from 0 to 60%). Interestingly, the results show that there is an improvement in both thermal and electrical behavior of the prepared samples up to 20-25% of filler content. In terms of the mechanical properties, both the tensile and impact strength increase with the content of biochar. The compressive strength, on the other hand, slightly decreases as the filler content increases.
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... Better competitive biochar for functional applications like composite reinforcement, energy storage, and sensors are being developed in recent years, thanks to a better understanding of carbonaceous structural evolution during biochar production and the development of more controlled processing methods [17][18][19][20][21][22]. Biochar carbon when used as reinforcement filler with polymer matrix gives excellent mechanical and thermal properties [23][24][25][26]. Some biochars have also shown good electromagnetic shielding and electrical conductivity [27,28]. ...
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