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Original article
Journal of Reinforced Plastics and
Composites
2022, Vol. 0(0) 1–42
© The Author(s) 2022
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DOI: 10.1177/07316844221112974
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Towards widespread properties of cellulosic
fibers composites: A comprehensive review
Mouad Chakkour
1
, Mohamed Ould Moussa
1
, Ismail Khay
1
,
Mohamed Balli
1
and Tarak Ben Zineb
2
Abstract
To meet the growing demand for the use of environmentally and friendly materials in various applications, many efforts have
been done in order to provide lighweighting, biodegradable, and renewable composites leading to a diminution of
greenhouse gas emissions. However, they are confronting some challenges that may limit their utilization in industries such
as poor interfacial adherence, high moisture absorption, low processing temperature, and impact strength. Several re-
search works have been carried out regarding the mechanical, thermal, and hydric properties of natural fibers reinforced
composites to deal with above underlined drawbacks. Actually, based on our analysis, there are two relevant concerns that
prevent their integration in other industries such as marine and renewable energy sectors. In this context, the current
critical review offers an up to date overview of the factors influencing the properties of the different components of natural
fibers composites (fiber, matrix, fiber/matrix interface) as well as their physicochemical, mechanical, thermal and hydric
behaviors. Recent works on the mechanical properties of nanobiocomposites based on cellulosic fibers, crystals, and nano-
clay fillers are reviewed. The future directions are also discussed to overcome the challenges confronted by these materials,
that is, fire resistance and moisture absorption, in order to widespread their utilization especially for wet and fire retardant
purposes.
Keywords
Cellulosic fibers, natural fiber composites, environmental aging, modeling
Introduction
Nowadays, the environmental consciousness has increased
and the use of ecofriendly materials that combine several
functional properties such as lightness and strength may be
encouraged.
1,2
Actually, the utilization of renewable and
lightweighting materials orient research activities toward
natural fibers that can reinforce polymers, bio-resins as well
as ceramics.
1,3
Indeed, natural fibers are characterized by
their important mechanical properties, low density, low cost,
biodegradability, renewability, and low environmental
impact.
1,2,4
These make them among the best candidates to
replace non-renewable and expensive synthetic fibers such
as glass, carbon, and kevlar fibers.
4,5
However, the low
temperature processing and the incompatibility with poly-
mers limit the utilization of natural fibers in industrial ap-
plications.
1,6
Currently, they are subdivided into three
categories based on their chemical compositions: Vegetal,
animal, and mineral fibers, where the latter are the most used
due to their abundance,
7
low cost
6
as well as attractive
mechanical properties.
6
Besides, the properties of renewable natural fibers de-
pend on several parameters such as: local climate (moisture,
temperature, and ultraviolet light), morphology, chemical
composition and extraction processes. Indeed, they are
highly influenced by the amount of absorbed water and sun
exposition during the plants growth in addition to the age of
plants.
8,9
These factors determine the polysaccharides
contents within plants especially the cellulose content and
its crystallinity.
6,8
Several characterizations could be used to
better understand the thermal, mechanical and physico-
chemical behaviors.
10,11
In fact, the morphology is one of the factors that in-
fluence the properties of natural fibers. The decrease of
mechanical properties is related to the increase of the mi-
crofibrilar angle (MFA) of fibers.
6
Bourmand et al.
6
reported
1
AMEEC team, Laboratory of Renewable Energies and Advanced Materials
LERMA, College of Engineering and Architecture, International
University of Rabat, Morocco
2
Laboratory for the Study of Microstructures and Mechanics of Materials
LEM3, University of Lorraine, France
Corresponding author:
Mohamed Ould Moussa, AMEEC team, Laboratory of Renewable Energies
and Advanced Materials LERMA, College of Engineering and
Architecture, International University of Rabat, Universit´
e Internationale
de Rabat Techn, sala al jadida 1100 AQ10, Morocco.
Email: mouad.chakkour@uir.ac.ma
that the flax and hemp fibers that exhibit high mechanical
properties have a small MFA comprised between 8° and 11°,
while sisal fibers displaying a low stiffness and strength
properties have a large MFA of about 20°. Likewise, the
local environment of natural fibers can also influence their
mechanical properties because of their high hydrophility.
Effectively, Rowell
12
studied the moisture adsorption be-
havior of some lignocellulosic fibers under 21°C and rel-
ative humidity RH of about 65%. They found that their
moisture adsorption ranges from 7% to 15%. Regarding
thermal resistance, the processing temperature of natural
fibers is limited due to the partial and complete degradation
of their chemical components at a certain temperature.
Actually, several studies
13,14
reported that the degradation
temperature of the most known natural fibers usually varies
between 215°C and 240°C. Concerning extraction pro-
cesses, the production of fine and strong fibers is technically
difficult
15
and dramatically depends on the used extraction
methods. For this occasion, Parnia zakikhani et al.
5
reported
the effect of different extraction methods on the tensile
properties of lignocellulosic bamboo fibers. They found that
the highest value is obtained for fibers extracted by com-
bining chemical and compression methods, while the lowest
one is exhibited by fibers extracted by using mechanical
rolling method. However, the roughness of the pure ex-
tracted fibers is small which make them incompatible with
considered matrices. Accordingly, different treatments ei-
ther chemical (NaOH, stearic acid, silane, and peroxide) or
physical (plasma, ozone, and corona) can be used in order to
enhance the compatibility and the interfacial adherence
between fibers and matrix by removing impurities and some
amorphous components present at the fibers surface.
1,5,16,17
In this case, Chin SC et al.
18
investigated the tensile strength
of lignocellulosic fibers, and, concluded that the tensile
strength of fibers treated with high NaOH concentrations
decreases when compared to pure ones, which is due to the
degradation of crystalline cellulose caused by the solution.
This review will provide a map for selecting the type of
fibers based on their physicochemical properties, cost, and
applications needs.
Besides, natural fibers are particularly used to reinforce
petrochemical-based resins including thermosets, thermo-
plastics and employed bio-resins matrices in bio-
composites.
8,9
Usually, petrochemical matrices are favored
to bio-resins due to their good mechanical properties,
1
abundance,
19
and low cost (approximately 10% lower
than bio-resins).
19
Regarding thermoplastic polymers, the
macromolecular chains are linked together with weaker
intermolecular forces such as van der Waals and hydrogen
bonds.
20
These polymers can change reversibly from vis-
cous to solid states under cooling and heating, respectively.
The advantages of utilizing thermoplastic polymers are their
important fracture resistance, high strain under failure, and
impact strength.
20
Also, their recyclability enables us to
reduce the waste amount. Polypropylene is one of the most
used thermoplastics because of its good mechanical prop-
erties (tensile and flexural strengths), small porosity, and
high dimensional stability which result in the enhancement
of the water absorption property.
21
The use of polyethylene
and polycarbonate are recommended because of their good
thermal and mechanical properties. Due to its excellent fire
and chemical resistance as well as low cost and good du-
rability, the polyvinyl chloride becomes the most appro-
priate material in building sector.
21
However,
macromolecular chains of thermosets matrices are linked by
hydrogen elements which are removed irreversibly upon
heating.
20
They are suitable for applications where the high
thermal stability and chemical reactions are needed.
20
In
addition, they exhibit a small viscosity at the initial stage of
processing because of their small molecular weight re-
sulting in a good wettability of fibers. Epoxy and polyester
are the most used thermosets because of their excellent
thermal, mechanical and chemical resistances in addition to
their high dimensional stability. Indeed, their hydrophobic
structure makes them the best candidate for composite
laminates and advanced composite surfaces.
21
Many studies have been carried out to investigate the
multiphysical properties of natural fibers reinforced com-
posites. Several researchers studied the effect of different
chemical treatments, fibers fraction, orientation and length
on the mechanical, hydric, and thermal properties. Most of
studies reported that the augmentation of fibers content
increases the void content in composites. The optimal
mechanical properties are generally obtained for 30 wt%
longitudinal fibers loading composites.
22,23,24,25
Regarding
hydric properties, the equilibrium moisture content and
absorption rate was found to increase while increasing the
fibers content. These results are explained by the important
void content within composites. The mechanical properties
are highly influenced by the moisture absorption which
induces swelling of fibers, leads to the development of shear
stresses at the interface and finally to the debonding of fibers
and matrix.
26
Some solutions presented in the literature such
as chemical treatments enhance the mechanical properties,
reduce partially the hydrophility, and moisture absorption of
biocomposites. However, they are still not sufficient to
address their use for wet applications. Other researchers
focused on the thermal properties of natural fibers com-
posites. Most of them reported that the thermal conductivity
rises when the fiber content increases due to the increase of
the porosity within composites that affects the capability of
heat transfer.
27,28
From modeling point of view, several works used finite
elements method FEM and analytical models to predict the
mechanical behavior of natural fibers reinforced composites
(NFRCs). In this way, Palombini et al.
29
used FEM to
investigate the effect of the different structural components
of the bamboo. Cao et al.
30
used some analytical models
such as Halpin-Tsai and the rule of mixture (ROM) to study
the mechanical properties of wood/flax reinforced
2Journal of Reinforced Plastics and Composites 0(0)
polypropylene composites. Potluri et al.
31
studied also the
mechanical properties of pineapple fibers reinforced epoxy
composites using analytical models.
As early reported, natural fibers are incompatible with
hydrophobic polymers due to their hydrophilicity. To en-
hance the fibers-matrix interface, the incorporation of
nanofillers such as nanoclays, SiO
2
,TiO
2
, and graphene
remains an efficient solution to improve the mechanical,
physicochemical, optical, and thermal properties of bio-
composites.
20
Nanoclays fillers have attracted the attention
of researchers due to their great aspect ratio, low cost, and
wide availability
32
in addition to their excellent mechanical
and fire retardancy behaviors. Preliminary investigations
reveal that the addition of nano-additives reduces the water
absorption while improving the stress-transfer between
matrix and fibers, until a critical filler content where the
mechanical properties drop due to the agglomeration of
fillers. Indeed, Haq et al.
33
have studied the effect of 1.5
wt.% of Cloisite 30B nano-additives on the mechanical
properties of hemp reinforced polyester hybrid composites.
They particularly reported that the tensile modulus and
strength increase by 6% and 20%, respectively. Arulmur-
ugan et al.
34
investigated the influence of montmorillonite
nano-clays contents on the mechanical properties of jute
fibers reinforced polyester nanocomposites and reported
that the optimal mechanical and vibration properties are
obtained for 5 wt.% nano-clays contents. Upon 5 wt.%
nano-clays content, the agglomeration of fillers prevents
a better load transfer between the composite components
weakening mechanical properties. However, few studies
have addressed the influence of nanofillers on the water
absorption and fire retardancy behaviors of biocomposites.
In general, the adoption of these renewable fibers in
industries would be of great interest due to their lighweight,
low cost, and non-toxicity.
35,36
Currently, automotive in-
dustry is leader in the use of natural fibers reinforced
composites (NFRCs) especially German companies
including Mercedes, Volkswagen, Audi, BMW as well as
Opel that encourage the utilization of natural fibers in
several interior applications such as decking, window
frames, floor trays, and the headliners.
1,37
In addition,
natural fibers are used in building, medical, and packaging
applications.
38
This current review paper aims to provide in a critical
way, the whole picture of natural fibers reinforced polymer
composites NFRCs and help to understand the issues for
their widespread integration into more applications mainly
for wet and fire-retardant targets. It is organized into dif-
ferent sections, as shown in the Figure 1. A detailed pre-
sentation of the factors influencing the different components
of NFRCs (fiber, matrix, fiber/matrix interface) as well as
the most used characterization techniques are provided in
the first part, then, the mechanical, hydric, and thermal
properties of biocomposites are reviewed while discussing
the encountered challenges. Afterward, progress works
regarding the mechanical properties of nanobiocomposites
are reported. Finally, applications and the future trends and
directions will be reported and discussed.
Composition of natural fibers reinforced
composites NFRCs
Natural fibers
In recent years, natural fibers have been widely used to
reinforce composite materials
1
with various origins such as
vegetal, animal, or mineral. Considered as renewable re-
sources, they exhibit attractive lightweight, mechanical, and
economic properties (see Table 1), which make them serious
alternatives to the expensive and non-renewable synthetic
fibers (glass, carbon, and kevlar fibers) at least for soft
structure applications.
39,40
Regarding vegetal fibers, they are namely composed of
sugar-based polymer components (cellulose and hemicellulose)
Figure 1. Graphical summary of the review paper.
Chakkour et al. 3
while protein is the main component of the animal ones.
8
The former can be categorized into eight groups based on
the nature of plants and plant part, they are extracted from
bast fibers (jute, flax, hemp, banana) from the skin of the
plants stems; leaf fibers (banana, pineapple) from leaves
plants; seed fibers (cotton, coir) from seeds; grass fibers
(bamboo, rice) and core fibers (corn, wheat stalk) from
stalks; wood fibers; foot fibers (cassava); and fruit fibers
(banana, coir).
8,19,41
Hence, according to above-mentioned
categories (wood and non-wood fibers)
1
they can be dis-
tinguished. The non-wood fibers are more lightweight and
show indeed high specific strength, high specific modulus,
flexibility during processing, fatigue and corrosion re-
sistance, world-wide abundance,
42
and higher cellulose
concentration compared to wood fibers.
9
In addition, there
are three types of animal fibers: Animal hairs fibers, avian
fibers, and silk fibers (for instance, spider silks). Bast fibers
(especially Flax) show rapid absorption and desorption of
water, low thermal conductivity, and the high acoustic in-
sulation.
1
The leaf fibers (like abaca fibers) present high
mechanical strength. Seed fibers are characterized by their
water absorption, strength, and concentration of lignin in
comparison to other natural fibers. Bamboo fibers (grass
fibers) are known by their excellent durability and good
properties such as stability, tenacity, flexibility, ultraviolet
radiation resistance, and small density.
1
Furthermore, sheep
animal fibers are known by their good thermal and acoustic
insulation, high deformability, and durability similarly as
the lightweight feathers fibers. Regarding mineral fibers,
they exhibit a good fire resistance.
19,41,43,44
Figure 2 summarizes the natural fibers classification il-
lustrated by some images.
However, the widespread utilization of the natural fibers
remains confronted to certain limitations
9,45
such as the high
moisture absorption, low compatibility with conventional
resins and low homogeneity compared to synthetic fibers
(glass, carbon, and kevlar fibers).
Nowadays, the vegetal lignocellulosic fibers are usually
encountered as reinforcement for composites in several
applications such as automotive, aeronautic, military,
building, and sport equipments.
9,47
This is due to their
mechanical performances compared to animal fibers. Cur-
rently, known as sugar-based plant fibers, lignocellulosic
fibers include mainly polysaccharides which are carbohy-
drates and associate several monosaccharides made of
carbon, oxygen, and hydrogen.
9
They are bonded by hy-
drogen links via the relevant hydroxyl groups in the
structure.
48
In fact, starch, cellulose, hemicellulose, and
proteins are the most widely usual polysaccharides. Sugar-
based plant fibers are composed mainly of cellulose,
hemicellulose, and lignin with some amount of pectines,
waxes, and impurities. Cellulose is considered as a rigid
armature surrounded by amorphous areas consisting namely
of lignin and hemicellulose with the other minor con-
stituents. It is reported that the properties of the vegetal
fibers are affected by the amount and the crystallinity of
cellulose, the polysaccharide composition of cell walls and
the microfibril angle which corresponds to the orientation of
cellulosic microfibrils with respect to the fiber axis.
6
These
key parameters are highly sensitive to the age, the plants
type, and the climate conditions (amount of sun radiation
and absorbed water during the plant growth).
6,9
According
to this particular microstructure of the vegetal fibers, it is
worth in the following sections to introduce more details
about the mentioned chemical components:
•Cellulose
It belongs to the family of periodic homopolysaccharides and
then appears as a sequence of linear macromolecules in which
the monomer is the glucose elements including repetitive
motif called cellubiose that is composed of two glucose
Table 1. Advantages and disadvantages of natural fibers.
Advantages of natural
fibers
Disadvantages of natural
fibers References
Low cost High moisture absorption
1
Low density Variation in fibers quality
19
Renewable Low maximum processing
temperature
19
Biodegradable Hydrophilic structure
1,6
Non-toxic Discontinued fibers
1,6
Figure 2. Classification of natural fibers with respect of their origin
1,46
(Google images reviewed on 2021).
4Journal of Reinforced Plastics and Composites 0(0)
molecules and six hydroxyl groups per monomer
5
(see
Figure 3). These hydroxyl groups ensure intra and in-
termolecular hydrogen bonds with other macromolecules
leading to crystalline arrangement of the cellulose macro-
molecules into microstructures called microfirbils.
49
The latter
are embedded in amorphous cellulose matrix showing small
packing density.
50
Moreover, the polymerization degree of
cellulose is 10–100 higher than the hemicellulose one.
5
•Hemicellulose
Hemicellulose is the second most widely organic material
found within the fibers in terms of quantities. It comprises
heterogeneous polysaccharides of xylopyranose, glucopyr-
anose, and mannopyranose chains.
51
Regarding the moisture
absorption, it is reported that hemicellulose components are
approximately 2.6 higher than the lignin ones.
9
•Lignin
Lignin is a 3D polymer composed of aromatic units strongly
intramolecular bonded and filling the space between cel-
lulose, hemicellulose, and pectins.
52
In addition, lignin acts
as a coupling agent between cellulose and hemicellulose
increasing the stiffness of fibers.
53
It is reported that the
specific tensile strength of plant fibers ranges between 1600
and 2950 MPa while Young’s modulus is comprised be-
tween 10 and 100 GPa.
53
The Young’s modulus of cellulose
is 140 GPa, and that for hemicellulose is equal to 8 GPa. The
evolution of these parameters is strongly influenced by the
temperature and the humidity. In fact, the presence of an
important quantity of lignin improves stiffness and rigidity
of the lignocellulosic fibers.
1,6
Figure 4 gives a sketch of the
morphology of a lignocellulosic bamboo fiber.
5
Nowadays, the majority of NF is produced in Asian
countries especially China, India, and Vietnam while small
quantities can be produced in South America (Brazil) and
Africa (Nigeria and South Africa)
1
(see Table 2). Abaca
fibers are species of banana that are cultivated as a com-
mercial crop in Asia, especially in the Philippines. They
exhibit an important tensile strength lying between 600 MPa
and 900 MPa and Young’s modulus ranging from 30 GPa to
50 GPa,
57
making them very promising for industrial ap-
plications. Sugarcane (Bagasse) attracts the attention of
researchers and scientists because of its biodegradability
and abundance. It is widely used for making furniture,
insulating boards, and filter muds.
55
Bamboo fibers are
abundant in humid equatorial zones, especially in Africa,
Asia, and South America. They are mainly used in
construction, agriculture, and wood-based panels.
55,58
Ba-
nana is obtained from the bast of the plant Musa sapientum
Linn. The high cellulose content and low microfibrillar
angle MFA indicate that banana fibers could be a good
reinforcement for hydrophobic polymer matrices.
55,59
Co-
conut palm is a member of the palm family that grows in the
Southeast Asia (Malaysia, Indonesia, and Philippines) and
tropical islands such as Seychelles, Mauritius, Polynesia,
Comoro, and Maldives. Date palm fibers are found in the
Middle East, Northern Africa, India, United States (Cal-
ifornia), and Pakistan. Flax is one of the most used natural
fibers because of its important specific mechanical prop-
erties which are comparable to those of glass fibers. Canada
has been the world largest producer and exporter of flax
since 1994. The latter are widely used as reinforcements for
composites in several industrial applications, including
automotive interiors.
55,60
Hemp (Cannabis Sativa) is a plant
native to Philippines and China. The corresponding fibers
are extensively used in textiles and automotive compo-
nents.
55,61
Jute is one of the cheapest natural fibers that
grows in China, India, and Bangladesh. It is known as the
“golden fiber”because of its biodegradability, excellent
thermal and electric insulations.
8
Pineapple is a short
tropical plant where related fibers are inexpensive and
exhibit a high specific strength and stiffness.
55,62
Sisal is one
of the widely used natural fibers because of its good me-
chanical and acoustic properties as well as easy cultivation.
The main producers of sisal fibers are Tanzania and
Figure 3. Monomer unity of cellulose polymer.
1,6
Figure 4. (a) Vascular bundle of bamboo, (b) elementary fiber
10–20 μm, (c) nano fibril 1–10 μm involves lignin, and
hemicellulose.
5,54
“Reprinted from Materials and Design 63, by
Parnia Zakikhani, R. Zahari, M.T.H. Sultan, D.L. Majid, Extraction
and Preparation of bamboo fiber-reinforced composites, pp 820–
828, Copyright 2021, with permission from Elsevier, (License
number: 5,118,180,687,934).”
Chakkour et al. 5
Brazilia.
55,63
Aliakbar et al.
1
report that the three fibers most
produced cellulosic are bagasse, bamboo, and cotton fibers.
The annual production reaches 75, 30, and 25 millions of
tons, respectively.
1
However, the effective cost of bamboo
and alfa fibers are the lowest among all lignocellulosic fi-
bers.
6
Actually, the bamboo price ranges between 0.3 and
0.5 $/kg which is 2.8–9.3 times cheaper than glass fibers.
Some of natural fibers such as sisal and hemp are more
expensive than others due to their quality, fineness, and
mechanical performance.
6
In this way, it is worth to
highlight that fluctuations of both supply and demand
dramatically affect the fibers price.
6
In addition to economic aspects, the mechanical prop-
erties of natural fibers are vital in their selection as
Table 2. Commercial prices
6
and world production
7,8,19,55,56
of the most used cellulosic fibers.
Fiber nature Producer Cost ($/Kg)
World production
(millions of tons) References
Abaca Philippines, Ecuador 0.45–1.3 0.07
6,19
Sisal Brazil, Kenya, Tanzania, China, Cuba, Haiti, Madagascar, India —0.378
6,8
Kenaf India, China, Malaysia, USA, Thailand, Vietnam 0.4–0.7 0.97
6,7
Jute India, Nepal, Bangladesh 0.5–1.15 3.45
6,8
Hemp China, France, Germany, UK 0.7–1.75 0.214
6,19
Bamboo China, Japan, India, Chile, Ecuador, Nigeria, Philippines, Pakistan 0.3–0.5 30
6,56
Cotton China, Brazil, India, Pakistan, Turkey 0.6–2.25 25
6,56
Elephant grass ——Abundant
55
Broom ——Abundant
55
Oil palm fruit ——Abundant
55
Wood —0.1–0.65 —
6
Ramie Tropical Asia 0.5–1.2 0.28
1,6
Flax China, Brazil, Philippines, India 1.75–2.7 0.83
1,6
Coir India, Thailand, Vietnam, Philippines, Indonesia 0.4–0.75 1.2
1,6
Alfa —0.2–0.4 —
6
Table 3. Averaged mechanical and physical properties of the most used natural fibers.
55,46,64,65,66,67
Fiber
Diameter
(μm)
Tensile
strength
(MPa)
Tensile
modulus
(GPa)
Elongation
(%)
Density
(g/cm3)
Specific tensile
strength
(MPa.cm3.g1)
Specific tensile
modulus
(GPa.cm3.g1)
Jute —393–860 13–60 1.5–1.8 1.44–1.52 258.5–597.2 8.5–41.6
Flax —345–1500 27.6–90 2.7–3.2 1.42–1.52 226.9–1056.3 18.2–63.4
Hemp —550–920 55–70 2–41.47–1.52 361.8–625.8 36.2–47.6
Bamboo —140–800 11–32 2.5–3.7 0.6 –1.1 127.3–1333.3 10–53.3
Bagasse 200–400 220–290 17–27.1 1.1 1.25 177.6–232 13.6–21.7
Feather —100–203 3–10 6.9 0.9 111.1–225.5 3.33–11.1
Sisal 50–300 468–790 9.4–25 2 –7 1.4–1.45 322.75–564.3 6.5 –17.2
Silk —100–1500 5 –25 15 –60 1.3 76.9–1153.8 3.8–19.2
Wool —50–315 2.3–5 13.2–35 1.3 38.46–242.3 1.77–3.85
Abaca leaf
fiber
114–130 418–486 12–13.8 - 0.83 503.61–585.54 14.45–16.62
Alfa —35 22 5.8 0.89 39.32 24.71
Banana 80–250 529–759 8.20 1–3.5 1.35 391.85–562.22 6.07
Cotton —287–597 5.5–12.6 3–10 1.6 179.37–373.12 3.43–7.87
Coir 100–460 108–252 4–615–40 1.15 93.91–219.13 3.47 –5.21
Curaua 170 158–729 —5 1.4 112.85 –520.71 —
Kenaf 81 250 4.3 —1.4 178.57 3.07
Oil palm 150–500 80–248 0.5–3.2 17 –25 0.7–1.55 51.62–354.28 0.32 –2.06
Piassava —134–143 1.07–4.59 7.8 –21.9 1.4 95.71–102.14 1.21–3.27
Pineapple
leaf
20–80 413–1627 34.5–82.5 0.8–1 1.3 317.69–
1251.53
26.53–63.46
Ramie 20–80 400–1000 24.5–128 1.2 –41–1.55 258–1000 15.8–128
6Journal of Reinforced Plastics and Composites 0(0)
reinforcement for composites. Several works have reported
such issues and analyzed the parameters that influence the
quality of the selected fibers.
5
The Table 3 shows the av-
eraged mechanical properties of the most widely used
natural fibers.
Usually, the specific mechanical properties of natural
fibers are generally important due to their small density. In
addition, one notes that hemp, jute, flax, and bamboo exhibit
tensile strength and Young’s modulus similar to those of
E-glass fibers that exhibit about 480–600 MPa.g/cm
3
and
28 GPa, respectively
68
(see Table 3). Sometimes, bamboo
fibers are considered as glass fibers due to their morphologic
longitudinal alignment.
Parameters influencing the mechanical properties of natural
fibers. The reported literature issues
6,9
have revealed re-
cently that the multiphysical properties of natural fibers
depend on several parameters such as extraction methods,
chemical composition, processing temperature, and con-
ditions of plant’s growth. Figure 5 shows the general factors
that influence the properties of fibers going from the plant
growth to the supplying stage. More details are highlighted
in following sections.
Chemical composition. Plant-based fibers are composed
mainly of cellulose, hemicellulose, lignin, and pectines as
mentioned early, in addition to waxes, fats and other in-
organic components which represent a small percentage of
fibers mass. Actually, the chemical composition varies with
the plant type, the growth conditions and the amount of
absorbed water and sun exposition. These factors are re-
sponsible especially for the change of cellulose amount and
crystallinity.
8
The latter represents the capacity of the cel-
lulose OH groups to form high-density hydrogen bonds
leading to compacted cellulosic chains. Hence, it is reported
that the crystalline cellulose shows higher Young’s modulus
that reaches 136 GPa and only 75 GPa for synthetic glass
fibers.
6
Table 4 shows chemical compositions of the most
used plant-based fibers.
Usually, more natural fibers contain cellulosic elements
more their mechanical properties increase except for the
cotton fibers.
6
Gurunathan et al.
9
showed that the specific
tensile strength of the plant-based fibers falls between
1600 MPa and 2950 MPa, while the specific tensile
modulus is comprised between 10 GPa and 130 GPa.
Besides, Cousins et al.
53
found that the Young’s modulus of
cellulose is almost 140 GPa whereas that of hemicellulose is
estimated to be only 8 GPa, provided, these values may
change with respect of the relative humidity conditions.
53
Further, Bledzki et al.
74
reported that the Young’s modulus
of crystalline cellulose lies between 134 and 136 GPa, while
that of the hemicellulose is about 8 GPa in dry atmosphere
(about 12% of relative humidity) and only 0.2 GPa at
ambient atmosphere.
75
Moreover, lignin that is considered
a compatibilizer or coupling agent between cellulose and
hemicellulose contributes to increase stiffness properties of
plant-based fibers.
8
According to above underlined works, it seems clearly
that crystalline cellulose and lignin dramatically contribute
mortally to enhance the mechanical behavior of vegetal
fibers.
6
Morphology. Morphological aspects including microfi-
brillar angle (MFA) and dimensions of fibers strongly affect
the mechanical properties of lignocellulosic fibers.
76
In fact,
the microfibrillar angle MFA characterizes the orientation of
cellulosic microfibrils in the secondary plants wall with
respect of the longitudinal direction.
6
In literature, several
authors used X-Ray diffraction method to measure mi-
crofibrillar angle which may change during tensile tests.
Wang et al.
77
showed that during tensile tests, cellulose
microfibrils are realigned allowing a reduction in microfi-
brillar angle. Trivaudey and Placet
78
investigate the effect of
tensile loading on the evolution of microfibrillar angle of
hemp fibers. They found that MFA decreased from 11° to
7.2° under a loading amplitude of about 0.5 N. Eder et al.
79
investigated the evolution of tensile stiffness of wood
samples and different extracted fibers with respect of mi-
crofibrillar angle MFA. They found that tensile stiffness of
wood samples varies between 8 GPa and 56 GPa, while their
correspondent MFA ranges between 10° and 40°. However,
stiffness does not exceed 30 GPa for MFA between 6° and
15° in the case of 65% of dried wood fibers obtained by
chemical extraction, while the lower Young’s modulus is
obtained for fibers extracted by other processes exhibiting
higher microfibrillar angles.
Regarding the fibers dimensions, Humphrey et al.
80
investigated the effect of aspect ratio of coconut, ba-
gasse, and oil palm fibers on the compressive and tensile
strengths of soil blocks. They opted for the following ex-
perimental protocol of block elaborating that exists to
maintain the maximum moisture content at 18%, to keep the
fibers immerged in water for 48 h before being beaten, use
a compound light microscope to determine the fiber di-
ameter, adapt the fiber length for considering 5 values of the
Figure 5. Factors influencing the fibers quality.
55
Chakkour et al. 7
aspect ratio between 25 and 125, mix fibers with soil
provided a volume fraction of fibers of about 1%, and
elaborate blocks with uniform section by imposing a con-
stant pressure of about 100 bars. They obtained that the
increase in the aspect ratio from 50 to 125 leaded to improve
the compressive strength by 26%, while the aspect ratio of
100 is the optimum value for blocks reinforced by oil palm
and bagasse fibers showing an increase of 14% and 32%,
respectively. At these maximum ratios, the tensile strength
increased by 61%, 24%, and 20% for blocks reinforced by
coconut, oil palm, and bagasse fibers, respectively. A.S.
Singha et al.
81
also investigated the effect of fiber length on
the tensile and compressive properties of agave fibers re-
inforced polystyrene composites. They used raw agave fi-
bers with 90 μm, short fibers with 3 mm and 8 mm,
respectively, of lengths were used as continuous fibers for
reinforcement. They concluded that reinforced composites
with particles exhibits excellent mechanical properties
where compared to long and short reinforcements.
Moisture adsorption. Hydrophilic features of cellulosic
fibers result in high water and moisture absorption.
82
In fact,
the resulting swelling alters their mechanical properties and
leads to poor interfacial adherence and to the interface
debonding phenomenon accordingly within natural fibers
reinforced composites when associated with hydrophobic
matrices.
Rowell
12
studied the equilibrium moisture content of the
widely used natural fibers at operating temperature of 21°C
and relative humidity RH of 65%. He showed that Abaca
and Pina fibers adsorbed almost 15% and 13%, re-
spectively. This corresponds to the highest moisture
contents whereas flax and bamboo fibers adsorb 7% and
8.9%, respectively.
Such behavior can be explained by the fact that cellulosic
fibers have a hydrophilic structure being responsible for the
high water and moisture adsorptions. Further, it is reported
that the hydrophilic structure of natural fibers lead to
a swelling after water or moisture absorption and causes
poor interfacial adherence with hydrophobic matrices. This
results in poor mechanical properties of natural fibers re-
inforced composites (NFRCs).
The equilibrium moisture content of the most widely
used natural fibers at a temperature of 21°C and relative
humidity RH of 65% are reported by Rowell and sum-
marized in Table 5
12
(see Table 5). Accordingly, Abaca,
Pina fibers adsorbed almost 15% and 13%, respectively,
which are the higher humidity contents whereas flax and
bamboo fibers have adsorbed 7% and 8.9%, respectively.
Chung KF et al.
83
have studied the effect of water ab-
sorption of two types of bamboo specimens. Their length
was about 1200 mm and the external diameter was esti-
mated to be 25 mm. Specimens are immersed in water inside
an oven at 105°C for various time periods: 1–2–4–8–12 h
and 1–2–3–7 days. They found that the compressive and
bending strengths of bamboo have decreased by half and one-
third after 7 days of immersion. During the same experiment,
the Young’s modulus has decreased by one-third. Godbole
et al.
84
have investigated the effect of water absorption on the
mechanical properties of bamboo samples. In their works,
specimens are immersed in distilled water at an ambient
temperature of 20°C for 144 h. They absorb 81.2% of their
weight. In addition, their Young’s modulus and tensile
strength have decreased by 48% and 37%, respectively.
To deal with these drawbacks, several studies devoted to
investigate the effect of chemical treatments on the moisture
adsorption and the mechanical behavior of natural fibers and
biocomposites. However, few researchers are focused on
the study of dimensional changes (swelling) of natural fibers
or natural fiber composites upon exposure to relative hu-
midity. It is very important to conduct research on this
aspect to constitute key data for the predictive modeling of
the hygromechanical behavior of biocomposites.
Table 4. Chemical composition of the most widely used lignocellulosic fibers.
55,69,70,71
Fiber Cellulose (wt%) Hemicellulose (wt%) Lignin (wt%) Waxes (wt%) Pectins (wt%) References
Abaca 56 –63 20 –25 7 –93 1
8,72,73
Coir 32 –43 0.15 –0.25 40 –45 —3–4
37,49
Cotton 85 –90 5.7 0.7 –1.6 0.6 0.1
37,49,70
Flax 71 18.6 –20.6 2.2 1.5 2.3
37,49,72
Jute 61 –71 14 –20 12 –13 0.5 0.2
37,49,70
Bamboo 26 –43 30 21 –31 ——
8,49
Hemp 68 15 10 0.8 0.9
37,49,69,72
Ramie 68.6 –76.2 13 –16 0.6 –0.7 0.3 1.9
37,71,72
Sisal 67 –78 10 –14 8 –11 2 10
37,49
Bagasse 55.2 16.8 25.3 ——
8,49
Agave 68.42 4.85 4.85 0.26 —
55
Banana 60 –65 6 –85–10 ——
55
Borassus 53.40 29.6 17 ——
55
Kenaf 31 –72 20.3 –21.5 8 –19 ——
55
Pineapple leaf 70 –83 —5–12.7 ——
55
8Journal of Reinforced Plastics and Composites 0(0)
Thermal resistance. The flammability of cellulosic fibers
leads to the loss of stiffness and strength which limits their
utilization under high temperatures.
85
It is reported that the
thermal decomposition of fibers depends on their chemical
composition.
85
Kozlowski et al.
85
reported that the increase
of crystalline cellulose within fibers structures decreases the
rate of the thermal decomposition of lignocellulosic fibers,
increasing then the fire resistance of natural fibers reinforced
composites.
Several authors observed that cellulose degrades in
a temperature range comprised between 300°C and 420°C,
while pectins and hemicelluloses between 250°C and
320°C
86–88
Due to the complex chemical structure of lignin,
there is a permanent discussion about its thermal de-
composition. Some works showed that lignin breaks down at
200°C according to Ref.
89
and 400°C according to Ref.
90,91
while Yang et al.
88
demonstrated that lignin can be decom-
posed in a large temperature range ranging from 160°C up to
900°C. It can be concluded that the high content of amor-
phous structures as hemicelluloses and pectins within fibers
decrease their fire resistance. However, chemical treatments
usually utilized for natural fibers can enhance their thermal
stability by removing amorphous parts. Ta ble 6 shows the
averaged decomposition temperature of the most used lig-
nocellulosic fibers. Figure 6 illustrates a typical TGA curve of
non-treated Agave Americana fibers.
Based on several studies, it seems that chemical treat-
ments enhance the thermal stability of fibers by eliminating
a significant amount of lignin and non-amorphous com-
ponents that degrade at low temperatures.
Extraction processes. Nowadays, it is well known
15
that
the production of fine and strength fibers is now technically
difficult because of limited and expensive involved pro-
cesses. Currently, the common extraction methods are me-
chanical such as steam explosion, retting, crushing, rolling,
and/or chemical such as those using alkaline treatments.
•Mechanical procedures
Steam explosion process
This process consists of cutting plant culms into different
specimens which are then placed in autoclave housing su-
perheated steam at a temperature of 175°C and a pressure
between 0.7–0.8MPafor60min.
5
This is cycled until cells
fracture. At the end of the process, the ash resulted is removed
by washing fibers using water at 90°C, then, the latter are
dried in an oven at a temperature of 105°C for 24 h. As
a consequence, lignin is condensed on the fibers surface
increasing their roughness. This negatively impacts the in-
terfacial adhesion between fibers and matrix leading to the
decrease of mechanical properties of the resulting composite.
Retting method
There are two types of retting such as dew retting and water
retting.
11
The first one consists in breaking down the cellular
tissues and the substances that surround the fibers in the
presence of bacteria, sunlight and dew. This method is
widely used in the industrial production of blast fibers.
However, in water retting, water molecules penetrate the
stems and cause the internal cells to swell resulting in
a bursting of the plant outer layer. In fact, the process in-
cludes maintaining the stem specimens in water for 3 days at
room temperature in order for it to be soft, and then, it is
beaten and separated using a knife or a razor. This operation
is repeated until the fibers are separate from each other.
96
Crushing method
This method consists to cut stems into several samples
which are crushed using a roll in order to extract fibers.
Then, the extracted fibers are washed with boiled water at
90°C for 10 h in order to remove fat traces.
97
Rolling mill method
This method is used to cut samples into several strips of
small thickness which are immersed in water for 1 hour in
order to be softened. Then, strips are passed through
a rolling mill at small rotation speed, soaked another time in
water for 30 minutes to ease their extraction and are sep-
arated using a knife or a razor.
98
•Chemical proceeding
Chemical extraction can affect the fibers microstructure by
modifying its composition. One distinguishes namely the
alkali procedure which consists to immerse plant strips into
container of NaOH with a concentration of 1.5 N (Nor-
mality) at 70°C for 5 hours. Then, alkali-treated strips are
pressed. The resulting fibers are washed in water and dried
in an oven.
99
It is reported that the main advantage of this
method is that fibers exhibit less damage.
99
•Combination of chemical and mechanical methods
The compression and roller mill techniques are usually used
after chemical treatments in order to extract fibers. How-
ever, the choice of the corresponding extraction method
may depend on the expected quality of extracted fibers.
Table 5. Some reported moisture content of some selected natural fibers at 21°C and RH 65%.
12,55
Fiber Moisture content (%) Fiber Moisture content (%) Fiber Moisture content (%)
Abaca 15 Hemp 9 Bamboo 8.9
Coir 10 Pina 13 Agave 7.69
Flax 7 Ramie 9 Bagasse 8.8
Jute 12 Sisal 11 Pineapple leaf 11.8
Chakkour et al. 9
Among all natural fibers, some studies have investigated the
effect of extraction processes on the mechanical properties
of bamboo fibers due to their high growth rate and their
ability to fix atmospheric carbon dioxide.
5,100
. Indeed,
Zakikhani et al.
5
investigated the mechanical behavior of
bamboo fibers using different extraction methods (see
Table 7). The combination of chemical and mechanical
methods enables to extract fibers with high tensile strength
ranging between 645 and 1000 MPa.
5
Furthermore, the
obtained fibers by steam explosion exhibits moderate
strength that change from 308 MPa to 862 MPa but remain
better than those extracted using the rolling mill method
where the strength did not exceed 270 MPa.
5
The morphology of fibers depends on the extraction
methods. Mechanical methods lead to a small global
roughness due to the presence of the cellulose surrounded
with lignin, hemicelluloses, and pectins. The local roughness
at the microfibril level remains important because of the
presence of some impurities and inorganic components at the
microfibril surface. The combination of chemical and me-
chanical methods shows small local roughness because of the
suppression of above-mentioned impurities, while the global
roughness increases as some of hemicelluloses and lignin are
removed.
1,5,102,101
Modification of natural fibers. The main drawbacks that would
restrict the utilization of natural fibers as reinforcements of
composites are their high moisture absorption and low thermal
stability.
103
In fact, the high hydrophility of the former makes
them incompatible with polymer matrices which lead to in-
terfacial debonding phenomena within composites.
8,19
Therefore, the surface of either natural fibers or some ma-
trices like bio-based resins needs to be treated in order to
improve the stiffness and the strength of composites.
8,104
One
can distinguish between physical and chemical treatments.
Physical modifications improve the mechanical adhesion
between matrix and natural fibers without changing the
chemical properties of the latter. Corona, plasma, and ultra-
violet (UV) are the mainly used physical treatments to change
the surface properties of natural fibers.
8
As in corona, plasma
treatments change the surface energy of cellulosic fibers under
high voltage at low temperature. Regarding the UV treatment,
it increases the polarity of fibers which allows a good in-
terfacial adherence with matrices.
105,106
In addition, chemical
treatments improve the adhesion between fibers and matrices
via chemical reactions.
107,108
The most used techniques are
described in the following sections.
Chemical modification
•Alkaline treatment
This method attacks the molecular structure of fibers by
decreasing the hydroxyl groups.
109
It involves a decrease in
Table 6. Decomposition temperature of some selected lignocellulosic fibers.
1,13,14,93,94,95
Fiber Decomposition temperature (°C) References
Bagasse 232
1,14
Bamboo 224
13,14
Cotton 232
1,13,14
Hemp 215
1,13,14
Jute 215
1,14
Kenaf 229
1,13,14
Pina 240
1,13,14
Chicken feather 350
93
Raw muntingia calabura 213
94
Silane-treated muntingia calabura 268
94
Alkali-treated muntingia calabura 254
94
Raw water hyacinth 289.22
95
Alkali-treated water hyacinth 294.67
95
Silane-treated water hyacinth 329.71
95
Citrifolia 248.44
92
Silane-treated citrifolia 274.71
92
Figure 6. TGA curves of raw and treated Agave Americana
natural fiber adapted to Madhu et al.
136
10 Journal of Reinforced Plastics and Composites 0(0)
the amount of non-cellulosic components such as hemi-
cellulose, lignin, pectins, and waxes.
19
The resulting ef-
fective surface area of fibers becomes cleaner allowing good
stresses transfer through cells. Further, given the optimum
concentrations, alkali treatment is the most widely used
treatment because of its simplicity and efficiency.
•Silane treatment
Silane forms a chemical link between fiber surface and
matrix via siloxane molecules.
8
First, silanol is created
using existing moisture and hydrolyzable alkoxy groups and
the free chemical bridges reacts with cellulosic OH groups.
This ensures molecular continuity along the interface and
the formed hydrocarbon chains that contribute to reduce
swelling.
19
•Acetylation treatment
In this treatment, fibers are immersed into acetic acid and
acetic anhydride, respectively, in order to accelerate re-
actions.
8
An esterification reaction occurs between OH
groups and carboxy/anhyride groups of natural fibers.
110
Accordingly, the fiber surface becomes consequently
smoother and the mechanical stress transfer toward cells
interfaces is improved.
1
•Benzoyl treatment
Benzoyl chloride is used to decrease the hydrophilic surface
property improving then the interfacial adherence and the
thermal stability of fibers. This treatment consists of two
process steps in which waxes and impurities are removed,
then, fibers are immersed in benzoyl chloride solution where
benzoyl groups are substituted to the OH groups.
1
•Potassium permanganate treatment
This method consists of immersing fibers in 0.5 wt.%
permanganate potassium and acetone for 30 min
11,112
The
resulting fibers are washed and air dried for 24 h at
a temperature of 105°C. Mohammed et al.
111
have treated
sugar palm fibers with 0.66 wt.% potassium permanganate
leading to an improvement of the tensile properties of
treated fibers and their composites. Sanjay et al.
112
have
reported studies dealing with the effect of 0.05 wt.% of
KMnO
4
treatment for 2 min at room temperature on the
morphological, tensile, flexural, and impact properties of
ramie reinforced polymer composites. The obtained results
show that there is an increase of the roughness of fibers
leading to better adhesion with matrix in addition to an
improvement of the tensile, flexural, and impact strengths.
•Stearic acid treatment
Regarding stearic acid treatment, a solution of 1% stearic
acid in ethyl alcohol is slowly poured into fibers which are
placed in a glass vessel.
11,112
Afterward, the treated fibers
are dried at 80°C for 45 min. Bagasse fibers were treated
with 3 wt.%, 5 wt.%, 7 wt.%, and 9 wt.% of stearic acid, and
then, dried at a temperature of 105°C for 45 min.
113
The
rheological studies revealed that stearic acid acts as a good
lubricant that minimizes shear viscosity. Kenaf fibers were
treated with 0.4 wt.% stearic acid, washed with distilled
water, and dried in an air oven at a temperature of 80°C.
Salem et al.
114
have reported that such a treatment reduces
the hydrophilicity of fibers by decreasing their water
absorption.
•Seawater treatment
Seawater treatment is the simplest and most economical
method for modifying natural fibers. After checking the pH
and the salinity of seawater, the fibers are immersed for
30 days, and then, washed with water and dried at ambient
temperature.
11,112
•Bleaching
The fibers are treated using calcium hypochlorite Ca(ClO)
2
for 45 min, then washed with deionized water, and oven
dried at 80°C for 24 h.
11
•Graft copolymerization
Regarding the graft copolymerization reaction, a monomer
(MMA) and an initiator are used once the fibers are im-
mersed into water. The grafting parameters such as time,
temperature, volume of solvent, and the initiator and
monomer concentrations should be controlled in order to
obtain the optimal grafting percentage.
11
Actually, it is reported that a pretreatment using hot water
can be utilized in order to dissolve undesirable hemi-
celluloses, waxes, and impurities from the surface leading to
good matrix/fibers compatibility.
1
In addition, many studies
have focused on effects of chemical treatments on the
mechanical properties of natural fibers. Doan, Thi Thu
Loan
87
studied the effect of alkali treatment with different
weight concentrations and immersion time on the tensile
strength of jute fibers.
87
The considered samples are
Table 7. Mechanical properties of extracted bamboo fibers by using different extraction methods.
5
Extraction method Tensile strength (MPa) Young’s modulus (GPa)
Steam explosion 308–862 17–36
Retting 503 36
Crushing 420 38
Rolling 270 —
Combined chemical+compression 645–1000 —
Combined chemical+roller mill 370–480 —
Chakkour et al. 11
composed of several elementary fibers. Tensile strengths of
jute fibers treated by 1% NaOH for 4 h, 5% NaOH for
10 min, and 5% NaOH for 12°h are compared with that of
pure jute fibers. It was found that the weight loss of fibers
increases upon large treatment periods and concentrations
of alkaline. The weight loss of treated fibers by 5% NaOH is
found to be higher than those treated with 1% NaOH at the
same immersion time. Doan Thi Thu Loan found that alkali
treatment with a weight percentage of 1% and immersion
time of 4 h was optimum for jute fibers. This can be ex-
plained that by the fact that the treatment removes a part of
amorphous components such hemicellulose and lignin
which causes a rearrangement of cellulosic fibrils in the
loading direction leading to high tensile strength due to the
load distribution. The 5% alkali-treated fibers for an im-
mersion time of 12 h show relatively small tensile strength
due to cellulosic fibrils damage and attacks by NaOH.
Furthermore, Chin SC et al.
18
studied the effect of alkali
treatment on the tensile strength of bamboo fibers. The
soaking time is maintained to 48 h, while the NaOH con-
centration was of 5%, 10%, and 15%, respectively. They
showed that untreated bamboo fibers exhibit the smallest
tensile strength and modulus, of about 140.1 MPa and
10.89, respectively. Using the above-mentioned treatments,
the tensile strength of fibers was found to increase up to
255.67 MPa, 319.52 MPa, and 212.96 MPa for 5%, 10%,
and 15% NaOH concentrations, respectively. The tensile
modulus increases until reaching 16.63, 27.93, and
25.99 GPa, respectively. The lowest tensile properties of 5%
NaOH treated fibers compared to 10% are attributed to the
incomplete delignification of fibers. However, it is worthy to
note that the tensile properties of fibers were improved by
the increase of NaOH concentrations until a critical value,
the solution is expected to damage the cellulose chains
reducing then the crystallinity of fibers
115
Moreover, the
10% alkali-treated fibers with soaking periods of 24 h and
72 h exhibit a tensile strength of about 267.44 and
176.90 MPa, respectively, while the tensile modulus is
found to be 18.78 and 24.29 GPa, respectively. The small
tensile strength is obtained for bamboo fibers immersed
within 10% NaOH solution for 72 h that could be attributed
to the elimination of lignin which links the cellulosic chains
together.
18
Table 8 shows a brief summary of the treatments
effects on the mechanical properties of some lignocellulosic
fibers and composites.
Osorio L. et al.
116
have also studied the tensile strength
and the tensile modulus of alkali-treated bamboo fibers with
the effect of 1%, 3%, and 5%, respectively, soaked for
20 min. They showed that the tensile strength of fibers
increases by 1%, 6%, and 4% for 1%, 3%, and 5% treated
fibers, respectively. The tensile modulus increases by 7%,
21%, and 19%, respectively, after chemical treatment. This
was explained by the removal of the weak non-cellulosic
components.
116
The effect of silane treatment has been
Table 8. Effects of treatment on the mechanical properties of some selected fibers and composites.
1,11,112,123
Fiber/composite Treatment Results References
Jute 1%, 5% NaOH Increase in tensile strength of 1% treated fibers compared to 5%
treated ones
87
Bamboo 5%, 10%, and 15% alkali
treatment
Increase in tensile properties compared to untreated fibers
18
Borassus 5% NaOH The tensile properties such strength and Young’s modulus were
improved
124
New cane 6% NaOH Improvement of tensile and flexural strengths
125
Bamboo 1%, 3%, and 5% NaOH The tensile strength and Young’s modulus increased after
treatments
116
Sisal fibers 5% benzoyl chloride The crystallinity index and thermal stability are improved
126
Areca fibers 6% NaOH Alkali treatment enhances the thermal stability of fibers
127
Coir-polyester composite 5% NaOH Better impact and flexural strengths
128
Hemp-polypropylene
composite
2%, 4%, and 6% silane The silane does not affect the tensile and flexural strengths
129
Hemp-polypropylene
composite
4% and 6% NaOH The treated fibers exhibited the highest tensile and flexural
strengths
129
Flax-polypropylene
composite
2.5% silane 6% and 3% enhancement in the tensile and flexural strengths
130
Kenaf-polypropylene
composite
2% NaOH Removal of hemicellulose
131
Bagasse-epoxy composite 5% KMnO
4
Improvement of tensile and thermal properties
132
Alfa- polypropylene 0.25 M NaOH The removal of non-cellulosic impurities and the increase of
microfibrillar angle MFA
112
Abaca-phenolic composite 50% acetylation The tensile and impact strengths were improved
14
Banana epoxy composite 1% NaOH Improvement of the tensile and flexural properties
133
12 Journal of Reinforced Plastics and Composites 0(0)
investigated by Tung Nguyen et al.
117
They found that the
tensile strength of raw steam exploded bamboo fibers is
higher than that of silane-treated bamboo.
Other investigations have reported on hygrothermal
treatments of natural fibers. Cuiyin Ye et al.
119
have studied
the effect of hygrothermal treatment at temperatures 160°C,
180°C, 200°C, and 220°C and relative humidity of 100% on
the porous structure of Moso bamboo and its mechanical
properties. They found that 220°C hygrothermal bamboo
sample appeared to be more brittle than untreated sam-
ples.
119
This was attributed to the loss of hemicellulose
leading to an increase of pores numbers after hygrothermal
treatment at 220°C. The total volume of pores and the
average diameter maximize at 160°C and is increased by
46% and 16%, respectively, compared to untreated ones.
This is due to the hemicellulose degradation and the sup-
pression of volatile components at high temperatures. At
160°C, the average pore size slightly decreases because the
hemicellulose degradation is still processing and the free
pores can be refilled by the chemical components flowing
under high temperatures. In addition, the number of hy-
drogen bonds between cellulose chains increases which
results in the reduction of the number of pores and their
sizes. Further, they showed that the relative crystallinity
reaches the maximum (36.92%) at a temperature of 180°C
being 11.1% higher when compared to untreated samples.
They explained this growth of crystallinity by the high
reactivity of the amorphous celluloses. The latter areas lost
water by condensation and produce hydrogen bonds en-
hancing consequently the crystallinity. Moreover, the re-
lease of hemicellulose parts led to the rearrangement of
amorphous cellulosic chains producing low energy bonds. It
is also reported that after overpassing a temperature of
180°C (200°C–220°C), the degradation of crystalline cel-
lulose begins leading to a decrease of the relative crystal-
linity. Yun et al.
118
and Cuiyin et al.
119
independently found
that relative crystallinity of Moso bamboo increased at
180°C by 5.67% in comparison to untreated samples.
Cuiyin et al.
119
showed that the microfibrillar angle
(MFA) decreases slightly during the hygrothermal treatment
and explained such a result by the softening of lignin and
hemicellulose around cellulosic fibrils. The amorphous
chains of cellulose are moved and reoriented when some
polysaccharides are degraded, resulting in a drop of the
microfibrillar angle. The reduction of the number of voids
and the microfibrillar angle as well as the increase of
crystallinity at 180°C was found to enhance both the elastic
modulus and hardness by almost 21%. At temperatures
higher than 180°C, they found that the crystalline cellulose
began to degrade on account to crystallinity decrease
leading to small elastic modulus.
Besides, there are various physical treatments that could
be used to change the surface energy of fibers and enhance
the fibers-matrix interlocking. Some reported treatments are
cited below.
Physical modification
•Plasma treatment
Plasma treatment is used to remove impurities and change
the surface energy in cellulosic fibers, using semi-industrial
prototype machines at ambient conditions.
122
Sanjay
et al.
112
have reported on jute fibers which are treated with
low pressured O
2
plasma environment for a duration of
15 min at several power ranges. The obtained results reveal
that plasma treatment enhances the fibers-matrix adhesion,
resulting in an improvement of the mechanical properties. It
is worthy to highlight that the increase of mechanical
properties of composites is proportional to plasma power.
•Vacuum ultraviolet irradiation treatment
Vacuum ultraviolet irradiation is a new method consisting of
removing impurities from the surface of plant fibers. Sanjay
et al.
112
have reported an interesting work revealing the
surface oxidation of fibers by vacuum ultraviolet irradiation.
Initially, the fibers are placed in a chamber of stainless steel
and high energy below 200 nm were used for irradiation.
•Ozone treatment
This process allows to generate surface oxidation on the
fibers surfaces using ozone gas at a temperature of 25°C and
aflow rate of 70 L/h, while the time of exposure ranges
between 5 min and 9 h.
11,112
Then, the treated fibers are
washed with distilled water and dried for 24 h at 60°C.
Sanjay et al.
112
have found that the degree of crystallinity
and crystallites sizes increase after ozone treatment. Avinc
et al.
20
have studied the effect of ozone treatment duration
on the burst strength of soybean fibers reinforced compo-
sites showing that the optimal strength is obtained for high
treatment duration.
•Corona treatment
Sanjay et al.
11
described the experiment set up of some
previous works devoted to studying the effect of corona
treatment on the cellulose fiber surface. For this purpose,
about 1 g of fibers is placed in the corona cell and treated for
1 min with an applied potential of 15 kV and frequency of
60 Hz at a temperature of 25°C and relative humidity of 50
RH%.
•Y-Ray treatment
The fibers are treated with NaOH solution (16 mol.dm
3
),
followed by tetramethylammonium hydroxide (1
3 mol.dm
3
). Afterward, the treated fibers are neutralized,
dried, and post-processed with 5 kGy/h dose rate. Some
findings
112,121
show that the 25 kGy γ-ray treatment im-
proves the fibers-matrix adhesion and enhances the physical
properties of sisal fibers composites.
Characterization of natural fibers. This section is devoted to
present the most widely useful characterizations techniques
for natural fibers.
Chakkour et al. 13
Spectroscopic characterization of Fourier transform infrared
spectroscopy. Fourier transform infrared spectroscopy (FTIR)
is used to determine the chemical composition of natural
fibers, composites reinforced natural fibers, and their func-
tional groups that interact with lignocellulosic fibers.
1,11,123
This method consists of imposing an infrared radiation to the
sample and measuring the wavelengths, thresholds, and the
intensity of the radiation absorption (see Table 9).
Mofokeng et al.
134
have investigated the spectroscopic
characteristics of randomly oriented sisal fibers reinforced
PLA composite. They found that O-H bonds became more
pronounced when increasing the fibers content, which is
explained by the presence of free hydroxyl groups at fibers
surfaces.
Marianne
135
reported the most common functions in the
study of natural fibers and polysaccharides.
Doan Thi Vi
87
studied the effect of 1% NaOH (1 im-
mersion day) treatment on bamboo fibers by infrared
spectroscopy FTIR. According to Ref.
87
the peaks corre-
sponding to carbonyl groups C=O (1750 cm
1
) and the
lignin characteristic group OCH
3
(1240 cm
1
) have
disappeared. Doan Thi concluded that alkali treatment
contributes to reduce the content of amorphous components
especially lignin and hemicellulose.
87
Chin SC et al.
18
reported a comparison between raw and
alkali-treated bamboo fibers using infrared spectroscopy
method.
18
They showed that the peak of the band at
895 cm
1
is due to the C-O stretching vibrations of
amorphous cellulose that increases with the NaOH treat-
ment. They showed that the intensity of hydroxyl group
peak of the band at 3446 cm
1
increases after alkali
treatment allowing a good fiber-matrix interaction. In ad-
dition, the intensity of bands peaks attributed to C-O, C-C
stretching (1036 cm
1
) decreases after chemical treatments
showing a reduction of hemicellulose and lignin con-
stituents of treated bamboo fibers. Table 9 gives an overview
of the infrared bands peaks and the associated chemical
bands of bamboo fibers.
135
P. Madhu et al.
136
have studied the chemical composition
of raw and chemically treated Agave Americana fibers
(AAF) with NaOH, stearic acid (SA), benzoyl peroxide
(BP), and potassium permanganate (PP).
136
They observed
that the band peak appearing at 2357 cm
1
is attributed to
waxes components that become more pronounced for un-
treated fibers compared to chemically treated ones. This was
explained by the partial removal of waxes within treated
fibers. The two peaks found between 3100 cm
1
,
3427 cm
1
and 2917 cm
1
, respectively, are expected to
characterize the cellulose respective hydroxyl and C-H
stretching groups.
X-ray diffraction. Vegetable fibers are mainly constituted
of sugar-based polysaccharides especially cellulose,
hemicellulose, and lignin. The cellulose which is the main
component of lignocellulosic fibers appears into two
chemical states which are crystalline (cellulose 1) and
amorphous (cellulose 2) celluloses. The content of cellulose
1 in natural fibers determines their mechanical
properties.
6,11,123
The crystallinity index (%) corresponds to
the amount of crystalline cellulose compared to the global
quantity of amorphous components.
6,135
Marianne Le-
troedec
135
have studied the crystallinity of hemp fibers
under NaOH and Ca(OH)
2
treatments. The obtained
crystalline peak (2θ) referring to the angle between the
incoming and the scattered X-Ray beam is comprised be-
tween 22° and 23° and corresponds to the crystallographic
plane family (002) of cellulose 1. Some depicted small
peaks corresponds to the plane (110). According to Ma-
rianne Letroedec,
135
if the content of crystalline cellulose is
high, the peaks remain distinct and if fibers contain a high
content of amorphous components, the peaks merge and form
a unique one. On the other hand, the obtained results showed
that NaOH treatment increases the crystallinity of hemp fibers
from 55% to 80%. This was explained by the removal of
a part of amorphous areas as hemicellulose, pectins, and
lignin in addition to other inorganic components. Further, the
hot water and calcium chloride treatments were found to have
Table 9. Overview of the principal infrared bands of bamboo fibers and their corresponding chemical bonds.
135
Wavelength (cm1) Chemical bonds Associated components
3300 O-H Polysaccharides
2885 C-H symmetric bending Polysaccharides
2850 CH2 symmetric bending Cires
1732 C=O Xylanes (hemicellulose)
1650–1630 OH Water
1505 C=C Lignin
1425 Deformation at the group plane C-H Pectines, lignin, hemicelluloses
1373 CH2 Polysaccharides
1335 C-O vibrations Cellulose
1240 Acetyl groups deformation (xylanes) Lignin, polysaccharides
1162 C-O-C Cellulose, hemicellulose
895 Glycosidic bonds vibrations Polysaccharides
670 C-OH vibration Cellulose
14 Journal of Reinforced Plastics and Composites 0(0)
a negligible influence on either cellulose or amorphous
parts.
135
Chin SC et al.
18
performed X-Ray diffraction analysis of
bamboo fibers with different NaOH concentrations and
soaking durations. The amorphous peak is characterized by
a low intensity at a 2θof 16°, whereas the peak corre-
sponding to the crystalline part is located at 2θ= 20°. Both
peaks are associated with the crystallographic plane (002).
Further, the results showed that the crystallinity index of
pure bamboo fibers is small (49.92 %) which is explained by
the high content of amorphous components in such fibers. In
fact, the crystallinity of specimens soaked within NaOH
solutions for 48 h has increased to 52.86, 58.73, and 53.68%
for 5%, 10%, 15% alkali-treated samples, respectively. The
increase of crystallinity index after chemical treatment is
due to the removal of waxes and a part of hemicellulose,
lignin, and pectines components.
Thermogravimetric analysis. Thermogravimetric analysis
(TGA) allows to determine the evolution of mass loss of
materials as a function of temperature.
11,123
Several studies
have addressed this point. For example, P. Madhu
136
have
explored thermogravimetric curves of Agave Americana
fibers. They divided the degradation of fibers into three
stages: The first stage is comprised between 50°C and
185°C corresponding to the moisture evaporation and the
removal of waxes components. The weight loss at this
degradation stage did not exceed 15%. The second stage
was found to be between 220°C and 340°C where the mass
loss was attributed to the degradation of hemicellulose and
cellulose components. The mass loss at this stage remains
under 60%. Further, they found that the third stage occurs
above 340°C and is related to the degradation of lignin and
other non-cellulosic components. Furthermore, Chin SC
et al.
18
have performed thermogravimetric analysis of pure
bamboo fibers and 5%, 10%, 15% NaOH alkali-treated
bamboo fibers for 24, 48, and 72 h.
18
They also observed
the above mentioned three degradation steps in the ther-
mogravimetric curve of non-treated and treated fibers. The
first stage between 26°C and 155°C is related to the
evaporation of moisture. The amount of adsorbed moisture
by the non-treated fibers (11%) is higher than the treated
fibers (7%–10%). This was explained by the effects of
alkali treatment on the hydrophilicity of cellulosic fibers
leading to the removal of amorphous zones. The second
decomposition stage ranges from 199°C to 379°C and is
essentially caused by the removal of holocellulose (cel-
lulose and hemicellulose) components. At this stage, the
mass loss of untreated bamboo fibers is much higher (52%)
due to the high content of hemicelluloses around cellulosic
fibrils. The final stage occurs between 364°C and 499°C
where the decomposition is due to the degradation of the
hardest components. Chin SC et al.
18
reported that alkali
treatment increases the content of lignin in fibers.
18
In
Refs.
18,136–139
all reported tests showed that alkali treat-
ment increases the thermal stability of fibers while three
degradation stages have been identified. Dehydration,
degradation of cellulose components, and final de-
composition of the fibers lead to the formation of char.
Figure 6 illustrates a typical TGA curve of a non-treated
Agave Americana fiber.
Density measurement. To measure the density, the fibers
are dried for 48 h in a desiccator that contains calcium
chloride, immersed in toluene for 2 h to eliminate micro-
bubbles, and then, kept in a pycnometer. The density of
fibers is calculated by the following formula
11,112,123
ρ¼m2m1
ðm3m1Þðm4m2ÞρT(1)
where, m
1
,m
2
,m
3
,m
4
, and ρ
T
are the mass of the empty
pycnometer, pycnometer containing fibers, pycnometer
filled with toluene, pycnometer filled with fibers and tol-
uene, and the density of toluene, respectively. Deeksha
et al.
123
reported several works in which fibers density is
measured using Archimedes’principle. In fact, several
solutions such as water and canoila oil are used to measure
the apparent loss in weight of natural fibers. Raja et al.
140
and Khan et al.
141
have estimated the density of Baobab and
Eleusine Indica fibers using Archimedes’principle with
ethanol as solvent and reported values of 1.1041 g/cm
3
and
1.1430 g/cm
3
, respectively.
Scanning electron microscopy and OM analyses. Scanning
electron microscopy (SEM) is usually used to study the
morphology of natural fibers reinforced composites. Several
studies have been carried out in order to analyze either the
morphology of fibers or the quality of fiber-matrix
bonding.
11,112,123
Madhu et al.
136
analyzed the surface
morphology of untreated and treated Agave Americana
natural fiber with sodium hydroxide, stearic acid, benzoyl
peroxide, and potassium permanganate. They noticed that
raw fibers are constituted of parallel microfibrils surrounded
by waxes, oils, and others impurities which make the fiber
surface less rough. However, they highlighted that chemical
treatments induce an increase in the number of pores on the
fiber surface while impurities such waxes are eliminated.
Chin SC et al.
18
investigated the effect of NaOH treatment
on the bamboo fiber surface. They observed that the fiber
diameter decreased by 25% after alkali treatment which is
explained by the dissolution of hemicellulose and the re-
moval of lignin and other impurities. Regarding the fiber-
matrix bonding, Pothan et al.
142
have investigated the effect
of fiber volume content on the morphology of banana-
polyester composites, and reported that the composite
with 40% fiber content has a better fiber-matrix bonding.
The adhesion between short bamboo fibers and poly-
propylene has been studied by Thwe and Liao
23
using
Scanning Electron Microscopy (SEM). The SEM images
showed that the fiber/matrix adhesion is relatively poor and
the fibers are completely debonded from the matrix.
Regarding the diameter of natural fibers, it is measured
by optical microscopy OM or a scanning electron
Chakkour et al. 15
microscope SEM.
11,123
These techniques allow measure-
ments with an accuracy of 10
6
μm. Several studies have
pointed out difficulties in measuring the diameter of fibers
because of their irregular shape and variable thickness.
Indeed, the presence of amorphous and inorganic com-
ponents makes the fibers cross-section uncircular. Hence,
the diameter is evaluated in 5 different locations along the
fiber length and the average value is considered. It is re-
ported that the diameter of pineapple leaf and ramie fibers
ranges between 20 μmand80μm.
55
The averaged fibers
diameters of the most used natural fibers are presented in
Tab le 3.
Moreover, the hierarchical structure of plant-based fibers
such as vascular bundles, fiber bundles, elementary fibers,
cellulosic microfibrils, cells, primary and secondary cell
walls could be investigated using optical microscope OM or
by SEM analysis.
11,112,123
The EDX analysis allows to determine the chemical
compositions of fibers surfaces.
11,123
Indeed, the reactions
of carbon C and oxygen O with sodium Na, aluminum Al,
silicium Si, and magnesium Mg could be detected, except
hydrogen H that represents the main constituent of natural
fibers. Najeeb et al.
143
have performed an elementary
composition analysis of Yankee pineapple fibers and have
reported values of 43.84 wt.%, 46.80 wt.%, and 9.36 wt.%
of carbon, oxygen, and potassium, respectively.
Chemical analysis. The chemical analysis permits to de-
termine the content of each natural fibers component. The
content of waxes and fats, pectins, lignin, holocellulose,
cellulose, and ash are determined according to the ASTM D
1107–56, ASTM 1110–56, ASTM D 1106–56, ASTM D
1104–56, ASTM D 1103–60 and ASTM D 1102–84, re-
spectively.
87
Sanjay et al.
11
have reported that the cellulose
is generated in an insoluble solution after treating the fibers
with 95% nitric acid solution and ethanol. The hemi-
cellulose content is estimated by exposing the fibers to
10 mL of cold neutral detergent solution and sodium sulfite
for 1 h, filtering the mixture through glass crucible and
washing the residue by hot distilled water. It is reported that
APPITA, TAPPI, and Conrad methods may be used to
determine the lignin, ash and waxes contents, respectively.
Najeeb et al.
143,123
have identified the chemical composition
of pineapple leaf fibers by TAPPI standards T20305 OS-74
and T222 OS-83 to determine the content of cellulose/
hemicellulose and lignin, respectively. The obtained com-
positions are 47.74% of cellulose, 15.98% of hemicellulose,
and 2.44% of lignin. Vijay et al.
144
have investigated the
chemical composition of Leucasaspera fibers by following
the APPITA method. The corresponding contents are
58.3%, 8.9%, and 4.5% of cellulose, hemicellulose, and
lignin, respectively.
Differential scanning calorimetry analysis. This analysis is
performed by DSC machine and permits the determination
of melting and glass transition (T
g
) temperatures of natural
fibers
11,123
The tests are performed with a heating rate of
10°C/min starting from 0°C until reaching the melting peak,
in nitrogen atmosphere. Palai et al.
145
reported that four
temperature peaks (76.3 °C, 154.75 °C, 242.75 °C, and
330.8 °C) are observed in the DSC curve of Crassipes fibers
and corresponds to the water removal, glass transition
temperature, hemicellulose, and cellulose degradations,
respectively.
Single fiber tensile test. As early mentioned, the tensile
properties of natural fibers are influenced by several factors
such as dimensions (length, width and thickness), envi-
ronmental conditions and plant type.
11,123
The test should
be performed by following the ASTM C1557-03 standard,
with a cell load of 5 kN or less and speed rate not exceeding
0.5 mm/min. It is worthy to note that each fiber edge needs
to be fixed with epoxy resin to avoid fibers damage. Vijay
et al.
144
and Najeeb et al.
143
have investigated the tensile
strength of Vachellia and pineapple leaf fibers by using
universal testing machine and recorded values of
33.075 MPa and 420.3 MPa, respectively.
Activation energy. This technique allows to determine the
minimum required energy to decompose the fibers.
123
It is
reported that a high activation energy corresponds to
a better thermal stability. Kumar et al.
146
and Manimaran
et al.
147
have investigated the activation energy Acacia
nilotica and Furcracea Foetida fibers and recorded values
of 69.73 kJ/mol and 66.64 kJ/mol, respectively. In
Refs.
145,148,149
the activation energy of Albizia Lebbeck
bark, Sida Cordifolia, and Eichhornia Crapssipes fibers is
89 kJ/mol, 73.1 kJ/mol, and 66.32 kJ/mol, respectively.
Ganapathy et al.
150
have reported that alkali treatment
increases the activation energy of banyan tree fibers from
72.45 kJ/mol to 72.65 kJ/mol. Similarly, alkali treatment
improves the thermal stability and activation energy of
treated Saharan Aloevera fibers when compared to un-
treated ones.
151
Matrices
In industrial applications, natural fibers are used as re-
inforcement of different matrices constituting the well-
known biocomposites. The main role of matrices is to
Figure 7. Classification of the used matrices in natural fibers
reinforced composites.
1
16 Journal of Reinforced Plastics and Composites 0(0)
ensure fibers protection and solid bonds with them allowing
mechanical resistance of the composite.
19,152
They can be
classified into two parts, namely, petrochemical-based
resins such thermoplastics, thermosets and bio-based res-
ins (see Figure 7).
Petrochemical-based matrix is a chemical product de-
rived from petroleum as coal and natural gas. The most used
one is the thermoplastic resin which is a polymer that can
change reversibly from the viscous to solid states under
cooling and heating respectively. Indeed, macromolecular
chains of thermosets matrices are linked by hydrogen el-
ements which are removed irreversibly upon heating.
However, each type of thermoplastic resins is characterized
by its own properties. The polyethylene (PE) exhibits high
ductility, impact strength, and good fatigue resistance, while
the polypropylene (PP) shows high temperature and di-
electric resistance.
153
Regarding thermosets resins, they are infusible materials
that can be cured by heating or by the use of a catalyst. In fact,
macromolecular chains of thermosets are linked by covalent
and non-covalent bonds making them sensitive to the heating
based reshaping when compared to thermoplastic ones.
154
In
addition, thermosets show higher modulus, good thermal
stability, and high chemical resistance. Among thermosets
resins, epoxy presents high thermal and mechanical prop-
erties, high water resistance and long working times.
155
Moreover, bio-resins are polymers based on renewable
resources and obtained from the cellulose and starch or by
the polymerization of plant-based sugars and oils
1,8,157
such
as polylactic acid (PLA) and polyethylene terephthalate
(PEET). Polylactic acid is a homopolymer of starch, while
the monomer unity of the PEET is a bio-ethylene glycol.
The latter is obtained from glucose fermentation and the
oxidation of the resulting bio-ethylene.
156
Furthermore, bio
polyethylene terephthalate, bio polypropylene, and bio
polyethylene are bio-based resins that are partially bio-
degradable.
8
The PLA exhibits a tensile strength and failure
elongation of about 4–10, respectively and 5–17 times
higher than the starch ones in addition to an important
flexural strength (see Table 10).
However, the development and the production of bio-
based resins still suffer from limitations. For example
a production of 3.4 million tons has been reported in 2011
and 235 million tons of petrochemical based ones in 2019,
19
in addition to the high moisture absorption and low thermal
stability of bio-resins.
8,158
Fiber-matrix interface
Actually, the mechanical properties of composite materials
mainly depend on the fiber content and the fiber-matrix
interface.
161–163
Indeed, the matrix transfers the load to the
fibers thanks to shear stress in order to ensure their reinforcing
role. However, to understand the adhesion phenomena, it is
important to consider the different types of bonding.
Mechanical bonding: This type results from the mechanical
interlocking and depends on the fiber roughness. The
principle is to introduce the matrix within the microvoids of
reinforcements in order to provide a good fiber-matrix load
transfer by means of the shear stress
163
(see Figure 8(a)).
Chemical bonding: The chemical bonds may be formed
thanks to the reactive sites of lignocellulosic fibers which
are the hydroxyl groups of cellulose.
164
Besides, several
coupling agents are utilized to form chemical bonding
between the hydrophobic matrices and vegetable fibers
19,165
(see Figure 8(b)).
Table 10. Relevant mechanical properties of the most used matrices including thermoplastics/thermosets and bio-resins
67,159,160
.
Type Resin
Tensile
strength
(MPa)
Young’s
modulus
(GPa)
Elongation
(%)
Flexural
strength
(MPa)
Flexural
modulus
(MPa)
Density
(Kg/m
3
)
Thermoplastic Polypropylene 26–41.4 0.95–1.77 15–700 40 1.5 890–910
Polystyrene 25–69 4–51–2.5 70 2.5 960–1040
Low-density
polyethylene
40–78 0.055–0.38 90–800 9 0.2 910–925
High-density
polyethylene
14.5–38 0.4–1.5 2–130 32 1.2 940–960
Polycarbonate 55–70 ———200 1.2
Polyvinyl alcohol 1600 ———6 1.19–1.31
Thermoset Polyester 41.4–89.6 2.07–4.41 2–2.6 70–110 2–4 1040–1400
Epoxy 55–130 3–62–10 110–150 3–4 1110–1400
Polyurethane 18 - 200–800 ——1250
Bioresin Starch 5–6 0.125–0.85 31–44 52 2.4 1000–1390
PLA 21–60 0.35–3.5 2.5–651–70 4.2 1210–1250
PHA 18–24 0.7–1.8 3.25 ———
Chakkour et al. 17
Electrostatic bonding: The electrostatic bonding results from
arrangement of charged particles with opposite polarities on
both matrix and fibers
163
(see Figure 8(c)).
Several studies were focused on the optimization of
lignocellulosic fibers-polymer matrix interface in order to
enhance the mechanical properties of the well-known bi-
ocomposites. In this way, chemical and physical treatments
were previously introduced in the section 1.1.2 to deal with
the fiber-matrix incompatibility.
9
Several works used chemical treatments such as NaOH
to increase the roughness of fibers for a better mechanical
interlocking with matrix. Others researchers used coupling
agents to further optimize the interface. Despite these
chemical treatments, a significant void content is still no-
ticed. Besides, the developments are directed toward the use
of nanoparticles, specifically, cellulose, clay, graphene,
silicone dioxyde SiO
2
, titane dioxyde TiO
2
, and calcium
carbonate CaCO
3
, to reduce the void content and enhance
the properties of biocomsposites. Indeed, the low cost and
ecofriendly character of clays and cellulose make them the
best candidate to reinforce composites.
Mechanical properties and theoretical
modeling of natural fibers reinforced
composites NFRCs
Mechanical and tribological properties:
Experiments side
Many research works investigated the mechanical proper-
ties of biocomposites depending on several parameters.
166
Indeed, Rivalani et al.
167
investigated the effect of alkali
treatment (20% by weight) for 1 h on the tensile, flexural,
hardness properties, and the water absorption behavior of
polyester composite reinforced by randomly dispersed sisal
and glass fibers. They tested the composites following five
configurations. The obtained composites by alternating
three layers of treated sisal fibers and four layers of glass
fibers have the highest tensile and flexural strength of
128 MPa and 184 MPa, respectively. Similar tests were
carried out on composites with untreated sisal fibers. In this
case, the tensile and flexural strength reach 112 MPa and
156 MPa, respectively, decreasing by 12.5% and 15.2%
compared to treated fibers reinforced composites . In ad-
dition, Rivalani et al. showed the water absorption behavior
of different composites and concluded that the increase in
sisal fibers layers increases the water absorption ca-
pacity. Moreover, the alkali treatment was found to
decrease the hardness of composites from 78 to 55.2
HRB, where the highest value is obtained for layered
composites by six untreated mats of sisal fibers and four
mats of glass fibers.
The mechanical properties of a polyvinyl butyral (PVB)
composite reinforced by kenaf fibers were studied by
Salman et al.
168
They found that the composite with 45/
45° oriented fibers exhibit the highest impact resistance
compared to the case of 0/90° orientation.
168
Yan et al.
169
investigated the mechanical properties of alkali-treated coir
fibers reinforced epoxy composites. The tensile, compres-
sive, and flexural strengths are 23.8 MPa, 2.98 MPa, and
40.4 MPa, respectively. The tensile, impact, and flexural
properties of sisal fibers reinforced epoxy composites were
studied by Yan et al.
170
using injection molding processing
technique. Their values reach 180.45 MPa, 46.5 MPa, and
191.37 MPa for the tensile, compressive, and flexural
strengths, respectively. Ramesha et al.
171
carried out the
tensile test of hemp fibers reinforced epoxy composites.
They showed that composites exhibited 55 MPa and
4.5 GPa for the tensile strength and Young’s modulus,
respectively. Codispoti et al.
172
studied the effect of the
matrix on the tensile properties of Hemp fibers reinforced
epoxy and polyester composites. The obtained tensile
strength reaches 63.12 MPa and 58.20 MPa, respectively,
for epoxy and polyester-based composites. Biswas et al.
173
explored the effect of fibers types (bamboo and jute) on the
mechanical properties of epoxy-based composites. They
showed that bamboo fibers reinforced composites exhibit
a high tensile strength while the jute fibers based ones
display the highest Young’s modulus. Mir et al.
174
high-
lighted the effect of temperature on the tensile properties of
jute fibers reinforced epoxy biocomposites, and showed
that their mechanical properties dropped by 50% at about
180°C. Ramesha et al.
175
investigated the effect of volume
fraction on the tensile and flexural properties of banana
fibers reinforced epoxy composites. They reported that
composites with 50% fibers volume fraction exhibited the
highest tensile strength while the optimal flexural strength
is obtained for composites with a fibers volume fraction of
almost 60%. Punyamurthy et al.
176
studied the effect of
fiber content on the impact strength of treated abaca fibers
reinforced epoxy composites, and concluded that a fiber
content of about 40 wt.% within composites shows the
Figure 8. Different types of bonding (a) Mechanical, (b) Chemical, and (c) Electrostatic bonding.
19,165
18 Journal of Reinforced Plastics and Composites 0(0)
maximum impact strength of 7.68 mJ/mm
2
. Sanjay MR
et al.
177
have assessed the effect of stacking sequences on
the tensile and flexural properties of jute/kenaf/glass woven
fabrics. Their results reveal that the laminates made only
from Glass exhibited a tensile strength of 332 MPa. The
laminates including only pure jute and pure kenaf fabrics
display low strength values, not exceeding 35 MPa and
45 MPa, respectively. However, their findings show that the
jute/kenaf hybridization slightly increases the tensile
strength of the laminates to reach 47 MPa. They have found
that the incorporation of glass fabrics as the outer layers
increases gradually the tensile strength of the laminates to
129 MPa. Similarly, they found that glass/kenaf/jute hybrid
laminates exhibit the highest flexural strength reaching
239 MPa. The authors highlighted that the flexural prop-
erties depend on the laminate stacking sequences. They
mentioned that having glass and kenaf as skin plies im-
prove the flexural strengths of hybrid laminates. Arpitha
et al.
178
have investigated the hybridization of sisal and
glass on the tensile and flexural properties of hybrid
laminates. The reported results show that the highest tensile
and flexural strengths are exhibited by glass laminates and
reach 346 MPa and 318 MPa, respectively. However, pure
sisal laminates display the lowest tensile and flexural
strengths remain under 33 MPa and 124 MPa, respectively.
The authors have pointed out that the glass/sisal hybrid-
ization increases the tensile and flexural strengths of
laminates to reach 168 MPa and 241 MPa, respectively. In
addition, Arpitha et al.
178
studied the effect of 0.72 wt.% of
silicon carbide on the mechanical properties of sisal lam-
inates. They revealed that the filler decreases the tensile
strength of laminates from 33 MPa to 31 MPa and increases
the flexural strength from 124 MPa up to 168 MPa. Athith
et al.
179
have studied the effect of tungsten carbide fillers
with 2–3μm size on the physical, mechanical, and tri-
bological properties of jute/sisal/E-glass reinforced natural
rubber/epoxy composites. Their findings reveal that the
incorporation of tungsten carbide fillers enhances the
density and the mechanical properties of composites. For
10 wt.% fillers content, the corresponding tensile strengths
are found to be 109.7 MPa, 71.38 MPa and 222.7 MPa for
glass/sisal-epoxy, jute/rubber-epoxy, and glass/jute-epoxy
laminates, respectively. The flexural strengths are 149 MPa,
35.38 MPa, and 100.3 MPa for glass/rubber-epoxy, glass/
sisal-epoxy, and jute/rubber-epoxy laminates, respectively.
Regarding tribological properties, the wear rate decreases
with the fillers content. Vinod et al.
180
have investigated the
influence of stacking sequences on the mechanical prop-
erties of jute/hemp reinforced bio-epoxy hybrid compo-
sites. They reported that the laminates with jute core layer
H/J/H exhibit the highest tensile strength of 65.44 MPa
when compared to other laminates. The optimal Young’s
modulus is obtained for pure hemp laminates H/H/H and
reaches 1.87 GPa. The maximum flexural strength and
modulus are observed for jute core layer laminates H/J/H
and pure hemp composites H/H/H reaching 121.2 MPa and
3.48 MPa, respectively. The high flexural and tensile
strengths for H/J/H laminates can be explained by the
presence of jute as a core that acts as a plasticizer due to its
high amorphous components which prevents the fracture
growth in laminates. Jothibasu et al.
181
have studied the
mechanical behavior of areca sheath/jute/glass fibers re-
inforced hybrid composites. The reported results show that
the jute/areca/glass fibers hybrid composites exhibit the
highest mechanical strengths compared to jute/glass and
pure areca/glass reinforced hybrid laminates. The tensile,
flexural, compression, and shear strengths reach
46.99 MPa, 5.4 MPa, 44.5 MPa, and 32.58 MPa, re-
spectively. For jute/areca/glass fibers composites, the areca
layer is sandwiched between high strength jute and glass
layers which explain their important mechanical properties
compared to other laminates. Krittirash et al.
182
have in-
vestigated the thermal, tensile, and impact properties of
kenaf/sisal fibers reinforced bio-epoxy hybrid composites
and reported that the incorporation of fibers decreases the
mechanical properties of composites which suggest that the
applied load is not effectively transferred from matrix to the
reinforcements. However, the hybrid composites show im-
portant resistance to weathering conditions exhibiting
moderate mechanical strengths when compared to neat bio-
epoxy. The optimal tensile and impact strengths are obtained
for pure kenaf fibers and kenaf/sisal/kenaf fibers reinforced
laminates and reach 23.56 MPa and 7.2 kJ/mm
2
,respectively.
The Tabl e 11 gives a summary of the mechanical properties
of the most relevant natural fibers reinforced polymer
composites. The Figures 9 and 10 show the tensile and
flexural strengths of some NFRCs, respectively.
On the other hand, during the relative motion of surfaces,
the wear and friction coefficients remain the main investigated
tribological properties.
183
The friction coefficient defines the
frictional behavior of composite materials while the wear one
corresponds to the progressive loss of the material during
mechanical and/or chemical processes.
183
Wei et al.
184
have
studied the effect of temperature on the tribological properties
of sisal fibers/phenol formaldehyde composites and concluded
that the wear rate increases with temperature. Mutlu et al.
185
have studied the tribological properties of rice straw and rice
husk dust reinforced pads and reported that the incorporation
of these fibers increases the tribological properties of the
corresponding composites. Similarly, Ojha et al.
186
showed
that wood apple shell particles reinforced composites exhibit
the lowest erosion wear compared to coconut composites. El-
Tay eb
187
have investigated and reported that the wear rate of
chopped sugarcane fibers composites is better than that of glass
fibers reinforced composites.
Comparative conclusion
Figures 9 and 10 show that the pure bamboo, sisal, and flax
fibers composites exhibit an important tensile and flexural
Chakkour et al. 19
Table 11. Mechanical properties of the relevant natural fibers reinforced composites.
Nature of composite Observation Reference
Randomly oriented sisal/glass fibers
reinforced polyester composites
Obtained composites by alternating three layers of treated sisal fibers and
four layers of glass fibers exhibit the highest tensile and flexural strengths
167
Kenaf fibers reinforced polyvinyl butyral
(PVB) composites
Composites with 45/45° oriented fibers exhibit the highest impact
resistance compared to the case of 0/90°
168
Poplar wood PW reinforced high-density
polyethylene HDPE
Decrease of flexural properties from 64 MPa for longitudinal fibers reinforced
composites to 34 MPa for composites with transversal fibers
188
Radiata pine RP reinforced high-density
polyethylene HDPE
Decrease of flexural properties from 48 MPa for longitudinal fibers reinforced
composites to 33 MPa for composites with transversal fibers
188
Rice husk RH reinforced high-density
polyethylene HDPE
Decrease of flexural properties from 39 MPa for longitudinal fibers reinforced
composites to 28 MPa for composites with transversal fibers
188
Randomly oriented kenaf fibers with silica
reinforced epoxy composites
The highest compressive and flexural strengths are obtained for composites
with 2 vol% of silica
189
Undirectional hemp fibers reinforced epoxy
composites
The obtained flexural strength at a mass fraction of 35% equal to 128.3 MPa
22
Alkali-treated coir fibers reinforced epoxy
composites
The tensile, compressive and flexural strengths were respectively 23.8 MPa,
2.98 MPa, and 40.4 MPa
169
Sisal fibers reinforced epoxy composites The obtained values are equal to 180.45 MPa, 46.5 MPa, and 191.37 MPa for
the tensile, compressive, and flexural strengths respectively
170
Hemp fibers reinforced epoxy composites Specimens exhibit a tensile strength of about 55 MPa
171
Hemp fibers reinforced epoxy composites Specimens exhibit a tensile strength of about 63.12 MPa
172
Hemp fibers reinforced polyester
composites
Specimens exhibit a tensile strength of about 58.20 MPa
172
Flax fibers reinforced epoxy composites The obtained tensile strength is almost 117.37 MPa
172
Flax fibers reinforced polyester composites The obtained tensile strength is almost 109.80 MPa
172
Bidirectional jute fibers laminates reinforced
epoxy composites
The incorporation of jute fibers increases the tensile, impact, and flexural
strengths of composites compared to neat matrices
191
Jute fibers reinforced composites The optimum tensile strength is obtained for unidirectional composites
compared to 45° and 90° oriented fibers within composites
115
Ramie and coir fibers reinforced epoxy
composites
The highest tensile, impact, and flexural strengths are obtained for ramie-
based epoxy composites
190
Banana fibers reinforced synthesized bio-
resins
Banana fibers increases the tensile strength, Young’s modulus, and flexural
modulus by 15%, 12%, and 25% compared to neat matrix respectively
192
Coir fibers reinforced epoxy composites The obtained maximum flexural strength for composites with a fiber length of
about 30 mm and a volume fraction of fibers of 40%
193
Jute and bamboo reinforced epoxy
composites
Bamboo fibers reinforced composites exhibit the highest tensile strength,
whereas the highest Young’s modulus is obtained by jute fibers reinforced
epoxy
173
Jute fibers reinforced epoxy biocomposites The tensile properties decrease by 50% at 180°C
174
Banana fibers reinforced epoxy composites 50% banana and 50% epoxy-based epoxy composites exhibit the highest
tensile strength, and the optimum flexural one is obtained for composites
with 60% of banana fibers
175
Treated abaca fibers reinforced epoxy
composites
The maximum impact strength is obtained for composites with a fiber loading
of about 40%
176
Jute/kenaf/glass woven fabrics The glass/kenaf/jute hybrid laminates exhibit the highest tensile and flexural
strengths reaching 47 MPa and 239 MPa, respectively
177
Sisal/glass hybrid laminates The addition of glass lamina increases the tensile and flexural strengths of
composites
178
Jute/sisal/E-glass reinforced composites The incorporation of tungsten carbide additives increases the density and the
mechanical properties of composites
179
Hemp/jute hybrid composites The laminates with jute core layer H/J/H exhibit the highest tensile and
flexural strengths
180
Jute/areca/glass fibers hybrid composites The jute/areca/glass fibers hybrid composites exhibit the highest mechanical
strengths compared to jute/glass and pure areca/glass laminates
181
20 Journal of Reinforced Plastics and Composites 0(0)
strengths ranging from 117 MPa to 180 MPa and between
191.4 MPa and 161.6 MPa, respectively. The lower values
are particularly obtained for radiata pine rice husk, coir, and
ramie fibers composites, and could be attributed to the small
cellulose content and microfibrillar angle of the reinforce-
ments. It can be clearly stated that the incorporation of glass
layers enhances the composites properties. Indeed, the
tensile strength reaches 168 MPa and 129 MPa for sisal/
glass and jute/kenaf/glass fibers reinforced hybrid compo-
sites, respectively. The flexural strength is 239 MPa and
184 MPa for jute/kenaf/glass and treated sisal/glass lami-
nates, respectively.
Theoretical modeling and prediction of the NFRCs
response to thermomechanical stimulus
Different theoretical and experimental approaches have
been proposed to understand the multiscale and multi-
physical deformation mechanism in NFRCs and partic-
ularly the debonding phenomenon of their matrix/fiber
Figure 9. Summary of the tensile strength of the relevant NFRCs.
Figure 10. Summary of the flexural strength of the relevant NFRCs.
Chakkour et al. 21
interface. In fact, some useful details of such issues are
addressed in the following sections.
Empirical models. Curve fitting with polynomial, expo-
nential, and trigonometric functions are the main used
tools to model the thermomechanical behavior of NFRCs.
Gupta et al.
194
predicted the effect of Young modulus and
tensile strength of NFRPC using multiple non-linear re-
gressions techniques based on available experimental data.
In the case where coupling phenomena exist in the
composite, the model is extended by adding a new as-
sociated parameter. Epaarachichi et al.
195
used two sim-
plified empirical models of the mechanical behavior of
randomly distributed short fibers in the matrix. In this
study, different short fibers are embedded in an elastic matrix
and the whole composite is then subjected to mechanical tests.
The obtained results show that predictions using empirical
models overestimate the evolution of elastic modulus as
a function of the fiber volume content. Furthermore, we
recommend to enrich these models by considering interfacial,
thermal, and moisture effects.
Finite element based models. One of the most commonly
used methods to simulate the multiphysical behavior of
materials and structures is the finite elements approach. It
predicts the response of NFRCs subjected to thermo-
mechnical stimulus on the basis of well-known parameters
such as Young’s modulus, Poisson ratio, density, thermal
conductivity, and specific heat capacity.
196
Palombini
et al.
29
used the finite element method to investigate effects
of structural components of the bamboo, such as paren-
chyma, sclerenchyma, and the large lumina of the xylem
and the phloem on the fiber behavior. Sliseris et al.
197
implemented a nonlinear plasticity model to simulate me-
chanical fracture in flax short fiber/polypropylene and flax
fabric/epoxy composites. Then, damage mechanisms such
as fiber breakage initiated at the defects neighborhood,
damage nucleation in polymer matrix and the fiber de-
bonding at fiber/matrix interface are addressed using so-
phisticated numerical tools. Furthermore, results show good
agreement with experimental ones.
Analytical models. This type of models
198
involves rule of
mixtures (ROM), inverse ROM (IROM), Cox model,
Hirsch model Halpin-Tsai model, Kelly-Tyson model, and
Bowyer-Bader model and leads in general to mathematical
expressions of the NFRCs properties with respect of those
of the matrix and the fibers separately. Cao et al.
199
used the
mentioned models to fit the mechanical response of the
wood/flax/PP composite. IROM is the most accurate
method to successfully mimic the experimental mechanical
behavior of composites. Potluri et al.
31
used analytical
models of the mechanical behavior in case of pineapple
fibers reinforced epoxy composite. On the basis of Chamiis
model and MROM, the predicted values of longitudinal,
transverse, and shear modulus are in agreement with ex-
perimental ones. The above underlined models are reviewed
in the following parts.
Rule of mixtures. Rule of mixtures are the simplest, ef-
fective methods that can be used. In fact, they are based on
matrices and fibers contents in order to predict the behavior
of the resulting composites. One can distinguish between
parallel model called ROM and series model called IROM.
The latter results from the Voigt’s assumption where both of
matrices and fibers exhibit the same strain. It is based on
Reuss’assumption in which the applied stress is equivalent
for both matrices and fibers
30
In fact, the ROM is usually used
to predict the mechanical properties of composites such as the
tensile and flexural strengths. Equations ((2), (3)) and ((4), (5))
show the expression of the strength and the modulus of the
composite following ROM and IROM models, respectively.
σcV¼ασ1V1þσ2V2(2)
EcV¼αE1V1þE2V2(3)
σc
V¼σ1σ2
σ1V2þσ2V1
(4)
Ec
V¼E1E2
E1V2þE2V1
(5)
where V is the volume fraction of fiber and matrix (V=V
1
+
V
2
), σis the strength, and E the modulus. The subscripts 1,
2, c refer to the fiber, matrix, and composite, respectively.
The parameter αis a harmonic coefficient which depends on
the orientation of fibers within matrix.
30,200
The Hirsh model. The Hirsh model is considered as
a combination of parallel and series rule of mixtures ROM.
The elastic moduli and tensile strength of the resulting
composite can be defined using the equations below.
30,133
σcV¼αðσ1V1þσ2V2Þþð1αÞσ1σ2
σ1V2þσ2V1(6)
EcV¼αðE1V1þE2V2Þþð1αÞE1E2
E1V2þE2V1(7)
Halpin-tsai model. Regarding this model, it considers the
matrix to fiber aspect ratio instead of the combination of
components content. It is usually used to determine the
mechanical behavior of continuous and discontinuous
oriented fiber-reinforced composites. The following equa-
tions (8) and (9) show the strength and modulus of the
composite following Halpin-Tsai model.
30,133
σc¼σ21þBηV1
1ηV1(8)
Ec¼E21þBηV1
1ηV1(9)
22 Journal of Reinforced Plastics and Composites 0(0)
where
η¼
σ1
σ21
σ1
σ2þB¼
E1
E21
E1
E2þB(10)
B¼2L
D(11)
where L is the fiber length and D its diameter.
Bowyer and Bader’s model. This model is developed to
analyze the tensile strength and elastic moduli of compo-
sites.
133,201
The following equations (12) and (13) are given
in terms of K1 which corresponds to the fiber orientation
(ranging from 0 to 1) and K
2
to the fiber length factor given
by equation. (14)
σc
V¼σ1K1K2V1þσ2V2(12)
Ec
V¼E1K1K2V1þE2V2(13)
K2¼lc
2lfor l <lc(14)
where lis the fiber length, l
c
the critical length, and σand E
are the tensile strength and elastic modulus, respectively.
Self-consistent and Mori-Tanaka models. These ap-
proaches are based on the Eschelby inclusion problem.
202
Let e be the deformation field in the volume V and E the
macroscopic deformation. The localization tensor A defined
by e(x)=A(x)Efor x2V, verifies < A>V¼I, where Iis the
identity tensor. The expression of the homogenized stiffness
tensor C
hom
is given by
Chom ¼<CA >V(15)
If the matrix (Subscript 0) reinforced by several in-
clusions (subscript i ranging from 1 to n), the expression of
the homogenized stiffness tensor C
hom
is given by
Chom ¼X
n
i¼1
fiCiAi¼C0þX
n
i¼1
fiCiC0Ai(16)
where f
i
is the volume fraction of inclusion i (ratio of its
volume to the volume of the VER), C
i
the stiffness
tensor, and A
i
the deformation localization tensor of
inclusion idefined by A
i
=< A>V
i
,whereV
i
is the
inclusion volume.
Self-consistent model. In this model, all reinforcements
are assumed to be immersed in the equivalent homogeneous
volume with an effective property described by C
SC
.
The deformation localization tensor for the re-
inforcement i is written using the Eschelby tensor Si
Esh
203
Ai¼hIþSi
EshCSC 1CiCSC i1
(17)
By replacing in equation (16), we obtain the expression
of the stiffness tensor C
SC
CSC ¼C0þX
n
i¼1
fiCiC0hIþSi
EshCSC 1CiCSC i1
(18)
Mori-Tanaka model. In this model, it is assumed that each
inclusion is embedded in the matrix of the composite. When
a stress is applied to the composite, an average stress σ
0
is
developed within it, resulting in an average strain ϵ
0
.
204
In this model, the localization tensor A
i
is written as
follow
Ai¼Ti X
n
j¼0
fjTj!1
(19)
Where
Ti¼hIþSi
EshC01CiC0i1
(20)
By replacing in equation (16), we obtain the expression
of the stiffness tensor C
MT
CMT ¼C0þX
n
i¼1
fiCiC0Ti X
n
j¼0
fjTj!1
(21)
Fracture based models. According to these models, crack
nucleation and propagation through the composite bulk or
along the interfacial area are predicted with regard to the
loading characteristics and the material fracture toughness.
Pupur et al.
205
used a shear lag model of wood fibers
combined with PLA or Tencel matrices to predict the effect
of fiber/matrix weight content on the elastic modulus and
the mechanical strength of the composite. The results were
compared with experimental data and reveal that in wood
fibers reinforced composites, the governing mechanism
changes in material stiffness and strength, is related to the
interaction between the fibers on one side and the interaction
between the fibers, and the matrix including porosities
effects on the other side. Keck and Fulland
206
investigate
the crack path through flax reinforced composites and
fracture behavior of several compact tensile specimens
corresponding to different fibers orientations and volume
fraction. The obtained results show that the latter strongly
induce changes in the crack path. Indeed, a large fiber
contents results in a fast crack path growth along the fiber.
Cohesive models. In fracture mechanics, cohesive zone
models allow to predict the crack nucleation and propa-
gation via a constitutive law giving the tensile normal stress
as a function of the relative displacement between the crack
lips.
207
Beakou and Charlet
208
used a bilinear cohesive zone
law to model the bundle strength in a flax fiber-reinforced
Chakkour et al. 23
composite. Results show good agreement with the exper-
imental data pointing out the efficiency of the cohesive
models of the matrix/fiber interface to model the fracture
properties of the bundles. Hbib et al.
209
investigated the
interfacial damage in a hemp reinforced composite using 1D
interface cohesive elements. A linear correlation is evoked
between the damage and the interfacial stiffness criteria and
evidences the great influence of the interface properties on
the damage evolution.
Hydric and thermal properties of natural
fibers reinforced composites (NFRCs)
Hydric properties
The hydrophilic structure of natural fibers is responsible for
the high moisture absorption of biocomposites. Several
studies revealed the water diffusion kinetic in some selected
biocomposites.
Chandrakanta Mishra et al.
210
studied the moisture
sorption behavior of kenaf/glass polyester hybrid composite
with different stacking sequences. They identified the
distinctive parameters such as: diffusion coefficient, (D
x
),
moisture content at equilibrium (M
m
), and the mechanism of
water transport. They found that composites made with
glass fibers only revealed low moisture gain (2.314%),
whereas composites made with kenaf fibers only shows high
moisture gain of about (10.560%). It is observed that the
water absorption of all specimens follows the mechanism of
Fickian’s diffusion.
210
They also noticed that there is a 51%
reduction of tensile strength for specimens made of kenaf
fibers only (from 65 MPa to 30 MPa). However, the in-
corporation of glass fibers was found to restrict the re-
duction to 37%. After drying samples, the obtained results
showed that 97% of tensile strength is recovered in case of
composites made with glass fibers only, whereas in case of
kenaf/glass hybrid composite, it is limited to 87.22%.
Venkatesha B.K et al.
211
investigated the moisture ab-
sorption behavior of woven bamboo/glass fiber-reinforced
epoxy hybrid composites with different laminate orienta-
tion. The volume fraction of fibers within composite
specimens ranges between 31.71% and 32.52%. The
specimens are immersed in different environmental con-
ditions: Distilled and saline waters. They found that the
moisture absorption rate in case of saline water is less than
that of distilled one. This was explained by the accumu-
lation of NaCl ions in the fiber surface immersed in saline
water which prevents further diffusion of moisture. The
obtained results revealed that the high values belonged to
composites with 60° laminate and showed 7.4% and 5.5%
for specimens immersed in distilled water and seawater
conditions, respectively.
211
They also noticed that the
tensile and flexural strengths decreased after being im-
mersed in the solutions. The maximum reduction in these
properties is observed in case of saline water environment.
This is explained by the physical damage or chemical de-
terioration that specimens exhibited.
211
Chittaranjan Deo
and Acharya
212
investigated the moisture adsorption be-
havior of lantana camara fibers reinforced epoxy composites
at different fiber contents and environmental conditions
(steam, saline water and sub-zero temperature). They found
that the moisture adsorption increases by increasing the fiber
content within composites. The moisture uptake is high for
composites immersed in steam environment. It is noticed
that moisture diffusion within all composites followed
Fick’s law.
212
The Figure 11 shows the typical water ab-
sorption of cellulosic banana fibers reinforced compo-
sites
213
at different fibers contents and reveals that the
equilibrium moisture content of composites increases with
respect of fibers content. The Table 12 gives a summary of
the hydric properties of the most relevant natural fibers
reinforced polymer composites. Figure 12 shows the typical
evolution of moisture absorption of some NFRCs as
a function of time. One can observe that the moisture
diffusion curves of cellulosic fibers composites are char-
acterized by a rapid diffusion rate during the first 60 h before
reaching the saturation stage after 200 h of water immersion.
The current research studies revealed that the chemical
treatments enhance the dimensional stability of fibers,
improve the interfacial adherence and decrease partially the
moisture absorption. However, these improvements are not
sufficient to integrate them in wet applications. The future
developments may focus on the integration of nanoparticles,
namely Mmntmorillonite nano-clays, nano silicone SiO
2
,
titane TiO
2
, and carbon nanotubes. Indeed, the preliminary
studies showed that the integration of nanoparticles de-
creased significantly the moisture absorption of
Figure 11. Typical moisture absorption curves of cellulosic
fibers composites.
24 Journal of Reinforced Plastics and Composites 0(0)
nanocomposites, leading to an increase in the mechanical
performance.
214–221
Moreover, many studies investigated the effects of water
on the mechanical behavior of biocomposites, but still now,
few researchers are interested on the effect of relative hu-
midity. It is very important to conduct research studies to
further understand the hygromechanical behavior of ad-
vanced biocomposites.
Recent works revealed a lack on analytical models ad-
dressing the prediction of hydric properties. It will be ef-
fective to implement micromechanical models such as
ROM, IROM, and Hirsh to predict the equilibrium moisture
Table 12. Hydric properties of the relevant natural fibers reinforced composites.
Nature of composite Observation Reference
Kenaf/glass reinforced polyester hybrid
composite
The water absorption of all specimens follows the mechanism of Fickian’s
diffusion. The composites made from kenaf fibers only exhibited high
moisture absorption
210
Woven bamboo/glass fiber-reinforced epoxy
hybrid composites
The moisture absorption rate in case of saline water is less than that of
distilled water
211
Lantana camara fibers reinforced epoxy
composites
The moisture adsorption increases by increasing the fiber content within
composites. It is higher for composites immersed in steam environment
212
Non-woven kenaf fibers reinforced polyester
composites
Alkali treatment enhances the water absorption resistance of composite
specimens
24
Bamboo fibers reinforced polyester composites At high temperatures, the moisture gain of specimens decreases
25
Randomly oriented oil palm empty fruit bunch
fibers reinforced epoxy composites
The moisture gain increases by increasing the fiber content in composites
222
Kenaf fibers reinforced unsaturated polyester
composites
The temperature increases the diffusion kinetic
223
Bagasse fibers reinforced epoxy composites The moisture uptake decreases from 12% to 8% after sodium hydroxide
treatment
224
Jute/sisal reinforced epoxy hybrid composites The water absorption decreased after chemical treatment. The optimal
moisture gain properties are related to 50sisal/50jute composites
225
Bamboo/sisal reinforced polyester hybrid
composites
The composite that possess higher content of bamboo fibers display lower
water absorption
226
Figure 12. Summary of the moisture absorption of the relevant NFRCs.
Chakkour et al. 25
content and the moisture diffusivity in the composite based
on the constituents’properties.
Thermal properties
This section aims to develop the reported works of the
thermal behavior of the relevant natural fibers reinforced
composites (NFRCs). Indeed, both their thermal conduc-
tivity and stability are addressed in the following sections.
Thermal conductivity. Nowadays, there is a significant de-
mand for new and improved housing, which may lead to
increased greenhouse gas emissions. To address these is-
sues, many studies have been conducted to improve the
global sustainability by incorporating natural fibers. Ac-
tually, Idicula et al.
227
studied the thermal properties of
banana/sisal reinforced polyester hybrid composites. An
amount of fibers are immersed in 10% NaOH and 5%
PSMA in toluene for 1 h and half an hour respectively. The
obtained results revealed that the incorporation of fibers
enhances the thermal conductivity of composites speci-
mens. It decreases from 0.153 W.m
1
.k
1
to
0.140 W.m
1
.k
1
for 20% and 40% (by volume) fiber
content composites. In contrast, the thermal diffusivity and
specific heat did not have a significant variation. In case of
40 vol.% fiber content, the thermal conductivity increased
and reached 0.201 W.m
1
.k
1
and 0.213 W.m
1
.k
1
for
NaOH and PSMA treated composites. This was explained
by the formation of pores due to the partial removal of non-
cellulosic components.
227
Takagi et al.
228
investigated the
thermal conductivity of unidirectional abaca and bamboo
fibers reinforced epoxy composites. For abaca composites,
the conductivity values equal to 0.298 W.m
1
.k
1
,
0.292 W.m
1
.k
1
and 0.273 W.m
1
.k
1
for neat epoxy,
13.4% and 24.9% fiber content composite (by volume)
respectively. In case of bamboo composites, it increased
from 0.298 W.m
1
.k
1
to 0.317 W.m
1
.k
1
for neat epoxy
and 22.42% of fibers volume fraction composites.
228
Bhaskar
et al.
229
investigated the potential of using natural fibers in
building thermal insulation. They studied the thermal con-
ductivity in longitudinal and transverse direction for banana/
palmyra fibers reinforced epoxy hybrid composites. They
showed that the incorporation of fibers enhances the thermal
insulation of the material. It is found that the thermal con-
ductivity in the longitudinal direction decreased from
0.363 W.m
1
.k
1
to 0.251 W.m
1
.k
1
for neat epoxy and 20%
palmyra/20%banana reinforced hybrid composite, re-
spectively. In the transversal direction, it increased from
0.363 W.m
1
.k
1
to 0.247 W.m
1
.k
1
respectively. They
found that the optimal value is related to 30%palmyra/10%
banana epoxy composites. In this case, the conductivity
reached 0.246 W.m
1
.k
1
and 0.240 W.m
1
.k
1
in the lon-
gitudinal and transversal direction, respectively.
229
.Figure 13
shows the thermal conductivity of some NFRCs.
However, few researchers are nowadays focused on the
development of fire resistance performances of NFRCs. It is
necessary to orient research studies in this area to expand
their utilization in aeronautic and marine industries. As
discussed in the previous sections, the integration of nano-
clays may be effective to further enhance the thermal
properties of biocomposites, due to their excellent fire re-
sistance.
230–232
It may also be important to address research
Figure 13. Summary of the thermal properties of some relevant NFRCs.
26 Journal of Reinforced Plastics and Composites 0(0)
on the thermo-hygromechanical behavior of biocomposites
due to the lack of data on this aspect.
Thermal stability. Krittirash et al.
182
have investigated the
thermal properties of kenaf/sisal fibers reinforced bio-epoxy
hybrid composites and reported that the neat epoxy and pure
kenaf laminates are degraded at 313.33°C while the max-
imum degradation temperature is extended to 315.83°C for
pure sisal fibers reinforced laminates. Vinod et al.
92
have
studied the effect of various chemical treatments on the
thermal properties of Morinda citrifolia fibers and their
corresponding composites. The obtained results show that
silane treatment increases the onset degradation temperature
of fibers from 248.44°C to 274.71°C which is explained by
the high ratio of cellulose weight related to the removal of
sensitive-hemicellulose components by the chemical
treatments. Besides, they reported that the highest thermal
expansion coefficient is obtained for raw fibers reinforced
composites due to the presence of high amorphous content
in the fibers, while the lowest value is related to the silane-
treated fibers composites and reached 90.82 ppm°C-1 and
64.62 ppm°C-1, respectively. Sanjay et al.
93
have studied
the thermal properties of chicken feather/Ceiba Pentandra
fillers reinforced bio-epoxy hybrid nanocomposites and
showed that these materials are suitable for applications up
to 310°C. Vinod et al.
94
reported that the chemical treat-
ments enhance the thermal stability of Muntingia Calabura
bark fibers. Indeed, the corresponding onset degradation
temperatures are about 213°C, 268 °C, and 254 °C for raw,
silane and alkali-treated fibers, respectively. Similarly,
Sumrith et al.
95
studied the effect of various chemical
treatments on the thermal stability of water hyacinth fibers.
The corresponding degradation temperatures are 289.22 °C,
294.67
°
C, and 329.71 °C for raw, NaOH and silane-treated
fibers, respectively.
George et al
159
have studied the effect of hybridization
on the thermal behavior of sisal/ramie/curaua fibers hybrid
composites and reported that the addition of the ramie/
curaua fibers enhances the thermal stability of the corre-
sponding composites compared to pure sisal laminates.
Radzi et al.
236
have investigated the thermal stability of
roselle/sugar palm fibers reinforced polyurethane compo-
sites. The reported results show that the incorporation of 75
wt.% of sugar palm fibers improves the thermal stability of
the corresponding laminates. Teixeira et al.
237
have reported
that the addition of alkali and silane-treated curaua fibers
enhances the degradation temperatures of composites. The
corresponding yield temperatures are 248°C, 273°C, and
293
o
C for pure polyester, alkali-treated and silane-treated
fibers reinforced polyester composites, respectively.
237
They reported that the first stage (at 200°C) corresponds
to the breakage of cross-links and the degradation of
polyester while the second stage occurs at 330°C and
corresponds to the decomposition of cellulose. Shanmu-
gasundaram et al.
238
have highlighted that the alkali
treatment improves the initial degradation temperature from
220°C to 250°C for untreated and treated mulberry fibers
reinforced polyester composites. They found that the
temperature related to the decomposition of cellulose,
hemicellulose and lignin increases when increasing the fi-
bers contents. It reaches 348°C, 353°C, and 357°C for 5
wt.%, 10 wt.%, and 15 wt.% fibers composites, respectively.
Jawaid et al.
239
have reported that the incorporation of jute
fibers improves the thermal stability of jute/oil palm hybrid
composites. Indeed, the first degradation temperature is
260°C, 283°C, and 288°C for pure oil palm composites, 1:1
Figure 14. Summary of the yield temperatures of the thermal stability of some natural fibers reinforced composites NFRCs.
Chakkour et al. 27
jute/oil palm hybrid composites and pure jute fibers com-
posites, respectively. Maou et al.
240
have studied the thermal
properties of data palm fibers reinforced high-density
polyethylene HDPE and reported that the acid hydrolysis
treatment enhances the onset and maximum degradation
temperatures of composites when compared to NaOH and
silane treatments. The onset temperatures reach 230.33°C,
188.76°C, and 194.72°C for acid hydrolysis, alkali and
silane-treated fibers composites. Fang et al.
241
noted that the
addition of 2 layers of jute fibers to 5 layers of polylactic
acid PLA showed an increase of the onset degradation
temperature of jute fibers reinforced polylactic acid PLA
laminates. The Jandas et al.
242
findings showed that the
presence of banana fibers enhances the stability yield
temperature of banana fibers reinfoced PLA composites to
reach 113.75°C. The high decomposition temperature of
jute fibers (almost 230°C) allows to improve the thermal
stability of PLA (30°C–60°C). The thermal stability of
some NFRCs is reported in Figure 14.
Comparative conclusion. Figure 13 illustrates the thermal
conductivity of some NFRCs. It can be clearly seen that
bamboo fibers epoxy composites show the highest value,
followed by abaca and abaca/palmyra epoxy composites.
The addition of carbon fibers to Agave Americana fibers
enhances the thermal insulation of the composites. In ad-
dition, hemp-polyurethane composites seem to be a good
insulator showing a thermal conductivity not exceeding
0.0407 W.m
1
.k
1
.Figure 14 shows that the chicken
feather/Ceiba Pentandra composites have the highest deg-
radation temperature of about 310°C. Indeed, the thermal
stability of these animal fibers exceeds 350°C and permits to
enhance the resistance of composites to fire. However, the
lowest degradation temperature is obtained for banana fibe AQ2rs
reinforced polylactic acid PLA composites which is due to
the poor thermal resistance of bioresin matrix. (Table 13)
Progress in mechanical properties
of nanocomposites
Biocomposites still suffer from the poor fibers-matrix
bonding due to the hydrophilic nature of the reinforce-
ments.
32
As early mentioned, these limitations can be
overcome by using chemical or physical treatments, leading
to a better fibers-matrix bonding.
123
However, several
works shed light on the effect of some nano-additives; for
instance, eggshell powder, titanium dioxide TiO
2
, nano-
clay, graphene, and silicon dioxide SiO
2
on the physical and
mechanical properties of nanocomposites.
243
Nanoclay fillers/polymer composites
Clay nano-additives attract the attention of researchers due
to their great aspect ratio, low cost, and availability.
32
Montmorillonite (MMT), kaolinite, smectite, chlorite,
kenyaite, and ilerite are the most known types of clays.
32
Recent research works regarding natural fibers/clays re-
inforced nanocomposites are summarized in the following
section. (Table 14)
Ramakrishnan et al.
244
have studied the effect of Cloisite
20A nano-clays on the morphological properties of jute
fibers reinforced epoxy nanocomposites and reported that
fibers-matrix interfaces bonding is improved after the in-
corporation of nano-additives. Hossain et al.
245
have in-
vestigated the effect of MMT K10 nano-clay on the water
Table 13. Thermal properties of the relevant natural fibers reinforced composites.
Nature of composite Observation Reference
Banana/sisal reinforced polyester hybrid
composites
Decrease of the thermal conductivity by increasing the fiber volume fraction
227
Unidirectional abaca fibers reinforced
epoxy composites
The incorporation of fibers decreased the thermal conductivity from 0.298 to
0.273 W.m1. k1 for neat epoxy and 24.9% fiber loading composite (by
volume) respectively
228
Unidirectional bamboo fibers reinforced
epoxy composites
The thermal conductivity increased from 0.298 W.m1. k1to
0.317 W.m1.k1 for neat epoxy and 22.42% fiber loading composites
228
Banana/palmyra fibers reinforced epoxy
hybrid composites
The incorporation of fibers enhances the thermal insulation of the material
229
Miscanthus natural fiber-reinforced
geopolymer composites
The thermal conductivity increased by increasing the fiber size and foaming
agent content
233
Bamboo fibers reinforced polypropylene
composites
The thermal conductivity increases with respect of fiber content, and reaches
0.1073 W.m1.k1 at 60 wt% fiber loading composites
27
Agave americana/carbon fiber hybrid
reinforced epoxy composites
The thermal conductivity of 10wt% treated composites is higher than that of
7.5wt% treated ones
28
Esculentus cyperus natura fiber-reinforced
polypropylene composites
The thermal conductivity decreases as the fiber volume content increases
234
Hemp fibers reinforced polyurethane
composites
The thermal conductivity increases with respect of the fiber loading. It is found
to be affected by water
235
28 Journal of Reinforced Plastics and Composites 0(0)
absorption and mechanical properties of woven jute fibers
biopol nanocomposites. The findings reveal that adding
clays decreases the drop in flexural properties after water
absorption. Arulmurugan et al.
34
have reviewed the me-
chanical properties of jute fibers reinforced polyester
nanocomposites at different MMT nano-clays contents and
found that the optimal mechanical and vibration properties
are obtained for 5 wt.% nano-clays contents. Deepak
et al.
246
have reported that the garamite nano-clays enhance
stresses transfer at the interface of coir fibers reinforced
polyester composites. Similarly, Borba et al.
247
showed that
the incorporation of 5 wt.% MMT nano-clays increases the
mechanical properties of curaua fibers reinforced styrene-
butadiene-styrene composites and reported that the ag-
glomeration of fillers leads to the decrease of the mechanical
properties of composites for high filler contents ( > 5 wt.%).
In a similar vein, Borba et al.
247
have reported that the
incorporation of 10 wt.% of MMT nano-clays in curaua
fibers reinforced polyester composites leads to a cluster
formation preventing effective load distribution. Saba
et al.
248
have studied the effect of organo-modified mont-
morillonite OMMT on the physical and mechanical prop-
erties of kenaf fibers reinforced epoxy and showed that the
corresponding fillers increase the density and the hardness
of composites. Venkatram et al.
249
have noted that the
addition of 3 wt.% of garamite nano-clays increases the
tensile, flexural and impact strengths of sisal-epoxy nano-
composites. Haq et al.
33
have studied the effect of 1.5 wt.%
of Cloisite 30B nano-additives on the mechanical properties
of hemp reinforced polyester hybrid composites. The ob-
tained results reveal that the tensile modulus and strength
increase by 6% and 20%, respectively. Prasad et al.
250
have
reported that the tensile strength and modulus of wild cane
grass reinforced polyester increases by 6.3% and 18.3%
after the addition of 4 wt.% MMT nano-clays, respectively.
Similarly, Wang et al.
251
showed that the tensile strength,
tensile modulus, flexural strength, and flexural modulus of
flax fibers reinforced epoxy composites increase by 14.3%,
7%, 20.7%, and 13.6% after the incorporation of 1.3 wt.%
of organo-modified OMMT nano-additives, respectively.
Biswal et al.
252
have investigated the effect of Cloisite 20A
nano-clays on the mechanical properties of pineapple fibers
reinforced polypropylene composites, manufactured using
compression molding. The obtained data reveal that the 3
wt.% filler reinforced nanocomposites exhibit the optimal
mechanical properties. The tensile strength, flexural
strength, tensile modulus and flexural modulus are im-
proved by 20%, 24.3%, 45.6%, and 38.57%, respectively.
Samariha et al.
253
have studied the influence of Cloisite 30B
addition on the mechanical properties of bagasse fibers
reinforced high-density polyethylene HDPE composites
manufactured using extrusion injection molding method.
Table 14. Yield temperatures of the thermal stability of the relevant natural fibers reinforced composites.
Nature of composite Observation Reference
Kenaf/sisal reinforced bio-epoxy The degradation temperature of sisal laminates is higher when compared to neat
epoxy and pure kenaf fibers laminates
182
Morinda citrifolia fibers reinforced
bio-epoxy
Silane treatment increases the thermal stability of the fibers and the corresponding
composites compared to alkali treatment
92
Chicken feather/ceiba pentandra
reinforced bio-epoxy
The incorporation of chicken feather fillers increases the thermal stability of the
composite reaching 310°C
93
Muntingia calabura bark fibers
reinforced green epoxy
Silane and alkali treatments enhances the thermal stability of the fibers and
composites when compared to raw fibers
94
Sisal/ramie/curaua fibers composites The addition of ramie/curaua fibers increases the thermal stability of the hybrid
composites
159
Sugar palm/roselle fibers reinforced
polyurethane
The maximum degradation temperature is obtained for 75.% sugar palm fibers
loadings
236
Curaua fibers reinforced polyester
composites
Silane treatment improves the degradation temperature of composites from 273°C
to 293°C
237
Mulberry fibers reinforced polyester
composites
The alkali treatment improves the thermal stability of composites from 220°C to
250°C
238
Jute/oil palm hybrid composites The addition of jute fibers increases the thermal stability of fibers of the hybrid
composites from 260°C to 283°C
239
Data palm fibers reinforced
polyethylene HDPE
The acid hydrolysis treatment enhances efficiently the onset degradation
temperature of composites when compared to alkali and silane-treated fibers
composites
240
Jute fibers reinforced polylactic acid
PLA
The addition of 2 layers of jute to 5 layers of PLA increases the thermal stability of
composites compared to other configurations
241
Banana fibers PLA composites The presence of 30 wt.% banana fibers enhances the decomposition temperature of
PLA composites
242
Chakkour et al. 29
Samariha et al. findings
253
show that the 2 wt.% filler re-
inforced composites exhibit the highest tensile and flexural
strengths while the optimal tensile and flexural modulus are
obtained for 4 wt.% loaded composites. Shahroze et al.
254
investigated the tensile, flexural, and impact properties of
sugar palm fibers/organo-modified montmorillonite OMMT
reinforced polyester composites processed through hot
pressing method. The composites reinforced with 4 wt.%
OMMT exhibited the optimal tensile properties while the
high flexural and impact strengths are obtained at 2 wt.%
OMMT content. Table 15 shows the mechanical properties
of various nano-clays reinforced biocomposites.
Cellulose fibers cellulose nanofibers and crystals
cellulose nanocrystals composites
polymer nanocomposites
There is a growing interest in the production of cellulose
nanocrystals composites (CNC) due to their attractive
mechanical characteristics, small density, biodegradability
and renewable nature.
255,56
CNC can be isolated from
different agricultural sources such as wood, cotton, bamboo,
jute, and sisal fibers using various chemical pretreatments to
fully/partially remove the amorphous hemicellulose and
lignin components from the lignocellulosic fibers.
256
Table 15. Mechanical properties of the relevant natural fibers reinforced nanocomposites.
Nature of composite Additives
Tensile
strength (MPa)
Tensile
modulus (GPa)
Flexural
strength (MPa)
Flexural
modulus (GPa) References
Basalt fibers epoxy 2 wt.% MMT 325 15.9 306 18.49
265
Curaua fibers-polyester 2.5 wt.% Organophilic
clay
36 —32.55 —
266
Banana fibers epoxy 3 wt.% Cloisite 173 10 88 8.1
267
Coccinia indica fibers epoxy 3 wt.% Cloisite 30B 38.29 —92.77 —
268
Jute fibers epoxy 5 wt.% Cloisite 20A 103.05 1.29 162.8 2.8
244
Jute fibers-polyester 1.5 wt.%
(MMT+Eggshell)
29.5 39.52 ——
269
Sugar palm fibers-
polyester
4 wt.% OMMT 24.56 3.68 68.12 3.78
254
Jute fibers-polyester 5 wt.% MMT 40.38 —234.98 —
34
Sisal fibers polypropylene 5 wt.% Cloisite 30B 55.95 1.7 ——
270
Curaua fibers-polyester 2.5 wt.% organophilic
clay
36 —32.55 —
266
Curaua fibers-polyester 2.5 wt.% organophilic
clay
36 —32.55 —
266
Jute/coir fibers-polyester 3 wt.% garamite 43 ———
246
Curaua fibers-styrene-
butadiene-styrene
2 wt.% Cloisite 10A 7.8 4.8 ——
247
Hemp fibers-polyester 1.5 wt.% Cloisite 30B 24 6 ——
33
Wild cane grass fibers-
polyester
4 wt.% MMT 99.57 2.26 221.61 4.19
250
Flax fibers epoxy 1.3 wt.% OMMT 87.5 7.55 140 6.2
251
Pineapple fibers
polypropylene
3 wt.% Cloisite 20A 45.14 6.45 65.01 4.46
252
Poplar wood fibers
polypropylene
3 wt.% MMT 41.7 —57.5 —
271
Sisal fibers epoxy 5 wt.% Cloisite 30B 57 25 ——
272
DCNC-epoxy 3.5 wt.% DCNC 72 2.25 ——
257
CNF-epoxy 15 vol.% CNF 109 5.9 ——
258
CNF-epoxy 0.1 wt.% CNF 6.2 0.42 ——
259
CNF-polyester 20 vol.% CNF 150 1.15 —125
261
CNF-phenol formaldehyde 12 wt.% CNF 17 —38 —
262
CNF-phenol formaldehyde 10 wt.% CNF 18 —36 —
262
CNF-phenol formaldehyde 8 wt.% CNF 16 —33 —
262
CNF-phenol formaldehyde 6 wt.% CNF 15 —28 —
262
CNF-phenol formaldehyde 4 wt.% CNF 13 —25 —
262
CMF-melamine
formaldehyde
5 wt.% CMF 142 16.6 ——
264
30 Journal of Reinforced Plastics and Composites 0(0)
Actually, they are extracted from the pretreated fibers using
strong acid hydrolysis leading to the dissolution of amor-
phous regions.
255
During this process, the hydronium ions
penetrate the amorphous regions and cause the cleavage of
glycosidic bonds resulting in the separation of individual
cellulose crystallites.
However, cellulose nanofibers CNFs are usually ex-
tracted from the pretreated fibers through mechanical
processes such as high-pressure homogenization, micro
fluidization, ball milling, and grinding.
255
Afterward, it is
reported to dry the obtained CNFs and CNCs by using
several methods such as evaporation of solvent, drying by
iyophilization or using supercritical state fluids.
255,256
Wang et al.
257
have studied the addition effect of
chemically modified CNC with dodecenyl succinic anhy-
dride to obtain hydrophobic CNC (DCNC) on the me-
chanical properties of cellulose nanocrystalls reinforced
epoxy composites. The reported results reveal that the
presence of 3.5 wt.% of DCNC enhances the tensile
strength, Young’s modulus, and strain at break by 82%,
21%, and 198% compared to the neat epoxy . This is ex-
plained by the homogeneous dispersion of nanofillers
DCNC in the matrix and the strong interactions between the
composite components. The values reach 72 MPa,
2.25 GPa, and 7.2% for tensile strength, Young’s modulus,
and strain at break, respectively. Ansari et al.
258
have in-
vestigated the influence of the incorporation of cellulose
nanofibers CNF mixed with acetone/epoxy/amine on the
mechanical performance of composites and revealed that
the nanocomposites exhibit a good tensile strength and
Young’s modulus at 15 vol.% fibers contents, reaching
109 MPa and 5.9 GPa, respectively. AL-Turaif
259
have
found that the addition of 0.1 wt.% of cellulose nanofibers
(CNF) improves the tensile strength, strain at break,
Young’s modulus and toughness by 121%, 73%, 64%, and
300% which are larger than the improvements by any other
reported nanocomposites reported in the literature.
260
Gao et al.
261
found that the addition of 20 vol.%
silane-treated cellulose nanofibers (CNF) increases the
tensile strength, flexural strength, shear strength, and
Young’s modulus of polyester nanocomposites by
117.7%, 38.4%, 38.7%, and 27.6%, respectively. They
are foun to be 150 MPa, 125 MPa, 123 MPa, and
1.15 GPa, respectively. It is reported that 12 wt.% of
cellulose nanofibers (CNF) induces a significant increase
of 142% in the tensile strength, 280% in flexural strength,
and 133% in impact strength for the Phenol Formalde-
hyde nanocomposites.
262
Theyexhibit17MPa,38MPa,
and28kJ/m
2
in tensile strength, flexural and impact
strengths, respectively. Nakagaito et al.
263
reported that
the flexural strength and Young’s modulus of 25 layers of
cellulose microfibers reinforced Phenol Formaldehyde
compositesare370MPaand16GPa,respectively.The
obtained results by Marielle et al.
264
show that the tensile
strength and Young’s modulus of cellulose microfibers
(CMF) reinforced melamine formaldehyde (MF) com-
posites increase by 50% and 30% at 5 wt.% fibers content,
when compared to neat matrix, respectively.
Applications and future trends of NFRCs
Because of their good mechanical properties, lightweight,
and sound attenuation, natural fibers based composites have
been utilized in the automotive, building, sport equipment,
and biomedical industries
37,65,273
(see Table 16). The
adoption of those fibers in several industries is encouraged
by their low cost and lightweight nature. They are usually
used to reinforce polyester and polypropylene matrices.
35
Table 16. Applications of natural fibers in different fields.
Fiber Applications References
Coir Containers and boxes, car seat covers, car door mats, helmet, car roof, sports applications
276,281,284
Flax Racing bicycle, packaging, car headliners, car side and back walls, car armrests, car trunk trim, car inner door
panel, car door trim panels, tissue engineering application
35,273,280,285
Hemp Cases for musical instruments, car side and back walls, car headliners, car armrests, car trunk trim, car inner door
panel, femur bone application
273,281,285
Kenaf Mobile phone casing, car door inner panel, car side and back walls, car headliners, car armrests, car trunk trim,
underbody shield applications
280,285
Wood Door and covered seatback panels in cars
286
Cotton Soundproofing, insulation
286
Abaca Car under door panels
286
Jute Packaging, containers, cladding applications
281
Bamboo Construction, interior design (roof, stairs, and window outlines of houses) and furnitures
274,282
Sisal Car inner door panel, car door trim panels, scaffolds in medical field
35,273
Coconut Seats of Mercedes a-class model
274
Bagasse Sustainable and thermal insulation materials
112
Banana Computer elements, door panels
112
Ramie Military applications
112
Chakkour et al. 31
However, these natural fibers could not replace glass fibers
in wet applications such as boats and kayaks because of
their high sensitivity to water.
37
The bast fibers reinforced
composites are found to exhibit high tensile strength and
a small impact one in contrast to cotton fibers. Sanjay et al.
showed that the combination of those two types of fibers
could be benefic for the high impact stressed applications
such as door panels of cars and safety helmets, combining
the high tensile properties generated by plants fibers and the
important impact strength of cotton fibers.
35
Nowadays, automotive industry is leader in the use of
natural fibers composites especially German companies
such as Mercedes, Volkswagen, Audi, and BMW that took
the initiative to use natural fibers in exterior as well as
interior applications such decking, window frames, and
headliners.
1,37
The first attempt to use natural fibers in
automotive industry was in 1941 by Henry Ford using flax
and hemp fibers as reinforcement for petrochemical poly-
mers.
1
The first commercial example of NFRC in the
automotive industry was the inner door panel of the 1999
S-Class Mercedes-Benz.
273
Indeed, it was made of 35% of
polyurethane PUR elastomer reinforced by 65% of hemp,
flax and sisal fibers. On the other hand, Mercedes-Benz
developed jute fibers reinforced epoxy composites in the
door panels of its E-Class vehicles in 1996. The company
used also coconut fibers as reinforcement for rubber latex
composites for the seats of the A-class model.
274
Similarly,
Audi has launched the A2 car in 2000 where door trim
panels are made of mixed flax/sisal reinforced polyurethane
composite.
35
Ford used kenaf fibers reinforced Poly-
propylene in the door panels of their “Mondeo”model,
whereas, flax is used in floor trays.
274
For Opel, the mixture
of flax and kenaf fibers is inserted in door panels for Vectras
model.
274
Volkswagen used cellulose to make the door
panels, seatbacks, boot-lid finish panels, and boot-liners of
all Passat, Golf, and Bora models. Holbery et al.
275
reported
that BMW group use a lot of natural fibers reinforced
composites in its automobiles. The group used about 10,000
tons of natural fibers in 2004. The BMW 7 series model is
composed of 24 kg of raw natural fibers including flax and
sisal in the interior door lining panels, cotton in the
soundproofing in addition to the wool and wood fibers in the
seatback parts.
274,275
On the other hand, Toyota used Bagasse fibers to re-
inforce used plastic polymers in the interior of cars.
273
Furthermore, the coir/polyester composites have been
also used to produce mirror casing, projector cover, helmet,
and roof.
276
It is worthy to note that 4000 tons of natural
fibers reinforced composites have been used in 2008 as
interior pieces for automotive industries.
277
In 2009, all Peugeots, 207 came out with an external
mirror fabricated using 30% hemp fibers.
277
Djamelhas
277
showed that the density of hemp and flax fibers reinforced
composites is 20%–30% much lower than glass fibers
reinforced ones. In the works of Yang et al.,
278
it was
reported that a reduction of 25% of automotive weight
would enable us to avoid 100 billion kilograms of CO
2
emission annually.
In addition, the Cambridge industry inserted flax fibers
polypropylene composites in its Chevrolet Impala model.
Moreover, plant-based fibers have attracted the attention
of researchers for the insulation in building.
277
The mixture
cement/lignocellulosic fibers could replace cement matrix/
Asbestos fibers in several applications as wall coverings and
piping.
277
Because of their thermo-acoustic properties,
hemp wool may be a good substituent to the glass one.
277
In
addition, Van de Weyenberg et al.
279
have shown that sisal
reinforced composites can be used in several building ap-
plications such as structural building, facades and span
roofing elements. Also, bamboo fibers are used as re-
inforcement in the construction of low cost houses.
274
In addition to the construction and automotive industries,
natural fibers composites can be implemented in other
applications including medical, pharmaceuticals, packag-
ing, bioenergy, and biofuels fields.
38
Currently, flax and
hemp fibers are used in racing bicycle and musical in-
struments,
280
coir and jute in packaging and containers
281
and kenaf in mobile phone casing.
274,280
Besides, bamboo is
also used in interior design (roof, stairs and window outlines
of houses), furniture, and spring chairs. One can note that
Barajas Madrid international airport was built using bamboo
strips.
282
Table 16 shows the different applications of
natural fibers.
Pending future developments, the application of natural
fiber composites remains limited, mainly in the automotive
and construction sectors. Further efforts should be made to
disseminate their use in sectors such as marine, aeronautic,
and renewable energy. Figure 15 shows the different ap-
plications of natural fibers reinforced composites.
Figure 15. Applications of natural fibers reinforced
composites.
283
32 Journal of Reinforced Plastics and Composites 0(0)
Conclusion and perspectives
The good thermo-acoustic properties, mechanical proper-
ties, ecofriendly character, low cost, biodegradability, re-
newability make natural fibers the best candidates to replace
non-renewable and expensive synthetic fibers such as glass,
carbon, and kevlar ones. They are nowadays used as re-
inforcements for petrochemical matrices including ther-
moplastics and thermosets especially in automotive
industries, building as well as in medical and pharmaceu-
tical fields. However, the utilization of those natural fibers
remains limited because of their sensitivity to different
parameters such as moisture adsorption, temperature in
addition to their incompatibility with conventional resins.
Several solutions have been presented in the literature but,
still now, biocomposites still suffer from the above limi-
tations. This paper has presented an up to date of different
parameters influencing the properties of natural fibers, used
matrices types to coat natural fibers, fiber-matrix interface,
mechanical, thermal, and hydric behaviors of natural fibers
composites. The current critical review presents the recent
developments and applications of natural fibers reinforced
composites and help to understand the issues for widespread
integration into more applications mainly for wet and fire-
retardant targets. The possible solutions and future ori-
entations are discussed as well. A brief update of analytical
and FEM-based models has been presented to predict the
mechanical response of natural fibers composites. On basis
of the current literature review and our own analysis, two
main trends that will address the future development of
NFRCs are discussed.
- Hybrid composites: One of the future trends that will help
NFRCs to overcome the lack of interfacial adhesion is to
combine different reinforcements in order to enhance the
properties of the final composite. Using more than one
reinforcement permit to counter any limitations by adding
other relevant constituents. For instance, to counter the
hydrophilic behavior of natural fibers, hybrid composites
are made of synthetic and natural fibers. Instead of the
chemical treatment of fibers, synthetic fibers are hydro-
phobic and therefore would give the best matrix/fibers
compatibility.
- Nanobiocomposites: Another new research attempt in the
NFRCs field is the use of nano reinforcement particles. The
latter possess a large surface to volume ratio that allows to
enhance the interfacial adhesion. Besides, nanoparticles such
as silicium oxide SiO
2
,titaneoxideTiO
2
, calcium carbonate
CaCO
3
, zeolite, and graphene may be the best candidates to
reinforce biocomposites. The preliminary studies reveal that
their incorporation decreases the void content and enhances the
stress transfer between fibers and matrix. However, few studies
have addressed their corresponding water absorption and
flame retardancy properties. Many efforts should be done to
understand the hydromechanical behavior of these nano-
biocomposites to extend their applications in wet conditions. In
fact, the hybridization of nanofillers would be an attractive
solution to improve the flammability and the water absorption
behavior of these bio-sourced materials. In addition, pre-
liminary investigations reveal that cellulose fibers and crystals
nanocomposites exhibit higher mechanical properties when
compared to macro and micro-cellulose composites; however,
they still suffer from limitations such as high moisture ab-
sorption, poor wettability, and incompatibility with most
polymeric matrices. For these reasons, scientists should ad-
dress research studies to the issues related to the modification
of nanocellulose to produce high-performance nano-
composites with a better hydromechanical and fire properties.
Furthermore, the main issue of the NFRCs behavior
modeling remains undoubtedly 3D multiscale predictions
which consider the mechanical properties of the matrix, the
fibers and their interfaces respectively. This allows to size
up the whole composite material and ensures a good in-
terfacial adhesion by selecting compatible matrix and fibers.
In addition, cohesive models enhance the knowledge re-
garding the prediction of crack initiation and propagation
under complex loadings as well as fatigue conditions. In
critical applications such as aerospace and aeronautics, it is
necessary to develop models for NFRCs that integrate their
thermomechanical dynamic properties. However, it is sure
that a big progress must be done in modeling the behavior of
NFRCs in order to finely characterize interfacial properties
and damage pronostic but also to understand the complex
geometrical structure of the natural fibers and relate it to the
mechanical, hydric, and thermal coupling effects on the
state of health of the composite material.
Acknowledgements
Mouad Chakkour, M. O, I.K and M.B acknowledge the In-
ternational University of Rabat. T.B acknowledges the Lorraine
University.
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with re-
spect to the research, authorship, and/or publication of this article.
Funding
The author(s) received no financial support for the research, au-
thorship, and/or publication of this article.
ORCID iDs
Mouad Chakkour https://orcid.org/0000-0002-4148-369X
Mohamed Ould Moussa https://orcid.org/0000-0001-8944-
1467
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Nomenclature
A The deformation localization tensor
C
MT
Mori-Tanaka stiffness tensor
CNC Cellulose nanocrystal
CNF Cellulose nanofiber
C
SC
Self-consistent stiffness tensor
E
2
Young’s Modulus of matrix
E
c
Effective Young’s Modulus of composite
E
1
Young’s Modulus of fiber
FTIR Fourier transform infrared spectroscopy
IROM Inverse rule of mixture
K
1
Fiber orientation factor
K
2
Fiber length factor
MFA Microfibrillar angle
NFRCs Natural fibers composites
NFs Natural fibers
OM Optical microscope
PLA Polylactic acid
PHA Polyhydroxyalkanoates
RH Relative humidity
SEM Scanning electron miscroscopy
Si
Esh The Eschelby tensor
TGA Thermogravimetric analysis
UV Ultraviolet treatment
V
1
Volume fraction of fibers
V
2
Volume fraction of matrix
XRD X-Ray Diffraction
σ
0
Average stress
ϵ
0
Average strain
αHarmonic coefficient
σ
1
Strength of fiber
σ
2
Strength of matrix
σ
c
Strength of composite
42 Journal of Reinforced Plastics and Composites 0(0)
