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*e-mail: panzera@ufsj.edu.br
Micromechanical Analysis of Hybrid Composites Reinforced with Unidirectional Natural
Fibres, Silica Microparticles and Maleic Anhydride
Leandro José da Silvaa, Túlio Hallak Panzeraa*, André Luis Christoforoa,
Juan Carlos Campos Rubiob, Fabrizio Scarpac
aDepartment of Mechanical Engineering, Federal University of São João Del Rei – UFSJ,
Brazil, Praça Frei Orlando, 170, São João Del Rei, MG, Brazil
bDepartment of Mechanical Engineering, Federal University of Minas Gerais – UFMG,
Belo Horizonte, MG, Brazil
cAdvanced Composites Centre for Innovation and Science, University of Bristol, Bristol, UK
Received: February 18, 2012; Revised: July 19, 2012
The work describes the analytical and experimental characterisation of a class of polymeric
composites made from epoxy matrix reinforced with unidirectional natural sisal and banana fibres with
silica microparticles and maleic anhydride fabricated by manual moulding. The analytical models,
ROM rule of mixtures and Halpin-Tsai approach, have been used in conjunction with a Design of
Experiments (DOE) analysis from tensile tests carried out on 24 different composites architectures.
The following experimental factors were analyzed in this work: type of fibres (sisal and banana fibres),
volume fraction of fibres (30% and 50%) and modified matrix phase by adding silica microparticles
(0%wt, 20%wt and 33%wt) and maleic anhydride (0%wt and 2%wt). The ROM approach has shown
a general good agreement with the experimental data for composites manufactured with 30%vol of
natural fibres, which can be attributed to the strong adhesion found between the phases. On the opposite,
the semi empirical model proposed by Halpin and Tsai has shown greater fidelity with composites
manufactured from 50%vol of natural fibres, which exhibit a weak interfacial bonding. The addition
of microsilica and maleic anhydride in the system did not enhance the adhesion between the phases
as expected.
Keywords: biocomposites, laminates, mechanical properties
1. Introduction
Biocomposites made from polymeric matrices and
natural fibres appeared during the last decade as a
sustainable alternative material for many applications
related to aerospace, automotive and structural engineering.
Interest towards an increased use of biocomposite structural
materials has grown from the surging demand for low cost
materials from environmental-friendly renewable sources,
and the possibility of finding an alternative to traditional
composites made of synthetic fibres1. The research and
development activity on the use and disposal/recycling
of synthetic fibres and resins derived from petroleum has
also been motivated by increasingly stringent requirements
by legal authorities, as well as by the high cost of the use
of synthetic fibres for some non high-end engineering
applications1.
A variety of natural fibres have been evaluated as
reinforcement phase in polymeric composites, such as
the bagasse from sugar cane, sisal, jute, curauá, flax,
piassava and banana plant2. Among them, the sisal fibre
constitutes perhaps the most promising due to its low cost,
high mechanical properties and market availability. Direct
extraction of banana fibres is not common practice3, however
this particular type of biofibre can be considered as a side
waste product from the cultivation of banana plants. The
fibres extracted from the pseudo-stem of banana plant exhibit
interesting mechanical properties for polymeric composites
reinforcements1.
One of the main difficulties when dealing with
natural composites is the adhesion between fibres and
matrices4,5, mainly due to the hydrophilic and hydrophobic
characteristics showed by the fibres and the polymers,
respectively. However, the chemical affinity between the
cellulose and the polymeric matrix can be improved by the
modification of the fibre surface6,7 or the polymer8-11, using
chemical additives like maleic anhydride.
Mishra et al.6, Naik and Mishra7, evaluated the effect
of adding maleic anhydride on the sisal and banana fibres
surface adhesion, observing a significant reduction of
the water absorption and an increase of the modulus of
elasticity, hardness and impact strength. An alternative
method to improve the mechanical performance of
biocomposites is by adding a second reinforcement phase.
In that sense, several studies involving the fabrication of
hybrid composites of polymeric matrices reinforced with
fibres and nano or microparticles of ceramic minerals have
been reported in open literature12-17. During a biocomposite
OI:D 10.1590/S1516-14392012005000134
Materials Research. 2012; 15(6): 1003-1012 © 2012
Silva et al.
failure, the crack initiates in the matrix phase and increases
the debonding between fibre and the matrix itself18. When
the crack propagation reaches the ceramic particles
along the fibre–matrix interface, the crack is impeded to
penetrate through the locations where the particles are
concentrated, because of the high strength provided by
the ceramics. Hence, additional effort is required for the
crack to propagate through the fibre-particle interface or
the particle-matrix interfaces (whichever is longer). This
additional effort not only reduces the crack propagation
velocity, but also increases the mechanical strength of the
composite. One of the major difficulties in developing
hybrid composites reinforced with fibres and particles is
the prediction of the effective mechanical and physical
properties19. The problem is more accentuated for structural
biocomposites, because their natural fibres exhibit
large variations in properties, due to the uncertainties
associated to the environmental conditions (moisture,
soil, temperature) in which these natural materials are
produced20. The absence of robust micromechanical
models predicting the mechanical properties of these
hybrid biocomposites is a major obstacle towards the
design of structural components using these novel types
of composite19.
Several models have been used in open literature to
predict the effective properties of composite materials
reinforced with long and short fibres, such as Rule of
mixture (ROM)19,20, Halpin-Tsai20,21, shear-lag analysis20,22
and Hashin-Strickman23. The rule of mixture (ROM) has
shown its effectiveness on predicting the tensile strength
of different natural fibres reinforced HDPE (high-density
polyethylene)24. Halpin-Tsai model is also found to be
the most effective equation in predicting the Young’s
modulus of composites containing different types of
natural fibres21.
The Rule of mixture (ROM) is the simplest available
micromechanical analysis model that can be used to predict
the elastic properties of a composite material19,20. As an
application of the ROM approach, Equation 1 shows how
to estimate the effective modulus of elasticity (E*) of the
composite, as a function of the properties of the fibres
and matrix materials, considering the direction of fibre
alignment. EF, EM, VF and VM are the modulus and volume
fractions of the fibre and matrix materials respectively.
*
Ef f mm
EV EV=+ (1)
The rule of mixture assumes a perfect bonded interface
between matrix and reinforcement(s). This assumption
may be unrealistic for the majority of real manufactured
composites, and therefore it is useful to adopt semi-empirical
models like the Halpin-Tsai one25, which compensates
for non-perfect interface conditions. The calculation of
the uniaxial tensile Young’s modulus for a unidirectional
composite according to Halpin-Tsai’s approach is illustrated
in Equation 2:
*
mf
f
E (1 V )
E1V
+ ξη
=−η (2)
Where η is given as:
fm
fm
EE
EE
−
η= +ξ (3)
The parameter ξ in Equations 2 and 3 is a shape fitting
variable to fit the Halpin–Tsai equation to the experimental
data, which describes also the packing arrangement and the
geometry of the reinforcing fibres20:
**
*
( )( )
( )( )
f m f fm
mf mf m
E E E VE E E
EEE VEE
−− −
ξ= −− − (4)
In this work it was evaluated the mechanical behaviour
of a polymeric composite reinforced with unidirectional
natural fibres, such as sisal and banana fibres, by the
use of micromechanical models and experimental
tests. The maleic anhydride was also investigated as a
coupling agent between the phases. The experimental
Young’s modulus was compared with the uniaxial one
predicted by rule of mixture and Halpin-Tsai equations.
From the micromechanical analyses presented, it is
possible to observe the effect of the hybridization and
chemical additive on the interfacial adhesion between the
constitutive phases.
Figure 1. Sisal fibres (a) and banana fibres (b).
1004 Materials Research
Micromechanical Analysis of Hybrid Composites Reinforced with Unidirectional Natural Fibres,
Silica Microparticles and Maleic Anhydride
2. Material and Methods
The polymeric composites were fabricated from
modified and non-modified epoxy matrix, supplied by
Resiqualy Company (São Paulo - Brazil), and from
dispersive phase of unidirectional sisal (Figure 1a) and
banana fibres (Figure 1b) supplied by Sisalsul Company
(São Paulo – Brazil). The fibres were extracted, washed
and combed by the supplier, with no chemical treatment.
The matrix phase was modified by the addition of
silica microparticles and maleic anhydride. The silica
microparticles were supplied by Moinhos Gerais Company
(Minas Gerais – Brazil), and classified by sieving process in
monomodal range of 400-500 US-Tyler (0.037-0.025 mm).
Table 1 exhibits the physical and mechanical properties
of the silica powder were provided by Moinhos Gerais
Company. The apparent density was determined using a
gas pycnometer by Micromeritics model AccuPyc 1330 and
the mechanical properties were estimated via dynamic ultra
micro hardness tester by Shimadzu model DUH-211. The
resin and the hardener were combined; afterwards the silica
microparticles were added and hand-mixed by 5 minutes in
room temperature around 22 °C.
Tensile tests were carried out according to ASTM
D3822-07[26] and ASTM D638-03[27] standards to determine
the tensile strength and modulus of elasticity of the fibres
and matrix phase, respectively. The test speeds were set as
3 mm/min for sisal and banana fibres and 2 mm/min for
polymeric matrices.
Figure 2 shows the samples for the non-modified and
modified matrices which were used to evaluate the physical
properties such as apparent density, apparent porosity and
water absorption based on BS 10545-3[28] standard. The
samples were fabricated manually by hand-mixed of epoxy
resin, silica particles and maleic anhydride, for 5 minutes
in room temperature around 22 °C. As a part of the overall
mechanical characterisation of the composites, the tensile
strength of the pure epoxy matrices was also determined
experimentally.
The Design of Experiment (DOE) activity was carried
out considering as experimental factors the type of
natural fibres (sisal and banana), volume fraction of fibres
(30% and 50%), maleic anhydride (0%wt and 2%wt) and
silica microparticles (0%wt, 20%wt and 33%wt). The
combination of these factors leads to investigate a total of
24 experimental conditions (see Table 2).
Preliminary tests were conducted in order to set the
upper volume fraction of fibres (50%) and silica particles
(33%wt) in the system, to obtain a suitable surface finishing
and lower porosity. A large percent of natural fibres
contributes to an overall lower cost of the composite (i.e. for
a 50/50%vol laminate, the cost of epoxy resin corresponds
nearly to ten times higher than the sisal phase) and also a
more sustainable composite material in terms of recycling
and sourcing. Mixture time (5 minutes), cure time (7 days),
room temperature (~22 °C) and the epoxy resin matrix were
kept constants during the DOE process.
The biocomposite laminates were fabricated aligning
manually the fibres by the aid of a metal frame. The manual
moulding process was carried out over a glass plate covered
by a cloth parting (Armalon), providing good surface
finishing to the lamina. The polymeric matrix (modified
and non-modified) was spread on the fibres by the use of
spatula and roller aerator. A glass fibre composite was used
to protect the specimen ends at the clamping area, avoiding
premature crack during the tensile testing (see Figure 3).
A scanning electron microscope (SEM - Hitachi T-3000)
was used to observe the cross section of the composites. The
tensile and flexural testing was carried out following the BSI
standard 2747[29] using an Autograph machine monitored by
a Topazium software with load cell maximum capacity of
20 kN. The test speed of the tensile tests was 2 mm/min.
A randomization procedure was adopted during the
sample fabrication and experimental tests. This randomization
let an arbitrary ordering of the experimental conditions,
avoiding that non controlled factors affect the responses30.
The effective modulus of the composites was estimated
using the ROM and Halpin-Tsai model. The particulate
phase was not directly considered in the micromechanical
analysis, using instead non-modified and modified matrices
mechanical properties in the models. The comparison
between experimental and predicted results allows verifying
whether the particles and/or the chemical additions
contribute to the fibre-matrix adhesion.
3. Results
Table 3 shows the physical and mechanical properties of
sisal and banana fibres evaluated within this work31. Table 3
shows the mean values of the properties with the respective
standard deviations. The banana fibres exhibited in general a
lower density and higher porosity than sisal fibres. The tensile
strength between the different types of fibres is quite similar;
however the banana fibres appear stiffer than the sisal ones,
showing a modulus of elasticity of 31.6GPa ± 2.8. The critical
constituent responsible for natural fibre strength and stiffness
are cellulose microfibrils. These microfibrils have a width
ranging from 5 to 30 nm, are highly crystalline materials
Table 1. Properties of silica particles supplied by Moinhos Gerais
Company.
Properties Unity Lower limit Higher limit
Apparent density kg/m32170 2220
Young’s modulus GPa 56 74
Tensile strength MPa 45 155
Compressive strength MPa 1100 1600
Figure 2. Non-modified matrix (a) and modified matrices by
addition of 2%wt of maleic anhydride (b), 20%wt of silica
microparticles (c) and 33%wt of silica microparticles (d).
2012; 15(6) 1005
Silva et al.
stress, achieving a maximum value close to 1000 MPa for
sisal fibres and 1200 MPa for banana ones.
Table 4 shows the physical and mechanical properties
related to the modified and non-modified termoset matrices.
Table 4 shows the mean values of the properties with the
respective standard deviations. It is apparent that, although,
the addition of maleic anhydride did not affect the physical
properties of the matrices, the addition of silica microparticles
increased the material´s density, which can be attributted to the
higher density of the silica particles (~2.2 g.cm–3). However,
the inclusion of silica in the composites did not affect the
apparent porosity and water absorption of the matrices.
Figure 5 shows the mechanical behaviour of the modified
matrices for 0%wt and 2%wt of maleic anhydride dispersions,
revealing an increase of the tensile strength and tenacity when
the chemical agent is added. However, the chemical additive
did not affect significantly the value of the modulus of elasticity.
From Figure 5 it is possible to observe that adding silica
Table 2. Experimental conditions.
Conditions Type of fibres Volume fraction (%) Maleic anhydride (%wt) Silica addition (%wt)
C1 Sisal 30 0 0
C2 Sisal 30 2 0
C3 Sisal 30 0 20
C4 Sisal 30 2 20
C5 Sisal 30 0 33
C6 Sisal 30 2 33
C7 Sisal 50 0 0
C8 Sisal 50 2 0
C9 Sisal 50 0 20
C10 Sisal 50 2 20
C11 Sisal 50 0 33
C12 Sisal 50 2 33
C13 Banana 30 0 0
C14 Banana 30 2 0
C15 Banana 30 0 20
C16 Banana 30 2 20
C17 Banana 30 0 33
C18 Banana 30 2 33
C19 Banana 50 0 0
C20 Banana 50 2 0
C21 Banana 50 0 20
C22 Banana 50 2 20
C23 Banana 50 0 33
C24 Banana 50 2 33
Figure 3. Tensile test specimens.
Table 3. Properties of sisal and banana fibres (mean values and
standard deviation).
Properties Sisal fibre Banana fibre
Diameter (µm) 192.5 (±26.3) 131.1 (±17.7)
Apparent density (g/cm3) 1.41 (±0.12) 1.35 (±0.09)
Apparent porosity (%) 76.21 (±2.01) 86.69 (±1.76)
Tensile strength (MPa) 887 (±143) 1063 (±259.5)
Modulus of elasticity (GPa) 16.4 (±2.5) 31.56 (±2.8)
formed by the aggregation of long thread like bundles of
molecules stabilized laterally by hydrogen bonds between
hydroxyl groups and oxygens of adjacent molecules32.
According to Joseph et al.33 the percent of cellulose in sisal
and banana fibres is nearly 70 and 83%, respectively.
Figure 4 shows the stress-strain behaviour of the sisal
and banana fibres under tensile loading. The stress-strain
curve of the sisal (Figure 4a) and banana (Figure 4b)
fibres can be approximately divided in four stages. During
stage (i), the stiffness reaches a maximum of 200 MPa,
increasing to a value of 450 MPa during stage (ii). The
450 MPa value was used to calculate the Young’s modulus
to be inserted in the Equations 1-4 for micromechanics
analysis. Stage (iii) features a large elongation of the fibres,
which can be attributed to the initial fraying effect. During
stage (iv) one can observe a significant increase in tensile
1006 Materials Research
Micromechanical Analysis of Hybrid Composites Reinforced with Unidirectional Natural Fibres,
Silica Microparticles and Maleic Anhydride
Table 4. Properties of polymeric matrices (mean values and standard deviation).
Setup Apparent density
(g/cm3)
Apparent porosity
(%)
Water absorption
(%)
Tensile strength
(MPa)
Modulus of elasticity
(GPa)
Epoxy resin 1.16 (±0.00) 0.30 (±0.07) 0.26 (±0.06) 31.99 (±2.72) 0.83 (±0.05)
2% of MA 1.15 (±0.00) 0.30 (±0.06) 0.26 (±0.06) 35.73 (±0.87) 0.81 (±0.03)
20% of silica 1.28 (±0.01) 0.30 (±0.06) 0.24 (±0.05) 26.26 (±1.36) 0.95 (±0.03)
33% of silica 1.34 (±0.02) 0.29 (±0.07) 0.22 (±0.05) 22.54 (±2.64) 1.10 (±0.07)
Table 5. Results obtained from tensile testing and micromechanical analysis.
Experimental
conditions
Experimental modulus
of elasticity (MPa)
Rule of mixture
(MPa)
E/N* Halpin-Tsai
(MPa)
E/N**
C1 5722 5502 1.04 2400 2.38
C2 5559 5487 1.01 2376 2.34
C3 5632 5583 1.01 2526 2.23
C4 6158 5423 1.14 2275 2.71
C5 5440 5690 0.96 2693 2.02
C6 5217 5766 0.90 2811 1.86
C7 5912 8616 0.69 4923 1.20
C8 5030 8605 0.58 4902 1.03
C9 4978 8673 0.57 5035 0.99
C10 5298 8559 0.62 4813 1.10
C11 4866 8750 0.56 5183 0.94
C12 4762 8804 0.54 5287 0.90
C13 9105 10050 0.91 3770 2.42
C14 8650 10035 0.86 3745 2.31
C15 8045 10131 0.79 3897 2.06
C16 7685 9971 0.77 3644 2.11
C17 7270 10238 0.71 4066 1.79
C18 7850 10314 0.76 4185 1.88
C19 8255 16196 0.51 8721 0.95
C20 7920 16185 0.49 8699 0.91
C21 6450 16253 0.40 8834 0.73
C22 6940 16139 0.43 8609 0.81
C23 7260 16330 0.44 8984 0.81
C24 6565 16384 0.40 9089 0.72
*Relation between experimental and predicted modulus of elasticity by rule of mixture model. **Relation between experimental and predicted modulus
of elasticity by Halpin-Tsai equations.
Figure 5. Stress/strain curves of the non-modified and modified
matrices with maleic anhydride and silica microparticles.
Figure 4. Tipical stress/strain curves for sisal and banana fibres.
2012; 15(6) 1007
Silva et al.
microparticles leads to a decrease of the tensile strength of
the matrices. As expected the Young’s modulus of the epoxies
increased by 14% and 32% when 20%wt and 33%wt of silica
were added, respectively. This behaviour can be attributed
to the high stiffness of the particulate phase, contributing to
increase the modulus of elasticity of the matrices. These results
are in accordance with previous works published in the open
literature13-14, where the Young’s modulus of the composites
increases by the addition of particles into polymeric matrix.
Table 5 contains the results related to the experimental
(E) and numerical (N) unidirectional Young’s modulus of
the fibres. The analytical values from the micromechanical
models, were calculated based on the individual mean
properties of the fibres and matrix phases from Tables 3
and 4. Table 5 shows the relation between the experimental
(E) and numerical (N) modulus of elasticity. If the value E/N
is higher than 1.0, indicates that the experimental value is
higher than the predicted one.
3.1. Composites fabricated with sisal fibres
Figure 6 shows the comparison between ROM,
Halpin-Tsai and experimental results for the C1 to C12 sisal
fibres composites.
Figure 6a shows the modulus of elasticity for the C1
and C7 composites; i.e. those composites manufactured
with 30%vol and 50%vol of sisal fibres, respectively,
Figure 6. Micromechanical analysis and experimental results for C1 to C12 composites.
1008 Materials Research
Micromechanical Analysis of Hybrid Composites Reinforced with Unidirectional Natural Fibres,
Silica Microparticles and Maleic Anhydride
Figure 7. Micromechanical analysis and experimental results for C13 to C24 composites.
with no silica and maleic anhydride addition. It can be
observed that the composite with 30%vol of fibres follows
the ROM model (E/N* = 1.04), while the Halpin-Tsai
approach underestimates the effective modulus of elasticity
(E/N** = 2.38). This result indicates a strong interfacial
adhesion for C1 composite.
On the opposite, the experimental results for C7
composite showed higher agreement with the Halpin-Tsai
equation (see Table 5), which suggests the presence of an
non-perfect bonding condition at the interface between
fibres and matrix. This behaviour can be attributed to the
small amount of matrix phase in the system (50%), which
affects the matrix wetting capacity around the fibres, and
consequently, the increase in porosity of the composites.
Figure 6b features the Young’s modulus of the
composites fabricated with maleic anydride addition,
corresponding to C2 and C8 composites (30%vol and
50%vol of sisal fibres, respectively). Similarly to the
composites with non-modified epoxy resin shown in
Figure 6a, the interface condition can be considered perfect
for the low level of sisal fibres (30%vol), exhibiting a E/N*
ratio of 1.01. The improved agreement provided by the
Halpin-Tsai approach (E/N** = 1.06) suggests also in this
case the existence of an imperfect interfacial condition was
2012; 15(6) 1009
Silva et al.
achieved for the composites with high level of sisal fibres
addition (50%vol).
The micromechanical analyses of the composites
made with 20%wt of microsilica addition (C3 and C9
composites) is shown in Figure 6c. It can be observed a
particularly good agreement (E/N* = 1.01) between the
experimental and rule of mixture modulus predicted for
the composite manufactured with 30%vol of sisal fibres
(C3 composite). On the opposite, the composite with
high level of sisal fibres (50%vol) revealed an imperfect
interface condition, exhibiting a large deviation of rule of
mixture prediction (E/N* = 0.57); while the Halpin-Tsai
model provided a higher fidelity prediction (E/N** = 0.99).
Figure 6d shows the estimated modulus for the composites
made with 20%wt of silica and 2%wt of maleic anhydride
addition (C4 and C10 composites). The results indicate that
the maleic anhydride affected the rheology of the system,
increasing the elastic modulus of the composites. The
increase of the fibre fraction reduced the interfacial adhesion
between the phases, a fact also confirmed by the low fidelity
of the ROM prediction in this case (E/N* = 0.62). Figure 6e
presents the behaviour of the C5 and C11 composites, i.e.
those composites manufactured with 30%vol and 50%vol
of sisal fibres and 33%wt of silica addition. The composites
made with 30%vol of fibres featured a higher agreement
between the rule of mixture model and experimental Young’s
modulus (E/N* = 0.96). However, the composite with 50%
of fibres was better described by the Halpin-Tsai approach
(E/N** = 0.94).
Although the addition of 33%wt of silica microparticles
increased the stiffness of the matrix (see Table 4), it was not
able to increase the modulus of elasticity of the composites.
This behaviour can be attributed to the increase of surface
area due to the silica particles addition, affecting not only the
rheology of the system but also the matrix wetting capacity
on the fibre surface. The addition of silica microparticles
also increases the porosity of the composites, therefore
contributing to the reduction of the mechanical properties.
Divergence between the experimental results and
predicted stiffness by ROM and Halpin-Tsai models is
shown in Figure 6f for the C6 and C12 composites, which
were manufactured with 30%vol and 50%vol of sisal fibres,
respectively, and 33%wt of silica microparticles and 2%wt
of maleic anhydride added into the polymeric matrix phase.
The modulus of elasticity of the matrix was increased by
silica and maleic anhydride addition (Figure 5). However,
the stiffer matrices did not originate stiffer composites. This
behaviour confirms the hypothesis of fibre-matrix interface
adhesion reduction provided by the addition of high content
of silica micro particles.
3.2. Composites manufactured with banana fibres
Figure 7 shows the analytical predicted and experimental
results related to the composites reinforced with banana
fibres (C13 to C24, see Table 2). The same discussions
performed for the composites reinforced with sisal fibres
(see section 3.1) can be extended for the composites
reinforced with banana fibres.
The composites manufactured with low level of banana
fibres (30%vol) showed a better agreement with the rule
of mixture model, while the composites with high level of
fibres (50%vol) were better described using the Halpin-Tsai
approach, indicating therefore a poor interface condition for
those composites compared to the ones of the previous case.
However, based on the results shown in Table 5, it is possible
to verify that the elastic moduli of banana fibre composites
are in general lower than the elastic moduli estimated by
the micromechanical analysis. Higher divergence between
experimental moduli and predicted moduli were observed
for the banana fibre composites in comparison to sisal fibre
composites, for both micromechanical models. This result
implies that the composites fabricated with sisal fibres show
in general a sounder fibre-matrix adhesion than the banana
fibre composites.
Figure 8 shows the backscatter mode SEM images at
100× of magnification featuring the failure surface of the
Figure 8. Backscatter electron imaging (BSE) at 100× of magnification for rupture surface of composites manufatured with sisal fibres
(a) and banana fibres (b).
1010 Materials Research
Micromechanical Analysis of Hybrid Composites Reinforced with Unidirectional Natural Fibres,
Silica Microparticles and Maleic Anhydride
sisal (Figure 8a) and banana (Figure 8b) fibre composites
after tensile testing. The sisal fibre composites shows a
fracture mode more brittle than the banana fibre composites.
Based on the investigations of Facca et al.20 and
Ku et al.24, the Halpin-Tsai model well described the
experimental modulus of HDPE (High-density polyethylene)
composites reinforced with different types of short natural
fibres and volume fractions varying from 0%wt up to 40%wt.
In contrast, the rule of mixture was not able to predict the
experimental data. In the present work, the composites were
fabricated with unidirectional banana fibres. The Young’s
modulus was better predicted by Halpin-Tsai model when
50%vol of fibres were added. However, when the composites
were fabricated with 30%vol of natural fibres, the rule of
mixture presented a better prediction, especially for the sisal
fibres. This result reveals the Rule of mixture (ROM) can
be acceptably applied to estimate the tensile modulus of
biocomposites with good interfacial adhesion.
4. Conclusions
The experimental and analytical Young’s moduli of
structural biocomposites based on sisal and banana fibres
were evaluated in this work. The experimental results were
generated through a Design of Experiments approach. The
main conclusions from this work are the following:
• Thebananabreshaveshownageneralhigherstiffness
than sisal fibres, however the sisal fibres exhibited a
superior tensile strength than the banana ones;
• Themechanicalbehaviourundertensileloadingisvery
similar for both natural fibres, featuring four different
stages in their stress-strain behaviour. A fraying effect
of fibres was observed when the stress is around
450 MPa, subsequently the stiffness is increased,
achieving a maximum tensile stress close to 1000 MPa
for sisal fibres and 1200 MPa for banana fibres;
• The addition of 2%wt of maleic anhydride into
matrix phase provided not only the increase of tensile
strength, but also the tenacity of the polymer itself;
• Theadditionofsilicamicroparticlesintothematrix
phase led not only to the reduction of tensile strength,
but also to the increase of the Young’s modulus of the
polymer;
• The micromechanical analysis provided some
indications about the interfacial conditions between
fibres and matrices within the natural composites.
The rule of mixture showed higher fidelity when
low levels of fibres (30%vol) enhancing the wetting
capacity of the matrix were added. The Halpin-Tsai
results were providing a higher correlation with
those composites fabricated with 50%vol of fibres,
revealing the presence of a poor interfacial adhesion;
• Thesisalbres are less porous than banana bres,
therefore absorbing less matrix phase. Low porosity
indicates better interfacial condition, and higher
fidelity of the predictions provided by both ROM and
Halpin-Tsai models;
• The addition of silica microparticles increases the
stiffness of the matrices, but does not seem sufficient
to improve the elastic moduli of the composites,
with a decrease of the level of adhesion between the
composites phases;
• The addition of maleic anhydride did not show a
relevant effect on the interfacial adhesion, featuring
instead a small increase of the Young’s modulus for the
composites C4 and C8 (sisal fibres – 20%wt of silica
addition) and C18 and C24 (banana fibres – 33%wt
of silica addition). Further investigation need to be
performed to assess the effect of this material as an
efficient coupling agent.
Acknowledgements
The authors would like to thank the financial support
of CAPES and the material suppliers: Resiqualy Company
(epoxy resin), Sisalsul Company (sisal fibres) and Moinhos
Gerais Company (silica particles).
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1012 Materials Research
