J Mater Sci (2020) 55:829–892
REVIEW
Review
A review of natural fiber composites: properties,
modification and processing techniques,
characterization, applications
Aliakbar Gholampour1
1
2
and Togay Ozbakkaloglu2,*
Department of Infrastructure Engineering, The University of Melbourne, Melbourne, VIC, Australia
Ingram School of Engineering, Texas State University, San Marcos, TX, USA
Received: 17 May 2019
ABSTRACT
Accepted: 4 September 2019
There has been much effort to provide eco-friendly and biodegradable materials for
the next generation of composite products owing to global environmental concerns
and increased awareness of renewable green resources. An increase in the use of
natural materials in composites has led to a reduction in greenhouse gas emissions
and carbon footprint of composites. In addition to the benefits obtained from green
materials, there are some challenges in working with them, such as poor compatibility between the reinforcing natural fiber and matrix and the relatively high
moisture absorption of natural fibers. Green composites can be a suitable alternative
for petroleum-based materials. However, before this can be accomplished, there are a
number of issues that need to be addressed, including poor interfacial adhesion
between the matrix and natural fibers, moisture absorption, poor fire resistance, low
impact strength, and low durability. Several researchers have studied the properties
of natural fiber composites. These investigations have resulted in the development of
several procedures for modifying natural fibers and resins. To address the increasing
demand to use eco-friendly materials in different applications, an up-do-date review
on natural fiber and resin types and sources, modification and processing techniques, physical and mechanical behaviors, applications, life-cycle assessment, and
other properties of green composites is required to provide a better understanding of
the behavior of green composites. This paper presents such a review based on 322
studies published since 1978.
Published online:
16 September 2019
Ó
Springer Science+Business
Media, LLC, part of Springer
Nature 2019
Introduction
The production of petroleum-based materials and
glass fibers results in the release of a significant
amount of greenhouse gases into the atmosphere [1].
Address correspondence to E-mail: togay.oz@txstate.edu
https://doi.org/10.1007/s10853-019-03990-y
The use of environmentally friendly green materials,
which are recyclable, biodegradable, and renewable,
has been recently considered to decrease the environmental impact of petroleum-based materials [2].
Fully green composites are a type of biocomposite
830
produced by a combination of biofibers and resins
from renewable agricultural and forestry feedstock
[1]. This type of composite is disposed of and composted at the end of their life without harming the
environment [3].
Biofibers have been extensively used as reinforcers
and fillers in composites in recent years [1]. They
come from biological origins and are classified into
non-wood fibers (natural fibers) and wood fibers [4].
Natural fibers have superior physical and mechanical
properties than wood fibers, and they are high in
cellulose content and crystallinity, and lighter in
weight than wood fibers [1]. Therefore, this type of
fiber could attract more attention in industry [5].
Some desirable properties of natural fibers include
high specific strength and modulus, flexibility during
processing, low self-weight, low cost, and substantial
resistance to corrosion and fatigue [6]. However,
natural fibers have drawbacks, such as high moisture
absorption, high anisotropy, low compatibility with
conventional resins, and less homogeneous than glass
and carbon fibers [3, 7].
Biocomposites are classified into fully and partially
green composites [4]. Petrochemical- and bio-based
resins are used as matrices in the partially and fully
green composites, respectively, and natural fibers are
used in both [4]. Although all biopolymers are generally compostable, not all biodegradable and compostable plastics are produced from biopolymers;
some are produced entirely from non-bio-based
materials [4]. Owing to the environmentally friendly
characteristic of green composites, they have being
increasingly used in industry [1]. Industrial applications include aerospace, automobile, military, construction, packaging, medical, sporting equipment,
and railway [3]. Table 1 summarizes the advantages
and disadvantages of fully green composites over
traditional composites [8].
The physical and mechanical properties of natural
fiber composites have been investigated in a few
previous review studies [9], which are summarized
in Table 2. However, detailed information for treatment and modification techniques, fire resistance,
thermal conductivity and analysis, durability properties, and time-dependent properties was not provided in these studies. Additionally, none of the
existing review articles published in journals provide
any information about creep, shrinkage, wear resistance, air content, compressive behavior, viscoelastic
properties, and life-cycle assessment of the natural
J Mater Sci (2020) 55:829–892
fiber composites. Therefore, to address the increasing
demand to use eco-friendly materials in different
applications, an up-do-date review is required to
allow a better understanding of the behavior of natural fiber composites manufactured using discontinuous natural fibers. This paper presents such a
review based on 322 studies published since 1978.
Natural fibers
In recent years, more renewable plant resources have
been discovered and used because non-renewable
resources are becoming scarce [1]. Figure 1 shows the
structural organization of a natural fiber cell wall
[24]. This type of fiber is strong, light, inexpensive,
and renewable [24]. These inexpensive natural fibers
can become a viable alternative for expensive and
non-renewable synthetic fibers (e.g., e-glass, which is
alumino-borosilicate glass, and carbon fibers) when
high elastic modulus is not required [25]. However,
the hydrophilic nature of natural fibers results in
swelling of fibers and the production of voids at the
interface between the matrix and fiber, leading to
poor mechanical properties of composites prepared
using these fibers [26, 27]. Therefore, suitable techniques must be utilized to produce natural fiber
composites with high-quality natural fibers.
Sources and types of fibers
Natural fibers are produced from plant-, animal-, and
mineral-based sources [1]. Fibrous plants are abundantly available in agricultural crops and tropical
areas [1]. Plant fibers are mainly composed of cellulose, whereas protein is the major component of
animal fibers [4]. Plant fibers are classified into primary sources, which are fibers produced as byproducts of other principal products (e.g., food,
feedstock, and fuel) for industrial usage, and secondary plants, which are produced as by-products
derived from manufacturing processes [4]. Table 3
shows the major natural fiber sources and their main
properties. Table 4 shows the main producers and
annual production of these fibers worldwide.
There are eight major types of plant fibers: bast
fibers (jute, ramie, flax, rattan, soybean, hemp, vine,
banana, and kenaf), collected from the skin and bast
around the plants stem; leaf fibers (abaca, banana,
sisal, and pineapple), collected from leaves; seed
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Table 1 Summary of
advantages and disadvantages
of fully green composites over
traditional petrochemicalbased composites
Advantages
Disadvantages
Less expensive
Lower weight
Higher flexibility
Renewable
Biodegradable
Good thermal and sound insulation
Eco-friendly
Nontoxic
Lower energy consumption
No residues when incinerated
No skin irritations
Lower mechanical properties (especially impact strength)
Higher moisture absorption
Lower durability
Poor fire resistance
Variation in quality
Restricted maximum processing temperature
Poor microbial resistance
Low thermal resistance
Demand and supply cycles
Table 2 Previous review studies on natural fiber composites
Year
Reference
2001 George et al.
[10]
2008 John and
Thomas [11]
2011 Zini and
Scandola [12]
2011 La Mantia and
Morreale [13]
2011 Ku et al. [14]
2012 Faruk et al. [4]
2012 Dittenber and
GangaRao
[15]
2012 Abdul Khalil
et al. [16]
2013 Koronis et al.
[17]
2013 Nguong et al.
[18]
2014 Faruk et al. [19]
2015 Satyanarayana
[20]
2015 Mohammed
et al. [21]
2016 Pickering et al.
[22]
2018 Sanjay et al.
[23]
Chemical
Physical
Processing
modification modification techniques
H
Water
Fire
Elastic
absorption resistance modulus
Tensile
strength
Flexural
strength
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Impact
strength
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
fibers (cotton, coir, and kapok), collected from seeds
and seed cases; grass fibers (corn, wheat, bamboo,
barley, and rice); core fibers (corn and wheat stalk),
collected from the stalks of the plants; wood pulp
H
H
H
H
fibers; root fibers (luffa, swede, and cassava); and
fruit fibers (borassus, tamarind, banana, and coir)
[1, 4, 28].
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Figure 1 Structural
organization of a natural fiber
cell wall [24].
There are mainly three types of animal fibers: animal hair, avian fiber, and silk fiber. The fibers of
animal hair (wool or hair) are taken from hairy
mammals and animals (e.g., sheep, goats, alpacas,
and horses) [1, 28]. Silk fibers are taken from the
dried saliva of bugs or insects during the preparation
of their cocoons. Avian fibers are taken from the
feathers of birds [1, 28].
Properties of fibers
Properties of a single fiber are dependent on the
shape, size, crystallite content, orientation, and
thickness of the cell walls [1]. The main characteristics of natural fibers are low energy consumption,
low density, non-abrasive nature, low cost, renewability, biodegradability, easy availability, and worldwide abundance [42]. Although plant fibers are
normally rigid, unlike brittle synthetic fibers, they are
not fractured during processing [43]. Plant fibers
exhibit comparable specific strength and stiffness
properties to glass fibers [43]. Bast fibers are extracted
from the stem ribbon using a retting process. This
type of fiber has moderately high tensile strength and
stiffness; it is inexpensive, exhibits high performance,
and is easily available [1]. This fiber is more applicable when strength, lightweight, and noise absorption are important, such as in the automotive and
building industries. Most of the plant fibers are categorized as eco-friendly fibers because they are
biodegradable and have no negative effect on the
environment, except those grown in temperate zone
with extensive use of agrochemicals (e.g., flax) [1]. On
the other hand, animal fibers have low stiffness balanced by their elastic recovery and high elongation
[1]. They are also less hydrophilic than plant fibers,
durable with moderate resistance, poor conductors of
heat, very sensitive to some alkalis, and able to provide reinforcement in multi-axial situations [1].
The cell walls of dried plant fibers are mainly
composed of lignin and sugar-based polymers (i.e.,
cellulose and hemicellulose) and small amount of
other materials such as starch, extractives, protein,
and inorganics [1]. Table 5 presents the chemical
components of the most used dried plant fibers in
industry with the average amount (in wt%) of the
compositions.
Cellulosic fibers have a hydrophilic nature (moisture absorbent) under natural conditions [1]. The
moisture content in fibers can negatively influence
the mechanical behavior of natural fiber composites.
Table 6 illustrates the equilibrium moisture amount
of natural fibers at the relative humidity (RH) of 65%
and temperature of 21 °C.
Grass (plant fibers)
Seed (plant fibers)
Leaf (plant fibers)
Up to 3000
Up to 200
Up to 1000
Up to 1500
Up to 350
100–650
20–32
Up to 90
Flowering plant in nettle family
(Boehmeria nivea)
Abaca plant (Musa textilis)
Pineapple leaf (Ananas magdalenae)
Agave (Agave sisalana)
Raffia palm (Raphia ruffia)
Coconut (Cocos nucifera)
Shrub (Gossypium)
Pentandra tree (Ceiba pentandra)
Grass pulp (Bambusoideae)
Ramie
Pina
Sisal
Raffia
Coir
Cotton
Kapok
Bamboo
Abaca
Up to 1900
6
Up to 4000
Hibiscus (Hibiscus cannabinus L)
Cannabis (Cannabis Sativa L)
Kenaf
Hemp
20a
10–20
11–22
–
12–25
–
200–400b
–
25–30
20–30
16–50a
17–20
Jute
Up to 4000
Fiber diameter (lm)
12–16
Fiber length (mm)
Up to 900
Flax
Bast (plant fibers)
Source
Herbaceous plant (Linum
Usitatissimum)
Vegetable plant in linden family
[Corchorus capsularis (white jute),
Corchorus olitorius (tossa jute)]
Fiber
Group
Table 3 Sources of major natural fibers and their main properties [1, 28–30]
Rapid absorption and desorption of
water
Low thermal conductivity, moderate
moisture regains, high insulation, high
anti-static properties, growing in
tropical areas with a humidity of
60–90%
Excellent durability
Heat conducting, good dying, good
ultraviolet-light blocking, natural antibacterial properties
Rapid absorption and desorption of
water, low elasticity, easy dying
High mechanical strength, buoyancy,
resistance to saltwater damage
Resistant to salt water, wear resistant
Coarse, hard, durable, strong, and
stretchable, not easily absorb
moisture, resistant to saltwater
deterioration, with a fine surface that
accepts a wide range of dyes
Rough
High concentration of lignin, high
strength, less flexibility than cotton,
unsuitability for dyeing, good
resistance to salt water damage and
microbial action
Rapid moisture absorption, high tensile
strength
Fluffy
Excellent durability, high stability, good
tenacity, good flexibility, good
ultraviolet radiation resistance,
excellent permeability
Main properties
J Mater Sci (2020) 55:829–892
833
Chicken, birds
Lambs
Indian cashmere goat
North African angora goat
Arabian dromedary and Northeast
Asian Bactrian camels
South America camels
Angora rabbit
Asbestos
Mixed silicates
Feather
Lambswool
Cashmere wool
Mohair wool
Camel hair
Apparent diameter when agrochemicals are not used
Apparent diameter
b
a
Mineral-based fibers
Saltwater clam
Dog hair
Muskoxen
Yak
Rabbits
Sheep
Byssus
Chiengora
Qiviut
Yak
Rabbit
Wool
Alpaca
Angora wool
Asbestos cloth
Glass
Maize (Maı́z)
Beech tree (Fagus)
Chinese mulberry silkworm
Cornstalk
Modal
Silk
Core (plant fibers)
Wood pulp (plant fiber)
Animal fibers
Source
Fiber
Group
Table 3 continued
to
to
to
to
50
390
115
125
Up to 150
–
12–300
–
Up
Up
Up
Up
3–13
–
25
50–80
16
–
Up to 152
Up to 3000
–
Up to 1500
Fiber length (mm)
12–29
12–16
0.03–0.035
5–24
15–20
14–19
25–45
15–23
5–50
10–50
–
15–20
16–90
14–16
16–40
40
200a
10–13
Fiber diameter (lm)
Soft, warm
Soft, good blending with other fibers
Fire resistant, lightweight
Fire resistant
Lightweight, strong
Lightweight, soft, wear resistant
Good absorbency, low conductivity,
easy dying finish
Lightweight
Lightweight, fluffy
Soft, does not shrink
Heavy, warm
Soft
Good thermal and acoustic insulation,
high deformability, high durability
Lightweight, good thermal and acoustic
insulation
Soft, warm, elastic
Soft
Durable, resilient, holding dyes well
Warm, lightweight
Main properties
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Table 4 Producers and production amount of most widely used natural fibers [1, 4, 31–41]
Production amount (9 103
ton)
Fiber
Producer
Abaca
Bagasse
Coir
Cotton
Flax
Jute
Kapok
Kenaf
Bamboo
Philippines (85%), Ecuador
70
Brazil, China, India, Thailand, Australia, USA
75,000
India, Sri Lanka, Thailand, Vietnam, Philippines, Indonesia, Brazil
1200
China, Brazil, India, Pakistan, USA, Uzbekistan, Turkey
25,000
France, Belgium, Netherland, Poland, Russian Federation, China
830
India (60%), Bangladesh, Myanmar, Nepal
3450
Philippine, Malaysia, China, South America, Indonesia, Thailand
101
India (45%), China, Malaysia, USA, Mexico, Thailand, Vietnam
970
China, Japan, India, Chile, Ecuador, Indonesia, Myanmar, Nigeria, Sri Lanka, Philippines, 30,000
Pakistan
China (80%), Chile, France, Germany, UK
214
China, Brazil, Lao PDR, Philippines, India
280
Brazil (40%), Kenya, Tanzania, China, Cuba, Haiti, Madagascar, Mexico, Sri Lanka, India
378
China (70%), Brazil, Bulgaria, Egypt, Madagascar, India, Thailand, Vietnam, Uzbekistan,
150
Turkmenistan
Australia, Argentina, China, Iran, New Zealand, Russia, UK, Uruguay
2100
China, Mongolia, Australia, India, Iran, Pakistan, New Zealand, Turkey, USA
20
Hemp
Ramie
Sisal
Silk
Wool
Cashmere
wool
Mohair wool
Camel hair
Alpaca
Angora wool
South Africa, USA
China, Mongolia, Afghanistan, Iran
Peru, Bolivia, Chile
China, Argentina, Chile, Czech Republic, Hungary, France
Table 5 Chemical
composition of the most
widely used plant fibers
[44–47]
5
2
7
3
Fiber
Cellulose (wt%)
Hemicellulose (wt%)
Lignin (wt%)
Waxes (wt%)
Abaca
Coir
Cotton
Flax
Jute
Kapok
Kenaf
Bamboo
Hemp
Pina
Ramie
Sisal
Bagasse
Oil palm
Curaua
Wheat straw
Rice straw
Rice husk
56–63
32–43
85–90
71
61–71
64
72
26–43
68
81
68.6–76.2
65
55.2
65
73.6
38–45
41–57
35–45
20–25
0.15–0.25
5.7
18.6–20.6
14–20
23
20.3
30
15
–
13–16
12
16.8
–
9.9
15–31
33
19–25
7–9
40–45
–
2.2
12–13
–
9
21–31
10
12.7
0.6–0.7
9.9
25.3
29
7.5
12–20
8–19
20
3
–
0.6
1.5
0.5
–
–
–
0.8
–
0.3
2
–
–
–
–
8–38
14–17
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Table 6 Equilibrium moisture content of major natural fibers at
RH of 65% and temperature of 21 °C [48]
Fiber
Moisture content (wt%)
Abaca
Coir
Flax
Jute
Bamboo
Hemp
Pina
Ramie
Sisal
Bagasse
15
10
7
12
8.9
9
13
9
11
8.8
degradability. In the fully and partially green composites, natural fibers are used with bio-based (as a
fully degradable matrix) and petrochemical-based (as
a partly degradable matrix) resins, respectively.
Petrochemical-based resins
A petrochemical-based matrix is a chemical product
derived from petroleum, which is obtained from
fossil fuels like coal and natural gas [4]. The two
major types of petrochemical-based matrices used for
green composites are thermoplastics and thermosets.
Polyethylene, polystyrene, polypropylene, and polyvinyl chloride (PVC) are utilized as thermoplastic
matrices, whereas epoxy, polyester, vinylester, and
phenolic (phenol formaldehyde) are used for thermoset matrices [1, 4].
Matrices for green composites
A matrix is used in composites to hold the reinforcing
materials together by surface connection. The main
responsibilities of the matrix are the environmental
tolerance, surface appearance, and durability of the
composite [1]. As the matrix is stressed, it transfers
the external load uniformly to the fibers, and it is
applied to resist the propagation of cracks and
damage [1]. In recent years, many studies have been
carried out in an attempt to find an alternative for
conventional petroleum-based matrices because of
limited fossil fuel resources and the environmental
impact of using petroleum-based matrices [4]. Figure 2 shows the classification of polymeric matrices
used in green composites based on their
Thermoplastics
Thermoplastic resins are based on polymers that can
be shaped easily in the viscous state and solidified by
cooling (physical change) [49]. In the melted condition, the viscosity of thermoplastic resins is approximately 500–1000 times higher than that of uncured
thermoset resins [4]. They are solid at room temperature and can be reformed and reshaped when
heated without chemical reactions [4]. Thermoplastic
resins have higher impact resistance (approximately
10 times), more reform-ability, higher damage tolerance, and higher processing temperatures and
Matrices for Natural Fiber Composites
Fully degradable
Natural based
• Polylactic acid
• Thermoplastic starch
• Cellulose
• Polyhydroxy alkanoate
Partly degradable
Oil based
• Aliphatic polyester
• Aliphatic-aromatic polyester
• Poly(ester amide)
• Poly(alkyene succinate)s
• Poly(vinyl alcohol)
Figure 2 Classification of polymeric matrices for natural fiber composites [1].
•
•
•
•
Polypropylene
Polyester
Polyethylene
Polyvinyl alcohol
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Table 7 Advantages and disadvantages of petrochemical thermoplastic resins [50]
Resin
Advantage
Disadvantage
Polyethylene
High ductility and impact strength
Good fatigue resistance
Lightweight
Low moisture absorption
Low cost
High temperature resistance
High dielectric resistance
Excellent chemical resistance
Good fatigue resistance
Good chemical resistance
Resistance to stress cracking
Very low moisture absorption
Weather resistance
High impact resistance
Versatility
Good chemical resistance
Flame retardant
Low cost
Good dimensional stability
Poor weathering resistance
Flammable
High thermal expansion
Polypropylene
Polystyrene
Polyvinyl chloride
Difficult to process
Comparatively expensive
Limited availability
Flammable
Low impact resistance
Brittle
Poor resistance to UV
Poor resistance at low and high temperatures
Table 8 Advantages and disadvantages of petrochemical thermoset resins [53–56]
Resin
Advantage
Disadvantage
Epoxy
High thermal and mechanical properties
High water resistance
Low curing shrinkage
Long working times ability
Easy to use
Lowest cost
More expensive than vinylester
Corrosive amine hardener
Difficult to process
Polyester
Vinylester
Very high chemical and environmental resistance
Higher mechanical properties than polyester
Phenolic
High fire resistance
pressures than thermoset resins [1]. Table 7 shows
the advantages and disadvantages of petrochemicalbased thermoplastic resins.
Thermosets
Thermoset resins are infusible and insoluble materials that are cured by heat or a catalyst [51]. Thermosets are completely different from thermoplastics;
they cannot be melted and reshaped by heating [52].
Owing to three-dimensional covalent bonds between
High curing shrinkage
Limited range of working times
Moderate mechanical properties
High styrene emissions in open molds
High curing shrinkage
More expensive than polyester
High styrene content
Requires post-curing for good properties
Difficult to process
the polymer chains (chemical change), this type of
resin has a higher modulus, improved creep resistance, higher thermal stability, and higher chemical
resistance than thermoplastic resins [4]. They are also
brittle at room temperature and show low fracture
toughness. Table 8 shows the advantages and disadvantages of petrochemical-based thermoset resins.
Bio-based resins
Bio-based resins are polymers that are fully or partially obtained from renewable resources [1]. Bio-
838
Table 9 Advantages and
disadvantages of bio-based
resins [57–60]
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Resin
Advantage
Disadvantage
Starch
Fully biodegradable
Low cost
PLA
High modulus and strength
Nontoxic
Relative low cost
High molecular weight
Fully biodegradable
Brittle
Difficult to process
Water-sensitive
Brittle
Relatively poor impact strength
Low thermal degradation temperature
Relative low decomposition temperature
Low stability
Brittle
Low deformability
More expensive than other bio-based polymers
High moisture absorbance
Relative low decomposition temperature
PHA
Cellulose
Abundant
Relative low cost
Ease to modify
Moderate impact resistance
Moderate heat resistance
based polymers can be produced from plants (e.g.,
starch and cellulose) or through the polymerization
of plant-based sugars and oils [e.g., polylactic acid
(PLA), polyethylene terephthalate, and polypropylene] [4]. Based on the physical properties, there are
three types of bio-based polymers: fully bio-based
and biodegradable, e.g., polyhydroxyalkanoates
(PHA) and starch, partially bio-based and
biodegradable, e.g., cellulose and PLA, and partially
bio-based and non-biodegradable, e.g., bio-polyethylene terephthalate, bio-polyethylene, and biopolypropylene [1, 4]. Table 9 shows the advantages
and disadvantages of different types of bio-based
resins.
It was reported that in 2011 approximately 3.5
million tons of bio-based polymers were produced in
the world, whereas 235 million tons of traditional
petrochemical-based polymers were produced in that
year [1]. The growth rate of bio-based polymer production has been high in recent years, and it has been
estimated that the production will reach approximately 12 million tons per year by 2020 [1]. However,
this amount is significantly lower than the production of petrochemical-based polymers, and it needs to
be increased further to decrease the negative environmental impact of petroleum-based materials [1]. It
has been shown that, for the same applications, some
types of PLA made from maize starch utilize up to
50% less oil and release 60% fewer greenhouse gases
into the atmosphere compared to petrochemicalbased polymers [1].
Fully bio-based and biodegradable polymers contain renewable carbon, which is taken from the
atmosphere and then returned to the atmosphere as
the polymer decomposes [4]. This renewable carbon
can be obtained by collecting the polymers, which are
currently used for disposable cutlery and flexible
food packaging, in which the plastics are disposed of
alongside the food waste [1]. Drop-in bio-based
polymers like bio-based polyethylene terephthalate,
which are chemically identical to petrochemicalbased polymers, can be traditionally recycled [1].
This type of polymer can be combusted to create
renewable energy when it is not possible to recycle it.
Bio-based resins offer several advantages over
petrochemical-based resins: they are energy efficient
in production (65% less energy required to produce),
safe (remain nontoxic as they degrade), recyclable
(break down faster with less energy), completely
renewable (as they are made from biomass), and
environmentally friendly (generate 68% less greenhouse gases) [61]. However, their production cost is
approximately 10% higher than that of petrochemical-based resins [62].
Modification of natural fiber composites
The interfacial adhesion across the phase boundary
(i.e., at the interface between two materials) is vital
for the mechanical behavior of composites [4]. Poor
bonding at the phase boundary leads to a composite
with weak mechanical properties. The main concern
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of the utilization of natural fiber composites is the
relative high moisture absorption of natural fibers,
which results in weak compatibility between fibers
and the matrix [4]. Although moisture absorption
mostly affects natural fibers, there are some resins
that absorb a large amount of moisture [1]. Therefore,
both the fiber surface and the matrix need to be
modified to enhance the adhesion of natural fibers to
the matrix for improving the strength and stiffness of
the natural fiber composite.
Fiber modification
Owing to the high sensitivity of natural fibers to
moisture, moisture absorption results in delamination between the matrix and fiber, severely reducing
the mechanical properties of the composite [4]. This is
attributed to the fact that because of the presence of
non-cellulosic components (i.e., pectin, lignin, and
hemicelluloses), natural fibers in nature are polar and
hydrophilic, and thus, they create active conditions
(i.e., accessibility to hydroxyl (OH) and carboxylic
acid groups) for water absorption [63]. Furthermore,
differences in the environmental conditions, such as
the amount of sun, rain, soil conditions, and the
amount of water the plant receives during the
growing period, as well as the processing and production conditions, may also affect the natural fibers
performance [1]. Therefore, performance and properties of natural fibers may be different in every
harvesting season and even in the same cultivation
population. The other limiting factor for the utilization of the natural fibers in composites is their low
thermal stability [4]. However, the problems regarding the utilization of natural fibers in the composite
can be solved by physical and chemical
modifications.
Physical modification
Physical modification on natural fibers enhances the
mechanical adhesion between the natural fiber and
matrix by enhancing the interface without changing
the chemical properties of the fibers [4]. Extensive
researches have been conducted to understand the
influence of physical treatments on natural fibers.
Physical treatment methods include corona, plasma,
ultraviolet (UV), fiber beating, and heat treatment.
These techniques are applied only to change the
surface properties of natural fibers.
The surface energy of cellulose fibers with corona
treatment is changed to improve the compatibility
between the hydrophilic fiber and matrix [64]. In this
approach, using a high voltage at low temperature an
atmospheric pressure plasma is generated [65, 66].
The surface energy of cellulose fibers is changed in
the plasma treatment method by a surface modification technique similar to that of the corona treatment
[67]. However, in plasma treatment, the gas type,
flow, pressure, and concentration are controlled; in
corona treatment they are not [67, 68]. The UV
treatment method increases the polarity of the fiber
surface, which leads to better wettability of the fibers
and increased strength of the composite [64, 69]. In
the fiber-beating approach, an increase in the surface
area, defibrillation of the fibers, and mechanical
interlocking can result in 10% increase in the strength
of natural fibers [70]. In heat treatment, fibers are
heated to a temperature close to the fiber degradation
temperature. This condition affects the physical,
mechanical, and chemical properties of the fibers,
including their water content, chemistry, strength,
cellulose crystallinity, and degree of polymerization
[71, 72].
Chemical modification
Chemical modification on natural fibers improves the
adhesion between the matrix and natural fibers via
chemical reactions. Extensive studies have been performed to understand the effect of chemical treatment on natural fibers. The hydrophilic nature of
natural fibers and the hydrophobic nature of matrices
are considered two different phases, resulting in
weak bonding at the interfaces of natural fiber composites [4]. The chemical treatment of natural fibers
decreases the inherent hydrophilic behavior of the
fibers and improves the adhesion properties of the
matrix and fiber [1]. Chemical treatment methods
include alkaline, silane, acetylation, benzoylation,
peroxide, maleated coupling agents, sodium chlorite,
acrylation and acrylonitrile grafting, isocyanate,
stearic acid, permanganate, triazine, oleoyl chloride,
and fungal treatments [19, 73].
Alkaline treatment The alkaline treatment approach
is one of the simplest and most economic and effective methods for improving the adhesion properties
of natural fibers to the matrix [74]. In this method, the
cellulosic molecular structure of the natural fibers is
840
modified using sodium hydroxide (NaOH) [74].
Alkaline treatment increases the speed of fiber fragmentation and disaggregation [75]. The orientation of
the highly packed crystalline cellulose order is
changed by creating amorphous regions in which
cellulose micro-molecules are separated and the
spaces are filled with water molecules [1]. Alkalisensitive OH groups are broken down and moved
out from the fiber structure, and a fiber cell–O–Na
group is created between cellulose molecular chains
by the remaining reactive molecules. Therefore, the
number of hydrophilic OH groups decreases, the
resistance of the fiber to moisture increases, and a
certain amount of hemicelluloses, lignin, pectin, wax,
and oil are taken out [1]. The surfaces of the fibers
become clean and uniform, which improves the stress
transfer capacity between the cells. An optimum
alkali concentration should be obtained to prevent
extra delignification of the fibers, but higher concentrations can weaken and damage the fibers. With an
optimum alkali concentration, the diameter of the
fibers is decreased, resulting in better adhesion due to
the increased effective fiber surface area and aspect
ratio (length/diameter) [76]. Based on the report by
Brigida et al. [77], alkaline treatment is the most
efficient approach for cellulose exposition of natural
fibers. They also found that alkaline treatment can
maintain the native hydrophilic characteristic of
green coconut fibers and increase their thermal stability. Figure 3 shows scanning electron microscopy
Figure 3 SEM micrographs
of longitudinal views of
a untreated and b 6% alkalinetreated kenaf fiber [78].
J Mater Sci (2020) 55:829–892
(SEM) micrographs of longitudinal views of
untreated and 6% alkaline-treated kenaf fibers [78].
Silane treatment In this approach, silane coupling
agents coat micro-pores on the fiber surface. Silane
forms a chemical link between the fiber surface and
the matrix via a siloxane bridge [4]. At the initial
stage of this treatment, silanols are created using
existing moisture and hydrolyzable alkoxy groups
[4]. One end of the created silanol reacts with a cellulose OH group, and the other end reacts with the
matrix functional group by the condensation process.
As a result, molecular continuity is formed across the
interface of the composite and a hydrocarbon chain is
obtained to restrain the fiber swelling into the matrix
[1]. It has been shown that fibers subjected to silane
treatment exhibit better tensile strength than those
subjected to alkali treatment [24]. Figure 4 shows
SEM micrographs of surface views of untreated and
2%, 4%, and 6% silane-treated hemp fibers [79].
Acetylation treatment In acetylation treatment, acid
catalysts are used to graft acetyl groups to the cellular
structure of the fibers [80]. The fibers are firstly soaked in
the acetic acid and then treated with the acetic anhydride
for 1–3 h at high temperature to accelerate the reaction
because acetic acid and acetic anhydride cannot separately react with the fibers [4]. In this approach, an
esterification reaction happens between OH and carboxylic/anhydride groups of the natural fibers [81].
Figure 5 shows SEM micrographs of surface views of
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Figure 4 SEM micrographs
of surfaces of a untreated and
b 2%, c 4%, and d 6% silanetreated hemp fiber [79].
100μm
100μm
100μm
100μm
untreated and 18% acetylation-treated flax fibers [80]. In
the acetylation treatment, the wax and cuticle on the
surface of the fiber are removed by the interaction with
acetyl, resulting in a smoother surface. Based on the
report by Tserki et al. [82], acetylation treatment of flax
and hemp fibers resulted in their reduced moisture
absorption, removal of the non-crystalline constituents of
the fibers, production of a smooth fiber surface, and
improvement of the stress transfer efficiency. In addition,
based on the study reported by Zafeiropoulos et al. [83],
acetylation changed the bulk properties of the flax fibers,
as well as their surface properties.
Benzoylation treatment To decrease the hydrophilic
nature of natural fibers, benzoyl chloride is used to
Figure 5 SEM micrographs
of surfaces of a untreated and
b 18% acetylation-treated flax
fiber [80].
improve the interfacial adhesion and thermal stability
of fibers [84]. First, in a pretreatment step, lignin,
waxes, and covering oil are extracted and reactive
OH groups are exposed on the fiber surface. The
fibers are then treated with the benzoyl chloride,
where the OH groups of the fibers are replaced with
benzoyl groups. Last, OH groups are attached to the
cellulose backbone [81]. Figure 6 shows SEM micrographs of surface views of untreated and benzoylation-treated flax fibers [85].
Peroxide treatment In peroxide treatment, peroxideinduced grafting of polyethylene is cohered to the
fiber surface and free radicals of peroxide react with
the OH groups of the fiber and matrix [86]. This
842
Figure 6 SEM micrographs
of surfaces of a untreated and
b benzoylation-treated flax
fiber [85].
J Mater Sci (2020) 55:829–892
10μm
10μm
(a)
method improves the adhesion of the fibers to the
matrix at the interface, decreases the moisture
absorption ability of the fibers, and increases the
thermal stability of the fibers [81]. Figure 7 shows
SEM micrographs of surface views of untreated and
peroxide-treated flax fibers [85].
Maleated coupling agents Maleated coupling agents
provide sufficient interaction between the functional
surface of the fibers and the matrix, and they can
form carbon–carbon bonds to the polymer chains of
the matrix [87]. The fiber surfaces are coated with
long-chain polymers due to the reaction of maleic
anhydride with the OH groups in the amorphous
region of the fiber cellulose structure [88, 89]. The
coating excludes the OH groups from the fiber cells,
reducing the hydrophilic tendency of the fibers,
which leads to the formation of a bridge interface and
efficient interlocking between the fiber and the matrix
because of the covalent bonding between OH groups
of the fibers and the anhydride groups of the maleic
anhydride [81, 90]. Figure 8 shows SEM micrographs
Figure 7 SEM micrographs
of surfaces of a untreated and
b peroxide-treated flax fiber
[85].
10μm
(b)
of untreated and maleic anhydride (MA)-treated jute
fiber surfaces [91]. The MA-treated fiber shows
improved fiber–resin adhesion at the interface, which
leads to failure of the fiber by tearing instead of
interfacial failure.
Sodium chlorite treatment In sodium chlorite treatment, fibers are bleached in an acid solution using
sodium chlorite (NaClO2) [92]. This approach
removes moisture from the fiber and increases their
hydrophobic nature, which increases the flexibility of
the fibers [81]. Figure 9 shows SEM micrographs of
surface views of untreated and sodium chloritetreated flax fibers [93].
Acrylation and acrylonitrile grafting In the acrylation
and acrylonitrile grafting method, more access is
provided for the reactive cellulose macro-radicals to
the polymerization medium by the reaction of acrylic
acid (CH2=CHCOOH) with the cellulosic OH groups
of the fibers [94]. Ester linkages with the cellulose OH
groups are produced by the carboxylic acid of
10μm
(a)
(b)
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843
Figure 8 SEM micrographs
of surfaces of a untreated and
b maleic anhydride-treated
jute fiber [91].
Figure 9 SEM micrographs
of surfaces of a untreated and
b sodium chlorite-treated flax
fiber [93].
coupling agents, and fiber moisture is eliminated by a
reduction of the hydrophilic OH groups of the fiber
structure [94]. Peroxide radicals start to graft with
acrylic acid on the matrix, producing oxygen–oxygen
bonds by extracting the hydrogen atoms of the
polymer chains [1]. Therefore, the bonding capacity
and stress transfer of the interface increase by the
coupling mechanism of the fibers and the matrix [81].
Figure 10 shows SEM micrographs of surface views
of untreated, acrylated, and acrylonitrile-graftedtreated oil palm fiber [95].
Isocyanate treatment Isocyanate treatment increases
the bonding properties between the fibers and the
matrix by providing strong covalent bonds between
them [96]. It also increases the moisture resistance
properties of the fibers [96]. The strong covalent
bonds (chemical bondage) and moisture resistance
are obtained by the reaction of the functional groups
(–N=C=O) of isocyanate with the OH groups of the
cellulose and lignin constituents of the fibers [81].
Figure 11 shows SEM micrographs of the surface
views of untreated and isocyanate-treated oil palm
fiber [95].
Stearic acid treatment In stearic acid treatment, the
water resistance of the fibers increases due to the
reaction of the carboxyl groups of stearic acid,
obtained from an ethyl alcohol solution, with the
hydrophilic OH groups of the fibers, thus eliminating
the non-crystalline constituents of the fiber structure
[97, 98]. Therefore, the fibers are dispersed better in
the matrix by breaking down the fiber bundles with
more fibrillation [81]. Figure 12 shows SEM micrographs of surface views of untreated and stearic acidtreated flax fibers [98].
Oleoyl chloride treatment Oleoyl chloride (fatty acid
derivate) reacts with the OH groups of the fibers and
improves their wettability and adhesion properties
[99]. This improvement is obtained by making the
fibers more hydrophobic by eliminating the hydrophilic OH groups from the external surfaces of the
fibers [81]. Figure 13 shows SEM micrographs of
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Figure 10 SEM micrographs
of surfaces of a untreated,
b acrylated-treated, and
c acrylonitrile-grafted-treated
oil palm fiber [95].
200μm
1μm
(a)
(b)
200μm
(c)
Figure 11 SEM micrographs
of surfaces of a untreated and
b isocyanate-treated oil palm
fiber [95].
1μm
(a)
Figure 12 SEM micrographs
of surfaces of a untreated and
b stearic acid-treated flax fiber
[98].
200μm
(b)
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Figure 13 SEM micrographs
of surfaces of a untreated and
b 24-h oleoyl chloride-treated
jute fiber in pyridine [99].
surface views of untreated and 24-h-oleoyl chloridetreated jute fibers in pyridine [99].
triazine with the hydrophilic OH groups of the cellulose and lignin constituents [81].
Permanganate treatment Chemical interlocking at the
interface between natural fibers and the matrix is
improved by treating the fibers with permanganate
[100]. In this approach, better adhesion between the
fibers and the matrix is provided by the reaction of
potassium permanganate (KMnO4) with the cellulose
OH groups and lignin constituents of the fibers, thus
improving the thermal stability of the fibers [81].
Figure 14 shows SEM micrographs of the surface
views of untreated and permanganate-treated flax
fibers [100].
Fungal treatment Fungal treatment is a new biological treatment in which specific enzymes are applied
to eliminate the non-cellulosic components of the
fiber surface [102]. Extracellular oxidase enzymes are
produced from white-rot fungi, which react with the
lignin peroxidase to remove lignin from the fibers.
Better interlocking between the fibers and matrix
creates fine holes in the fiber surface via producing
hyphae [81]. Figure 15 shows SEM micrographs of
surface views of untreated and fungal-treated hemp
fibers [102].
Triazine treatment Triazine (C3H3N3) treatment
increases the adhesion properties of the fibers and
matrix by providing covalent bonds between them,
and it enhances the moisture resistance of the fibers
[101]. The covalent bond and moisture resistance are
obtained by the reaction of the functional groups of
Figure 14 SEM micrographs
of surfaces of a untreated and
b permanganate-treated flax
fiber [100].
Matrix modifications
In general, the concern of moisture absorption in
composites stems mainly from the natural fibers.
However, some matrices (especially bio-based resins)
such as soy protein resins absorb a large amount of
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J Mater Sci (2020) 55:829–892
Figure 15 SEM micrographs
of surfaces of a untreated and
b fungal-treated hemp fiber
[102].
moisture [1]. Soy protein concentrate resin was
modified by Chabba and Netravali [103] through
adding glutaraldehyde in one case and vinyl alcohol
in the other. Both methods resulted in approximately
2% less moisture absorption as compared to that of
the unmodified resin. Kumar and Zhang [104, 105]
improved the moisture absorption of soy protein
resin made with different superplasticizers using
benzilic acid. They found that immersing the soy
protein resin in 0.5% benzilic acid for 26 h resulted in
61% (with thiodiglycol superplasticizer) to 76% (with
formamide superplasticizer) decrease in the moisture
absorption of the resin. Newill et al. [106] found that
a 0.254-mm-thick polymeric coating resulted in an
approximately 47% reduction in the moisture
absorption of the matrix. Doherty et al. [107] illustrated that the moisture absorption of a matrix could
be reduced by 61% by applying a natural ligninbased coating with a 0.204-mm thickness.
Processing techniques
The main parameters influencing the processing of
natural fiber composites are moisture, fiber type, fiber
volume fraction, and temperature of the composite.
The moisture of both the fiber and the matrix must be
controlled before processing, and the required modifications must be performed if moisture is present
[1]. The length, aspect ratio, and chemical composition of the fibers also have great effects on the processing, sustainability, and performance of
composites [1]. An increase in the fiber volume fraction of the composite increases the composite’s stiffness, strength, and water uptake and decreases its
deformability [1]. The processing temperature is
another important factor. The maximum temperature
that can be used in the processing period to avoid
degradation of the most of natural fibers is 200 °C
within 20 min [108, 109]. Any temperature above this
limit can cause degradation, shrinkage, and low
performance of the natural fiber composite because
the chemical, physical, and mechanical properties of
the natural fibers are changed by depolymerization,
oxidation, hydrolysis, decarboxylation, dehydration,
and recrystallization [110]. The major manufacturing
methods for natural fiber composites are compression molding, extrusion molding, injection molding,
and resin transfer molding techniques. Table 10 presents the advantages and disadvantages of the processing techniques for manufacturing natural fiber
composites.
Processing techniques of thermoplastic
composites
Compression molding
Compression molding has been used since 1990s for
thermoplastic composites as the demand for lightweight and high-performance materials increased [1].
In this technique, preheated materials are initially
placed in the molding cavity. Then, they are compressed and deformed by the core side of the mold
while subjecting the cavity to high pressure [1].
Before opening the mold and removing the composite, the high pressure is maintained until the composite solidifies. The important parameters that must
be considered with this technique are the amount of
material, heating time, pressure applied to the mold,
and cooling time [109].
Sheet molding, as a type of compression molding
technique, is one of the main processing approaches
for composite production. Figure 16 shows the process diagram of sheet molding technique [123]. In this
technique, a measured amount of a specific resin is
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Table 10 Advantages and disadvantages of different processing techniques [109, 111–122]
Technique
Advantage
Disadvantage
Compression
molding
Fast setup time
Low wasted material
Low cost for large and complex composites
Good surface finish
Even pressure distribution on the composites
Excellent part reproducibility
Very high volume production ability
Low labor requirements on a production level
Fast setup time
Low initial setup costs
Low production costs
Low production speed
Suitable only for flat or moderately curved composite shapes
Sheet molding
Extrusion
molding
Injection molding
Hand lay-up
Resin transfer
molding
Resin infusion
molding
Low operational cost
Low cost in mass production
High throughput
Flexibility to make parts with complex shapes
High precision
Simple principles to teach
Low tooling cost
Wide choice of material types and suppliers
Flexibility in material design
Better product consistency than that of
compression molding
Tighter tolerance and more intricate parts than
injection molding
Fast setup time and low setup costs
Low maintenance costs
Excellent surface quality on both sides
Short process time
Highly even quality and material thickness
dispensed with a paste reservoir into the plastic carrier film, which is passed under a chopper to cut the
fibers onto the surface [123]. Another sheet is added
on top of the layer after placing the fibers in the resin
paste. The sheets are then compacted and sent
through to the take-up roll for storing the product.
The carrier film is removed, and the material is cut
into charges and molded under heat and pressure to
prepare the composite based on the requested shape.
Finally, the product is removed from the mold after
full curation [1].
Suitable only for the preparation of composites with a low fiber
volume fraction
Moderate production speed
Suitable only for the preparation of composites with a uniform
cross section
Mediocre precision
High initial setup costs
Labor intensive
Styrene emission from unsaturated polyester and vinylester
resins
Dependency of the quality of the laminate to the skill of
laminators
Resins need to be low viscosity to be workable by hand
More material is wasted than with compression molding
Production speed is lower than that of injection molding
Slow cycle times and high consumable costs
Extrusion molding
Extrusion molding is one of the most widely used
procedures in the manufacturing process of natural
fiber composites. This technique is preferred because
of the high stiffness and strength of the composites
and because the formation of the composites with
this technique is very easy [1]. This technique begins
with storing the thermoplastic material in the form of
pellets or granules in a hopper. Then, they are
delivered to a heated barrel to be molten. The molten
plastics are subsequently used for the required shape
of the composite. The final stage is cooling the product [1].
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J Mater Sci (2020) 55:829–892
Figure 16 Process diagram of
sheet molding technique for
composite production [123].
Figure 17 Process diagram of
resin transfer molding
technique for composite
production [123].
Injection molding
Injection molding is the most common approach
utilized in the manufacturing of mass-produced
composites. This method is initiated by putting the
polymers in the form of pellets or granules into the
hopper and heating them to be molten. The molten
materials are then injected into a chamber formed by
a split die mold and kept in the mold. The melt is
chilled down to solidify before opening the mold
[120].
Processing techniques of thermoset
composites
Hand lay-up
Hand lay-up is the most common, simplest, and
cheapest technique for production of composites [15].
In this technique, a release agent as an anti-adhesive
agent is initially applied to the open mold, and the
fibers are then placed in the mold. Resins are applied
to the fibers by pouring and brushing with a roller or
brush. Lay-up is made by building layer upon layer
till the desired thickness. Entrapped air in the laminate is removed manually with squeegees or rollers.
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Table 11 Density of major natural and synthetic fibers and
polymers [15, 22, 129]
Fiber and polymer type
Density (kg/m3)
Cotton
Jute
Flax
Hemp
Kenaf
Ramie
Sisal
Coir
Pina
Abaca
Bamboo
Bagasse
Banana
Oil Palm
Curaua
Pulp
Silk
Feather
Wool
Carbon
Glass
Polypropylene
Low-density polyethylene
High-density polyethylene
Polystyrene
Polyester
Nylon 6
Nylon 6,6
Vinylester
Epoxy
Phenolic
Starch
PLA
PHA
1520–1560
1440–1520
1420–1520
1470–1520
1435–1500
1450–1550
1400–1450
1150–1220
1440
1500
600–1100
1250
1350
700–1550
1400
1500
1300
900
1300
1800–1840
2550–2600
890–910
910–925
940–960
960–1040
1040–1400
1120–1140
1130–1150
1200–1400
1110–1400
1160–1210
1000–1390
1210–1250
1180–1260
Vacuum is often used to avoid air bubbles and to
help draw the resin into the cavity [116].
Resin infusion molding
Resin transfer molding
In resin transfer molding (RTM), the resin is preheated and loaded into the holding chamber instead
of loading the polymer into an open mold [124]. This
technique is most suitable for medium-volume production of large components. Figure 17 shows the
process diagram of RTM technique [123]. Layers of
the textile are prearranged in the solid mold, and the
resin is then injected to impregnate the preforms.
Physical properties
Unit weight and density
Table 11 shows the densities of most widely used
natural and synthetic fibers and polymers, showing
that the natural fibers have a lower density than
synthetic fibers.
Air content
Over several years of producing synthetic composite
materials, it is now possible to control and optimize
the void content to be lower than 2% by volume of
the composites [130]. Baley et al. [131] reported different techniques to determine the void content of
60
Flexural Strength (MPa)
Laminates are then left to cure under standard
atmospheric conditions [113].
Resin infusion molding (RIM) is a double-mold process with medium flow as one type of vacuum-assisted RTM, in which the second mold face is
replaced with a flexible membrane [125]. The process
takes place on timescales where the fibers swell
during infusion [126, 127]. It is initially started by
laying dry fibers in the metal face, and the flexible
tool face is then pressed down over the composite
object. The liquid resin is injected under moderate
pressure into the mold and saturates the fibers. The
mold can be subjected to a vacuum to minimize air
pockets to enhance the quality of the composite [128].
50
40
30
20
10
0
0
10
20
30
40
50
60
70
Void Content (%)
Figure 18 The effect of void content on the flexural strength of a
vacuum-treated random-oriented wood pulp fiber/starch
composite manufactured using hot pressing [135].
850
composites. The void content in natural fiber composites is a concern because of the voids inherent in
natural fibers, which affect the transverse failure of
the composites [132]. Air or other volatile substances
can be trapped inside the composites during insertion of the fibers into the resin, and micro-voids can
form after curing of the composites. Voids can form
during compounding or melt-flow processing, or
because of uneven shrinkage owing to thermal gradients during the solidification process (cooling). An
increase in the cooling rate results in an increase in
the void content of composites. Micro-voids create
poor mechanical properties, which result in sudden
failure of the composites [87]. The fiber volume
fraction and fiber length in the composites are the
main factors for void or bubble formation. Higher
fiber content and length in the composite increase the
probability of void formation [133]. It was reported
that a high void content in graphite fiber/polyimide
and glass fiber/polystyrene composites (i.e., over
20% by volume) results in low fatigue resistance
[133, 134]. The shapes of the voids can be changed
from spherical to elongated, and the void content can
be decreased by a post-extrusion hot-drawing
approach [133]. Dong and Takagi [135] assessed the
effect of void content on the flexural properties of a
vacuum-treated random-oriented wood pulp fiber/
starch composite manufactured using hot pressing
(see Fig. 18). Judd and Wright [136] studied quantitatively the effect of voids on the mechanical properties of fiber/resin composites and reported that,
regardless of resin, fiber type, and fiber surface
treatment, the interlaminar shear strength of a composite decreases about 7% for each 1% void. Ghiorse
[137] showed that laminate lay-up type affects the
void distribution, and void length, area, and aspect
ratio increase with an increase in the void content.
J Mater Sci (2020) 55:829–892
Tensile properties
The tensile properties of natural fiber composites are
mainly affected by the interfacial adhesion between
the resin and fibers [1]. Physical and chemical modification of the fiber and resin enhances the tensile
properties of composites [14]. Tables 13 and 14 present the tensile properties of natural fibers and
polymers (thermoplastics and thermosets), respectively. The tensile properties of natural fiber composites are highly dependent on the fiber volume
fraction in the matrix resin. Although a number of
researchers showed irregular trends for tensile
properties of composites as a function of fiber volume
fraction, it is generally true that by an increase in the
fiber volume fraction below an optimum value, the
load is distributed to more fibers and the matrix can
carry the applied load after fibers fracture. This can
lead to a higher tensile strength for the composite
[14]. With further increases in the fiber volume fraction after the optimum amount, brittle fracture occurs
in the fibers and the matrix cannot support the
additional load from the fibers. Under these conditions, the low tensile strength eventually leads to
failure of the whole composite [160]. The contrary
and irregular trends for the tensile properties can be
because of many factors, including incompatibility
between the fibers and matrix, fiber degradation, and
incorrect manufacturing processes [14]. Figures 19
and 20 illustrate the influence of fiber content on the
tensile strength and elastic modulus of flax (manufactured using extrusion and injection molding) and
hemp fiber/high-density polyethylene (HDPE) composites (manufactured by compression molding)
[14, 93, 161]. Figure 21 shows the influence of the
fiber content by weight on the elongation at break of
cross-plied flax fiber/HDPE, cotton fiber/corn starch
(CS), and jute fiber/polybutylene succinate (PBS)
composites [14, 93, 145, 162].
Mechanical properties
Flexural properties
In general, the mechanical properties of natural fiber
composites are lower compared to those of synthetic
fiber composites. However, these properties can be
enhanced by proper modifications of the natural
fibers and matrices through the application of techniques summarized in previous sections. Table 12
presents a summary of the existing studies on the
mechanical properties of natural fiber composites.
Flexural stiffness is one of the key parameters for
measuring the resistance of a composite against
bending deformation. Flexural properties are mainly
dependent on two parameters: the modulus of the
composite material and the moment of inertia [4].
Table 15 presents the flexural properties of polymers
used in natural fiber composites. An increase in the
fiber content up to the optimum amount increases the
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Table 12 Summary of existing studies on the mechanical properties of natural fiber composites
Reference
Composite
Apparent fiber
diameter (lm)
Manufacturing
method
Observation
Zampaloni et al. Random-oriented hemp, flax, kenaf, sisal, –
[138]
and coir fiber/polypropylene composite
Compression
molding
Joseph et al.
[139]
Cross-plied abaca fiber/phenolic
composite
–
Compression
molding
Biswas et al.
[140]
Random-oriented coir fiber/epoxy
composite
–
Hand lay-up
Gao and Mader
[141]
Random-oriented jute fiber/
polypropylene composite
22
Injection
molding
Karmaker and
Schneider
[142]
Shibata et al.
[143]
Cross-plied jute and kenaf fiber/
polypropylene composite
–
Injection
molding
Random-oriented kenaf and bagasse
fiber/corn-starch composite
Kenaf: 140a
Bagasse: 394a
Compression
molding
Sharma and
Cross-plied banana fiber/polyurethane
Kumar [144]
composite
Liu and Hughes Woven flax fiber/epoxy composite
[145]
80a
Compression
molding
Resin infusion
Zhang et al.
[146]
Unidirectional flax fiber/phenolic
composite
–
Li et al. [147]
Unidirectional flax fiber/epoxy composite 5–35a
40% by weight hemp fiber composite
exhibited a higher tensile and flexural
strength than 40% by weight kenaf,
flax, coir, or sisal fiber composites
Fiber length of 30 mm was optimum for
achieving maximum tensile, flexural,
and impact strength
Fiber length of 30 mm was optimum to
achieve maximum tensile and flexural
strength. An increase in fiber length
resulted in an increase in the impact
resistance of the composite
Fiber length is an important parameter
that can affect the tensile behavior of
the composite
Superior mechanical properties for the
jute fiber composite than kenaf fiber
composite
60% and 66% increase in flexural
modulus of kenaf and bagasse fiber
composite with increasing fiber
volume fraction, respectively
15% fiber content by weight resulted in
the maximum flexural strength
Fracture toughness is mostly dependent
on the fiber volume fraction rather
than reinforcement architecture
Crack propagation was accompanied by
extensive fiber bridging in the analysis
of interlaminar fracture energy
Approximately 30% increase in the
interlaminar fracture energy of the
composite using 1% by weight of
multi-walled carbon nanotubes on the
surface of flax fibers
Improved resistance to crack
propagation when fiber treated by
styrene and composite cured by an
out-of-autoclave curing process
Maximum increase in fracture
toughness of the composite by adding
both micro-rubber and nanosilica
particles
–
Compression
molding
Resin infusion
Kafi et al. [148] Woven jute fiber/polyester composite
–
Compression
molding
Kinloch et al.
[149]
17
Resin infusion
Woven flax fiber/epoxy composite
852
J Mater Sci (2020) 55:829–892
Table 12 continued
Reference
Composite
Apparent fiber
diameter (lm)
Manufacturing
method
Observation
Hsieh et al.
[150]
Random-oriented rubber fiber/epoxy
composite
1.24
Resin infusion
Agunsoye and
Aigbodion
[151]
Wong et al.
[152]
Random-oriented bagasse fiber/
polyethylene composite
13
Compression
molding
175% increase in the fracture toughness
with the addition of 20% by weight
silica nanoparticles
Fracture toughness steadily decreased
with increasing fiber content
Random-oriented bamboo fiber/polyester
composite
300–380a
Hand lay-up
Alamri and
Low [26]
Random-oriented recycled cellulose/
epoxy composite
5–10
Compression
molding
Muralidhar
[153]
Unidirectional flax fiber/epoxy composite –
Hand lay-up
Bledzki et al.
[154]
Woven jute and flax fiber/epoxy
composite
Jute: 37
Flax: 20
Compression
molding
Cao et al. [155] Bagasse fiber/polyester composite
390a
Compression
molding
Biswas et al.
[34]
Sisal fiber/polyester composite
–
Compression
molding
Dhakal et al.
[156]
Random-oriented hemp fiber/polyester
composite
20
Hand lay-up
Jandas et al.
[157]
Random-oriented banana fiber/polylactic
acid composite
–
Compression
molding
Liang et al.
[158]
Unidirectional flax fiber/epoxy composite –
Hand lay-up
a
The highest fracture toughness was
achieved using 50% volume fraction
and a fiber length of 10 mm
Fracture toughness increased with an
increase in the fiber content up to 46%
by weight
Compressive strength and stiffness of
the composite was because of
reinforcing the matrix rather than the
fiber/matrix synergy
An increase in the fiber content
enhanced the impact strength of the
composite. Flax fiber composite had
higher impact resistance than jute fiber
composite, and an increase in the
number of voids decreased the impact
strength of composite
Impact strength of the composite 30%
increased with an increase in the fiber
content up to an optimum amount of
65%
At 0.5% fiber volume fraction, sisal
fiber/polyester composite had
comparable impact strength to glass
fiber/polyester composite
Increasing the hemp fiber volume
fraction up to 10% led to a 228%
increase in the impact strength
An increase in the fiber volume fraction
up to 25% resulted in the maximum
impact strength
Compressive strength of the composite
15–30% decreased by applying 10 J
impact loading
Diameter of a fiber bundle
flexural strength of natural fiber composites [4].
However, further increases in the fiber content
decrease the flexural strength, owing to defects in the
wetting of fibers that can induce stress concentration
points in the composites. An increase in the fiber
content also increases the flexural modulus of the
composites [4].
The fracture toughness of natural fiber composites,
which means a composite’s resistance to crack
propagation, is mainly influenced by the intrinsic
properties of the matrix, the fiber volume fraction,
853
J Mater Sci (2020) 55:829–892
Table 13 Tensile properties
of most widely used natural
fibers [14, 15, 22, 159]
Fiber
Tensile strength (MPa)
Elastic modulus (GPa)
Elongation (%)
Cotton
Jute
Flax
Hemp
Kenaf
Ramie
Sisal
Coir
Pulp
Pina
Abaca
Bamboo
Bagasse
Banana
Oil Palm
Curaua
Silk
Feather
Wool
287–800
393–860
345–1500
550–920
195–666
400–938
468–790
135–240
1000
413–1627
430–760
140–800
222–290
500
80–248
87–1150
100–1500
100–203
50–315
5.5–12.6
13–60
27.6–90
55–70
53–66
61.4–128
9.4–25
4–6
40
34.5–82.5
6.2–20
11–32
17–27.1
12
0.5–3.2
11.8–96
5–25
3–10
2.3–5
7–12
1.5–1.8
2.7–3.2
2–4
1.3–5.5
3.6–3.8
2–7
15–40
4.4
1.6
1–10
2.5–3.7
1.1
1.5–9
17–25
1.3–4.9
15–60
6.9
13.2–35
Table 14 Tensile properties of polymers used in natural fiber composites [129, 159]
Resin
Tensile strength (MPa)
Elastic modulus (GPa)
Elongation (%)
Type of resin
Polypropylene
Low-density polyethylene
High-density polyethylene
Polystyrene
Nylon 6
Nylon 6,6
Starch
PLA
PHA
Polyester
Vinylester
Epoxy
Phenolic
26–41.4
40–78
14.5–38
25–69
43–79
12.4–94
5–6
21–60
18–24
41.4–89.6
69–83
55–130
50–60
0.95–1.77
0.055–0.38
0.4–1.5
4–5
2.9
2.5–3.9
0.125–0.85
0.35–3.5
0.7–1.8
2.07–4.41
3.1–3.8
3–6
4–7
15–700
90–800
2–130
1–2.5
20–150
35–300
31–44
2.5–6
3–25
2–2.6
4–7
2–10
1
Thermoplastic
and the interfacial bonding strength [4]. The energy
dissipation is influenced by pull out of fibers in the
wake of crack propagation of the composites [149].
Compressive properties
The compressive behavior of natural fiber composites
is highly dependent on the reinforcement architecture and fiber volume fraction of the composite
[4, 163]. With increasing fiber volume fraction below
the optimum amount, higher compressive strength is
Thermoset
obtained for the composite, owing to a decrease in the
number of voids [4]. Increased compressive strength
is also attributed to good homogeneity and high
compaction between the fibers and the matrix.
However, as the fiber volume fraction exceeds the
optimum value, the compressive strength decreases.
Generally, when the fiber volume fraction exceeds
1.5%, a reduction of compressive strength of
approximately 8.5% occurs for every 0.5% increase in
fiber volume [164, 165]. However, it was shown that
the compressive strength of natural fiber composites
854
J Mater Sci (2020) 55:829–892
Tensile Strength (MPa)
50
40
30
20
10
Flax-HDPE
Hemp-HDPE
0
0
10
20
30
40
50
Fiber Volume Fraction (%)
Figure 19 The effect of fiber content on the tensile strength of
flax (manufactured using extrusion and injection molding) and
hemp fiber/high-density polyethylene (HDPE) composites
(manufactured by compression molding).
Elastic Modulus (GPa)
8
6
4
2
Flax-HDPE
Hemp-HDPE
0
0
10
20
30
40
Figure 20 The effect of fiber content on the elastic modulus of
flax (manufactured using extrusion and injection molding) and
hemp fiber/high-density polyethylene (HDPE) composites
(manufactured by compression molding).
Elongation (%)
Flax-HDPE
100
Impact strength
The impact behavior of natural fiber composites
mainly depends on the bonding level between the
matrix and fiber [4]. This property plays a major role
during the service life of natural fiber composites [4].
The impact properties of natural fiber composites can
be improved by modification methods. The impact
loading may be the result of bumps, crashes, and
falling objects and debris. Some of the influential
parameters on the impact strength of composites are
the level of adhesion, favorable bonding, fiber pullout, and energy absorption [4].
50
Fiber Volume Fraction (%)
120
architecture parameter on the compressive behavior
of a hemp fiber/polyester composite tube manufactured by filament winding technique with fiber orientation of 10°, 30°, 45°, 60°, and 90°. Figure 22 shows
the macro- and micro-images of the tubes with fiber
orientation of 45° and 90° [163]. They found that a
fiber orientation of 10° with the loading direction
resulted in the highest compressive strength and
modulus, and the observed fracture modes were
diamond-shaped buckling, micro-buckling, concertina-shape buckling, and progressive crushing.
Cotton-CS
Effect of fiber/matrix treatment
on the mechanical properties
Several researchers have assessed the effect of fiber or
matrix treatment on the mechanical properties of
natural fibers composites. Table 16 presents a summary of the existing studies on the effect of
fiber/matrix treatment on the mechanical properties
of natural fiber composites.
Jute-PBS
80
Effect of crystallinity on the mechanical
properties
60
40
20
0
0
5
10
15
20
25
30
35
Fiber Content by Weight (%)
Figure 21 The effect of fiber content on the elongation at break
of cross-plied flax fiber/HDPE, cotton fiber/corn starch (CS), and
jute fiber/polybutylene succinate (PBS) composites.
gradually reduced with increasing fiber volume
fraction [165, 166]. Wecławski et al. [163] assessed the
influence of the fiber orientation as a reinforcement
Crystallinity restricts the mobility of the molecular
chains of the material and plays a major role in the
physical and mechanical behavior of natural fiber
thermoplastic matrix composites [207]. The structure
of a cellulose material composes amorphous and
crystalline regions. The amorphous region absorbs
chemicals, such as dyes and resins, whereas the
compactness of the crystalline region makes it difficult for chemical penetration [207]. Chemical treatments on the fiber increase its crystallinity, owing to
the removal of lignin and hemicellulose after
855
J Mater Sci (2020) 55:829–892
Table 15 Flexural properties
of the polymers used in the
natural fiber composites [1]
Resin
Flexural strength (MPa)
Flexural modulus (GPa)
Polypropylene
Low-density polyethylene
High-density polyethylene
Polystyrene
Nylon 6
Nylon 6,6
Polyester
Vinylester
Epoxy
Phenolic
Starch
PLA
PHA
40
9
32
70
85
103
70–110
130–140
110–150
80–135
52
51–70
94
1.5
0.2
1.2
2.5
2.3
3.1
2–4
3
3–4
2–4
2.4
4.2
2.7
Figure 22 a A diagram with
positioning of cross section
and fiber orientation, and
macro- and micro-images of
hemp fiber/polyester
composite with fiber
orientation (h) of b 45° and
c 90° [163].
(a)
Hemp fibers
Section A-A
500μm
(b)
Hemp fibers
Hemp fibers
Section A-A
500μm
(c)
Section B-B
500μm
856
treatment [208, 209]. A limited number of studies
assessed the effect of crystallinity on the mechanical
properties of natural fiber composites. Rong et al. [72]
investigated the effect of different fiber treatments on
the crystallinity and found that heat treatment (at
150 °C for 4 h) of unidirectional sisal fibers led to
slightly higher crystallinity index (66%) than those
under acetylation (65%) and alkaline treatment (61%).
They found that alkaline-treated sisal fiber/epoxy
composites manufactured by compression molding
technique had the highest tensile strength and modulus, acetylated-treated sisal fiber/epoxy composites
had the highest flexural strength, and heat-treated
sisal fiber/epoxy composites had the highest flexural
modulus. Barone [210] investigated the influence of
the degree of crystallinity of the polymer on the
tensile strength and modulus of random-oriented
keratin feather fiber/polyethylene composite manufactured by compression molding technique. They
found that an increase in the crystallinity of the
polymer obtained from heat treatment (at 170 °C for
15 min) led to an increase in the tensile strength and
modulus of the composite because of the increased
fiber/polymer interaction. Zafeiropoulos et al. [211]
and Joseph et al. [87] reported that cooling rate is one
of the main parameters affecting the crystallinity
degree of the natural fiber-reinforced composites.
They found that lower cooling rates result in smaller
fragment lengths of random-oriented flax fiber/
polypropylene and sisal fiber/polypropylene composite, indicating a better stress transfer-ability and
stronger interface by slow cooling. Suryanegara et al.
[212] reported that random-oriented pulp fiber/PLA
composite manufactured by compression molding
technique with 3% fiber weight fraction and fully
crystallized polymer (heat-treated at 100 °C for 1 h)
had 4% and 16% higher tensile strength and modulus
compared to those with fully amorphous polymer,
respectively. However, they found that the crystallinity of polymer increased the brittleness of the
composite. Figure 23 shows the axial tensile stress–
strain curves of pulp fiber/PLA composite under
amorphous and crystallized states [212].
When a fiber is embedded into a semicrystalline
polymer, the fiber may act as a nucleating site for the
growth of spherulites [213]. If many nucleation sites
exist along the fiber surface, the resulting spherulite
growth will be restricted in the lateral direction and a
columnar layer, known as trans-crystallinity, develops and encloses the fiber [213]. Although several
J Mater Sci (2020) 55:829–892
studies have investigated the trans-crystallinity of
conventional composites, only a few studies investigated the influence of the trans-crystallinity on the
mechanical properties of natural fiber composites.
Zafeiropoulos et al. [214, 215] studied the effect of
trans-crystallinity on the interface of flax fiber/
polypropylene composite and found that the induced
fiber trans-crystallinity from stearation surface modification resulted in 81% increase in the interfacial
shear strength of the composite, owing to the
improved interfacial adhesion between fiber and
polymer by the presence of a trans-crystalline layer.
Sawpan et al. [216] assessed the influence of the fiber
trans-crystallinity from alkaline treatment on the
interfacial shear strength of random-oriented hemp
fiber/PLA composite manufactured by extrusion and
injection molding techniques and reported that
alkaline-treated fiber composite had 18% higher
interfacial shear strength than that with no fiber
treatment. They attributed their observation to the
positive influence of the fiber crystallinity at the
fiber/polymer interface, which is shown in Fig. 24
[216].
Simulation and modeling
Analytical models
Six analytical models have been used for prediction
of the mechanical behavior of natural fiber composites with discontinuous fibers, including rules of
mixture, series and parallel, Hirsch model, Halpin–
Tsai equation, Bowyer–Bader model, and shear lag
model [217]. The fundamental properties used in
these models are elastic modulus of the fiber (Ef),
elastic modulus of matrix (Em), tensile strength of
fiber (rf), tensile strength of matrix (rm), fiber volume
fraction (Vf), and matrix volume fraction (Vm). In
these models, the micro-defects in the polymer and
fiber/polymer interface are not considered [218].
Virk et al. [219] used rules-of-mixture model to
predict the elastic modulus and tensile strength of
unidirectional jute fiber/epoxy composite manufactured by resin infusion molding technique and
reported that using the common method to measure
the cross-sectional area of the fibers by linear measurements of fiber diameter and an assumption of
circular cross section results in an overestimation of
the cross-sectional area and hence leads to low values
of key mechanical properties of natural fibers. They
857
J Mater Sci (2020) 55:829–892
Table 16 Effect of fiber/matrix treatment on mechanical properties of natural fiber composites
Reference
Treatment
Composite
Apparent
fiber
diameter
(lm)
Manufacturing
method
Observation
Gassan and
Gutowski [64]
Fiber corona
treatment
Tossa jute fiber/epoxy
composite
–
Compression
molding
Ragoubi et al.
[167]
Fiber ultraviolet
(UV) treatment
Fiber corona
treatment
30% increase in the flexural strength
under optimum corona treatment
condition
30% increase in the flexural strength
Random-oriented hemp
fiber/polypropylene
composite
Flax and hemp
fiber/tannin and hexamine
composite
Random-oriented
miscanthus fiber/polylactic
acid and polypropylene
composite
Random-oriented flax fiber/
polyester composite
Random-oriented sisal fiber/
high-density polyethylene
composite
Woven jute fiber/highdensity polyethylene
composite
Random-oriented jute fiber/
polyester composite
–
Extrusion
molding
Improved Young’s modulus,
stiffness, and tensile strength
–
Compression
molding
–
Extrusion–
compression
molding
Improved tensile strength and
modulus by optimum duration of
corona treatment of 10 min
Improved mechanical properties
20
Compression
molding
Compression
molding
Improved permeation and
mechanical properties
25% and 18% increases in the tensile
strength and modulus, respectively
30a
Compression
molding
30a
Hand lay-up
35% and 30% increases in the
interlaminar shear strength and
flexural strength, respectively
14% increase in the flexural strength
100a
Hand lay-up
Pizzi et al. [168]
Ragoubi et al.
[169]
Marais et al. [67] Fiber plasma
treatment
Martin et al. [68]
Seki et al. [170]
Sinha and
Panigrahi
[171]
Seki et al. [172]
Woven jute fiber/polyester
composite
Jang et al. [173]
–
Random-oriented coir fiber/ –
polylactic acid composite
Random-oriented kraft fiber/ 25
polypropylene composites
Jute fiber/vinylester
–
composite
Compression
molding
Extrusion
molding
Compression
molding
Aziz et al. [78]
Random-oriented hemp and
kenaf fiber/polyester
composite
Compression
molding
Cao et al. [155]
Random-oriented bagasse
fiber/polyester composite
Hemp:
180a
Kenaf:
110a
390a
Prasad et al.
[174]
Random-oriented coir fiber/
polyester composite
150–300a
Hand lay-up
Beg and
Pickering [70]
Ray et al. [74]
Fiber-beating
treatment
Fiber alkaline
treatment
Compression
molding
72% and 129% increases in the
interlaminar shear strength through
the low- and radio-frequency
oxygen plasma treatment,
respectively
Improved mechanical as well as the
thermal behavior
10% enhancement in the tensile
strength
35%, 23%, and 19% increases in the
flexural strength, modulus, and
laminar shear strength, respectively
25% and 150% increases in the
flexural strength of hemp and kenaf
fiber composite, respectively.
13% and 65% increases in tensile
and impact strength, respectively,
with a 1% alkaline fiber treatment
40% increase in the impact and
flexural strength by 5% alkaline
treatment for 72 h
858
J Mater Sci (2020) 55:829–892
Table 16 continued
Composite
Apparent
fiber
diameter
(lm)
Manufacturing
method
Observation
Ray et al. [175]
Random-oriented jute fiber/
vinylester composite
–
Hand lay-up
Onal et al. [176]
Random-oriented carpet
waste jute yarn composite
Woven pina fiber/polylactic
acid composite
10
Compression
molding
Compression
molding
20% enhancement in the flexural
strength by 5% alkaline treatment
for 4 h
Improved impact strength
Reference
Treatment
Huda et al. [177]
50a
Qin et al. [178]
Unidirectional ramie
fiber/cellulose composite
–
Compression
molding
Bledzki et al.
[179]
Unidirectional flax and
hemp fiber/epoxy and
polypropylene composite
Random-oriented banana
fiber/epoxy composite
–
Filament
winding
150a
Hand lay-up
Random-oriented hemp
fiber/polypropylene
composite
Banana fiber/polylactic acid
composite
–
Compression
molding
–
Compression
molding
Xu et al. [181]
Random-oriented kenaf
fiber/polystyrene
composite
50–120a
Compression
molding
Pothan and
Thomas [182]
Cantero et al.
[183]
Abaca fiber/polyester
composite
Random-oriented flax fiber/
polypropylene composite
50–250a
Compression
molding
Extrusion
molding
Abdul Khalil
and Ismail
[184]
Ismail and
Abdul Khalil
[185]
Sgriccia et al.
[186]
Oil palm empty-fruit-bunch –
and coir fiber/polyester
composite
180a
Oil palm empty-fruitbunch/natural rubber
composite
Hemp and kenaf fiber/epoxy 160a
composite
Eng et al. [187]
Random-oriented oil palm
fiber/polylactic acid
composite
Venkateshwaran
et al. [180]
Suardana et al.
[79]
Jandas et al.
[157]
Fiber silane
treatment
–
–
Compression
molding
Compression
molding
Compression
molding
Melt blending
26% improvement in the flexural
strength through 40% alkaline
treatment
23% improvement in the tensile
strength through 9% alkaline fiber
treatment for 1 h
Improved flexural strength, tensile
strength, tensile modulus, and
elongation
20, 12, 132, and 131% increases in
the flexural strength, flexural
modulus, tensile strength, and
tensile modulus through 1%
alkaline treatment, respectively
2% and 4% increases in the flexural
and tensile strength through 6%
silane treatment, respectively
136% and 49% enhancement in the
tensile and impact strength,
respectively
45% and 25% increases in the
storage modulus and tand (the ratio
of viscous to elastic properties)
through 5% silane fiber treatment,
respectively
Improved storage modulus
6% and 3% increases in the tensile
and flexural strength through 2.5%
silane treatment, respectively
Improved tensile strength
Improved tensile strength, modulus,
tear strength, fatigue life, and
hardness
Silane-treated fiber composite had a
higher flexural modulus than
alkaline-treated composite and
similar to glass fiber composite
18% improvement in the tensile and
flexural strength
859
J Mater Sci (2020) 55:829–892
Table 16 continued
Reference
Treatment
Composite
Apparent
fiber
diameter
(lm)
Manufacturing
method
Observation
Bledzki et al.
[80]
Joseph et al.
[188]
Fiber acetylation
treatment
Random-oriented flax fiber/
polypropylene composite
Random-oriented abaca
fiber/phenolic composite
100a
Injection
molding
Compression
molding
Up to 35% increases in the tensile
and flexural strength
81, 700, and 8% enhancement in the
tensile strength, Young’s modulus,
and impact strength through 50%
acetylation treatment, respectively.
Higher improvement in the tensile
strength, Young’s modulus, flexural
strength, and impact strength of the
composite by acetylation treatment
than those by silane treatment
Improved stress transfer efficiency at
the interface of fiber and resin by
determining the optimum treatment
time
Decreases in the flexural properties
240a
Hill and Abdul
Khalil [189]
Random-oriented coir and
oil palm fiber/polyester
composite
Coir:
336a
Oil Palm:
408a
Compression
molding
Zafeiropoulos
et al. [98, 190]
Flax fiber/polypropylene
composite
20
Compression
molding
Luz et al. [191]
Random-oriented bagasse
fiber/polypropylene
composite
Sisal fiber/polystyrene
composite
10
Injection
molding
–
Compression
molding
91% improvement in the tensile
strength
–
Manikandan
et al. [192]
Flax fiber/low-density
polyethylene composite
Random-oriented sisal fiber/
polystyrene composite
6% enhancement in the tensile
strength
Improved storage modulus and
damping properties
Joseph et al. [86] Fiber peroxide
treatment
Sreekala et al.
[95]
Sapieha et al.
[193]
Sisal fiber/polyethylene
composite
Random-oriented oil palm
fiber/phenolic composite
Cellulose fiber/polyethylene
composite
–
Compression
molding
Combined
compression/
extrusion
molding
Compression
molding
Compression
molding
Compression
molding
Abdul Razak
et al. [194]
Hong et al. [91]
Kenaf fiber/polylactic acid
composite
Random-oriented jute fiber/
polypropylene composite
–
Manikandan
et al. [84]
Fiber
benzoylation
treatment
Wang et al. [85]
Yang et al. [195]
Fiber treated by
a maleated
coupling agent
Random-oriented rice husk
fiber/polypropylene
composite
–
150–500a
–
45a
209a
Compression
molding
Compression
molding
Injection
molding
Improved tensile strength
Improved impact resistance and
flexural and tensile properties
Dicumyl peroxide-treated
composites exhibited superior
mechanical properties than
treatment benzoyl peroxide-treated
composites
Improved tensile and flexural
strength
Improvements in the Young’s
modulus and dynamic storage
modulus through maleic anhydride
treatment
Improved tensile strength through
the positive influence of maleated
polypropylenes on the interfacial
bonding
860
J Mater Sci (2020) 55:829–892
Table 16 continued
Reference
Treatment
Li et al. [196]
Keener et al.
[197]
Liu et al. [198]
Gassan and
Bledzki [199]
Wielage et al.
[200]
Li et al. [93]
Fiber bleaching
Misra et al. [201]
Vilay et al. [94]
Sreekala et al.
[202]
Joseph et al.
[203]
Paul et al. [204]
Kalaprasad et al.
[205]
Torres et al.
[206]
Li et al. [100]
Pickering et al.
[102]
Fiber acrylation
and
acrylonitrile
grafting
Composite
Apparent
fiber
diameter
(lm)
Manufacturing
method
Observation
Random-oriented
hildegardia fiber/
polypropylene composite
Random-oriented jute and
flax fiber/polypropylene
composite
Random-oriented abaca
fiber/high-density
polyethylene composite
Woven tossa jute fiber/
polypropylene composite
–
Injection
molding
Improved tensile strength through
maleated polypropylenes
10–20
Injection
molding
60% increase in the tensile and
flexural strength
–
Injection
molding
–
Compression
molding
Random-oriented flax and
hemp fiber/polypropylene
composite
Random-oriented flax fiber/
polyethylene composite
Random-oriented sisal fiber/
polyester composite
–
Injection
molding
–
Injection
molding
Hand lay-up
Improvement in the moduli and
strength through maleic anhydridegrafted styrene treatment
Improved dynamic modulus and
specific damping capacity through
maleic polypropylene anhydride
treatment
Improved dynamic mechanical
properties through maleic acid
anhydride-grafted treatment
Improved tensile strength through
sodium chlorite treatment
Improved tensile, flexural, and
impact properties through sodium
chlorite treatment
Superior tensile and flexural
properties for acrylation-treated
composite than those with alkaline
treatment
Improved tensile strength, Young’s
modulus, and elongation at break
of the acrylic acid-treated fibers
Improved tensile properties
50–240
Random-oriented bagasse
fiber/polyester composite
10
Vacuum
bagging
Random-oriented oil palm
fiber/phenolic composite
–
Compression
molding
100–300
Injection
molding
Compression
molding
Compression
molding
Improved tensile and flexural
strength
Improved tensile strength and
Young’s modulus
Compression
molding
Injection
molding
23% increase in the shear strength
through 3% stearic acid treatment
Improved tensile strength through
potassium permanganate treatment
Injection
molding
22% increase in the tensile strength
through fungal treatment, 32%
increase in the tensile strength
through combined fungal/alkaline
treatment
Fiber isocyanate Random-oriented sisal fiber/
treatment
polyethylene composite
Fiber stearic acid Banana fiber/polypropylene
treatment
composite
Random-oriented sisal fiber/
low-density polyethylene
composite
Sisal fiber/polyethylene
composite
Random-oriented flax fiber/
Fiber
high-density polyethylene
permanganate
composite
treatment
Fiber fungal
Random-oriented hemp
treatment
fiber/polypropylene
composite
125–150a
100–300
223
–
20
861
J Mater Sci (2020) 55:829–892
Table 16 continued
Reference
Treatment
Chabba and
Resin treatment
Netravali [103]
by
glutaraldehyde
and vinyl
alcohol
a
Composite
Apparent
fiber
diameter
(lm)
Manufacturing
method
Observation
Unidirectional flax fiber/soy
protein composite
–
Compression
molding
Glutaraldehyde improved
mechanical properties and vinyl
alcohol improved the toughness
Diameter of a fiber bundle
proposed a fiber area correction factor to accurately
predict the elastic modulus and tensile strength of the
composite. Kalaprasad et al. [220] conducted a comparison between prediction of tensile properties of
random-oriented sisal fiber/low-density polyethylene composite manufactured by injection
80
Axial Stress (MPa)
Amorphous Polymer
60
40
0% Fiber
3% Fiber
5% Fiber
10% Fiber
20% Fiber
20
0
0
0.02
0.04
0.06
0.08
Axial Strain
(a)
80
Axial Stress (MPa)
Crystallized Polymer
60
40
0% Fiber
3% Fiber
5% Fiber
10% Fiber
20% Fiber
20
0
0
0.02
0.04
0.06
0.08
Axial Strain
(b)
Figure 23 Axial tensile stress–strain curves of pulp fiber/
polylactic acid composites under a amorphous and
b crystallized states.
molding technique obtained by series and parallel,
Hirsch, shear lag, Halpin–Tsai, and Bowyer–Bader
models and reported that all models except the series-and-parallel model showed good agreement with
experimental tensile strength and modulus of longitudinally oriented composites. They also reported
that Hirsch and Bowyer–Bader models had a good
agreement with experimental results of randomly
oriented composites. Facca et al. [221] predicted the
Young’s modulus of random-oriented hemp fiber/
HDPE composite manufactured by compression
molding technique using rules of mixture, Halpin–
Tsai equation, and shear lag model. Their results
showed that Young’ modulus predicted by Halpin–
Tsai equation had a higher accuracy than that by
other models because the efficiency factor in the
Halpin–Tsai equation is normally derived from
experimental measurements of the material to be
modeled. Beckermann and Pickering [222] predicted
the tensile strength of hemp fiber/polypropylene
composite manufactured by extrusion and injection
molding techniques by Bowyer–Bader model and
reported that fiber aspect ratio and orientation are
two main parameters that should be considered in
the prediction of the mechanical behavior of natural
fiber composites. Migneault et al. [223] predicted
tensile elastic modulus of random-oriented pulp
fiber/HDPE composite manufactured by injection
molding technique using shear lag model and Halpin–Tsai equation and reported that the predictions
considering an orientation factor in the modeling had
a good agreement with the experimental results.
Munde and Ingle [224] compared the modeling
results of Hirsch model, Halpin–Tsai equation, and
Bowyer–Bader model with the experimental results
of the tensile strength and elastic modulus of
862
J Mater Sci (2020) 55:829–892
Figure 24 Optical light
micrographs of hemp fiber/
polylactic acid composite with
a untreated fiber and
b alkaline-treated fiber [216].
50 μm
50 μm
(a)
random-oriented coir fiber/polypropylene composite
manufactured by compression molding technique.
They found that Hirsch model developed a higher
accuracy in prediction of the tensile strength and
modulus of the composite than Halpin–Tsai and
Bowyer–Bader models.
Finite element simulation
Finite element (FE) method has been used to model
the properties of natural fiber composites, because it
is possible to efficiently study the effect of different
parameters, such as fiber reinforcement type, fiber
volume fraction, fiber aspect ratio, and fiber orientation, on the properties of natural fiber composites
[225]. Among the FE methods, representative volume
element (RVE) FE, as the most popular homogenization-based multi-scale method and most effective
and accurate method for mechanical properties of
composite materials, has been considered for prediction of the mechanical properties of composites.
Two RVE methods of direct RVE and orientation
averaging methods have been used to simulate natural fiber composites with discontinuous fibers. In
direct RVE method, matrix surrounds a number of
natural fibers, whereas in the orientation averaging
method, a single fiber is embedded in the matrix (as a
block unit cell (UC)). A few studies predicted the
mechanical properties of natural fiber composites
with discontinuous fibers by FE method. Modniks
and Andersons [226] used RVE orientation averaging
method to predict the Young’s modulus of randomoriented flax fiber/polypropylene composite manufactured by extrusion molding technique (shown in
Fig. 25). They assumed that the UC is transversely
isotropic and determined the elastic constants of the
composite from five loading nodes. They showed that
(b)
the modeling predictions were in a good agreement
with the experimental results. Modniks and Andersons [227] also used RVE orientation averaging
method for prediction of the nonlinear deformation
of the random-oriented flax fiber/polypropylene
composite (shown in Fig. 26). They first conducted
the simulation under six load conditions of transverse tension, transverse compression, pure tension,
axial tension, shear tension, and equi-biaxial tension
to determine deformation parameters, and then they
input the parameters into the RVE orientation averaging method for prediction of the nonlinear deformation of the composite. They reported a good
agreement between the stress–strain curve predictions and experimental results. Kern et al. [225]
developed a direct 3D RVE model for investigating
the influence of fibers and micro-cellular voids on the
tensile behavior of random-oriented wheat straw
fiber/polypropylene composite (shown in Fig. 27).
They found that the composite with voids had a
shorter fracture path than that with no void. Their
predictions were in a good agreement with the
experimental results. Sliseris et al. [228] developed a
direct 3D RVE model for simulation of the tensile
behavior of random-oriented flax fiber/polypropylene composite considering the influence of the fiber
aspect ratio, bundles, and defects. Flax fiber was
modeled as a linear isotropic elastic material and
polypropylene resin was modeled as a nonlinear
plastic material. To consider the influence of the fiber
defect in the modeling, they modeled the fibers and
the interface between the fiber bundles as a brittle
material with a continuum damage mechanic model
(shown in Fig. 28). They found that plastic deformation of the composite started at the fiber endings and
around fiber defects, which was in agreement with
the experimental results.
863
J Mater Sci (2020) 55:829–892
Figure 25 Schematic of a the
unit cell (comprising a fiber
embedded in a block of
matrix) and b loading nodes of
the unit cell for prediction of
the Young’s modulus by
representative volume element
(RVE) orientation averaging
method [226].
Longitudinal modulus
Transverse modulus
Shear modulus
Shear modulus
Orthogonal cross-secons
Poisson’s rao
(a)
Figure 26 Schematic of the unit cell for prediction of the
nonlinear deformation of a random-oriented fiber composite [227].
Durability properties
The low durability of natural fiber composites results
in a decrease in the mechanical properties and the
growth of fungus and bacteria in the composite [229].
Several studies have been conducted to investigate
the key durability-related properties of water
Figure 27 Meshing of
arbitrarily distributed wheat
straw fibers embedded in
a solid polypropylene and
b polypropylene with microcellular voids in a direct 3D
representative volume element
(RVE) model [225].
(b)
absorption, wear resistance, and weathering in natural fiber composites. Table 17 presents a summary
of the existing studies on the durability properties of
natural fiber composites [230–240].
All natural fibers are hydrophilic in nature, and
their saturation moisture content ranges from 3 to
13% [241]. The water absorption of the natural fiber in
natural fiber composites is a serious concern for their
potential outdoor application [241]. The absorption of
moisture in natural fibers results in the swelling of
fibers and a reduction of the adhesion between fibers
and matrix, dimensional instability, matrix cracking,
and weak mechanical properties of the composites
due to debonding between the fibers and the matrix
[139, 242]. The water absorption property of natural
fiber composites depends on the fiber volume
864
J Mater Sci (2020) 55:829–892
Figure 28 Modeling of the
failure mechanism considering
the damage at fiber defects
using 3D representative
volume element (RVE) model
for simulation of the tensile
behavior of random-oriented
flax fiber/polypropylene
composite [228].
fraction, fiber orientation, temperature, exposed surface area, permeability of the fibers, void content, and
hydrophilicity of the composite components. The
water absorption increases with increasing natural
fiber volume fraction of composite. All three dimensions of composites increase by the fiber water
absorption. Pressurization at low fiber volume fractions can improve the water absorption of natural
fiber composites. Chemical treatment of fiber surfaces
can also remove the hydrophilic OH bonds [85, 243].
Symington et al. [244] found that the mechanical
properties of flax and coir fiber composites decreased
more than those of kenaf, jute, and abaca fiber composites under fully soaked conditions. They also
showed that abaca and kenaf fibers had the highest
capacity to absorb moisture (164% and 150% by
weight, respectively), which resulted in severe damage to the structure of the fibers. Wear resistance
refers to the ability of the composite materials to
withstand scraping, skidding, rubbing, and sliding
objects on its surface [233]. The extended exposure of
natural fiber composites to sunlight and UV radiation
results in physical and mechanical weakening of the
composite [237].
Time-dependent properties
Creep
The extended exposure of natural fiber composites to
stress results in a decrease in the strength and elastic
modulus of composites [1]. The damage mechanisms
in composites due to creep result in fiber fracture,
fiber pull-out, matrix yielding, interfacial debonding,
and crack coalescence. An imperfect interface leads to
repeated frictional sliding in cyclic loading under
fatigue conditions. Three important parameters that
can affect the creep behavior of natural fiber composites are fiber loading, temperature, and styrene
concentration [245]. A number of studies have
assessed the effect of the creep on natural fiber
composites. Alvarez et al. [246] investigated the
influence of fiber content on the creep behavior of
random-oriented sisal fiber/starch composites manufactured by injection molding technique through
short-term flexural creep tests and found improved
creep resistance by an increased fiber content in the
matrix. Acha et al. [247] found the same result for a
woven jute fiber/polypropylene composite. Xu et al.
[248] showed that a random-oriented bagasse fiber/
PVC composite had better creep resistance than
random-oriented bagasse fiber/HDPE composites
manufactured by extrusion molding technique at low
temperatures. Amiri et al. [249] presented a time–
temperature superposition principal to generate a
creep compliance master curve for a flax fiber/vinylester composite. Yang et al. [250] reported that the
increase in banana fiber content up to 60% by weight
in a PLA matrix in the random-oriented direction
resulted in the highest creep resistance. The influence
of fiber treatment on the creep behavior of woven jute
fiber/epoxy composite manufactured by hand lay-up
method was investigated by Militký and Jabbar [251]
through a three-point bending test. They showed that
a plasma-treated fiber composite exhibited a lower
creep strain than an untreated fiber composite.
865
J Mater Sci (2020) 55:829–892
Table 17 Summary of existing studies on the durability properties of natural fiber composites
Reference
Composite
Masoodi and Woven jute fiber/epoxy
Pillai [27]
composite
Apolinario
Unidirectional flax fiber/
et al. [230]
polyester composite
Assarar et al. Unidirectional flax fiber/
[231]
epoxy composite
Apparent
fiber
diameter
(lm)
Manufacturing
method
Observation
103a
Compression
molding
Infusion
molding
Hand lay-up
26% increase in the water absorption of the composite by
incorporating 40% by weight jute fiber
9% decrease in the water uptake of 1% silane-treated fiber
composite under saturation conditions
Water aging considerably decreased the elastic modulus (i.e.,
40%) and increased the elongation (i.e., 61%) of the
composite compared to the glass fiber composite
Composites made with non-dry fibers (with moisture content
of 10%) exhibited lower moisture absorption, degree of
swelling, and shrinking than those made with dry fibers
(dried in an oven at 80 °C)
The use of fibers with length of 1–1.5 mm and diameter of
350 lm significantly reduced the specific wear rate (i.e.,
by about three to four times) of the neat polyester resin
Longer fibers (* 5 mm) had higher wear resistance because
less pulling-out of fibers occurs at that length. When the
ends of the fibers in the matrix were perpendicular to the
counterface and the anti-sliding direction (i.e., anti-parallel
orientation), the composites exhibited higher wear
resistance as compared to other directions
Coir fiber composite had higher wear resistance than
sugarcane and oil palm fiber composites
Presence of kenaf fibers oriented in the normal direction
enhanced the wear performance of the resin by * 85%
15
–
Lu and
Vuure
[232]
Unidirectional flax fiber/
polyester composite
10
Vacuumassisted resin
infusion
Yousif and
El-Tayeb
[233]
El-Tayeb
[234]
Random-oriented oil
palm fiber/polyester
composite
Random-oriented
sugarcane fiber/
polyester composite
350
Hand lay-up
1200–2700a
Hand lay-up
Yousif [235]
Coir fiber/polyester
composite
Unidirectional kenaf
fiber/epoxy composite
215–304a
Compression
molding
Vacuum
bagging
Random-oriented palm
leaf fiber/polypropylene
composite
–
Injection
molding
Singh and
Gupta
[238]
Unidirectional jute fiber/
phenolic composite
–
Pultrusion
technique
Dash et al.
[239]
Joseph et al.
[240]
Unidirectional jute fiber/
polyester composite
Random-oriented banana
fiber/phenolic
composite
–
Compression
molding
Compression
molding
Chin and
Yousif
[236]
Abu-Sharkh
and Hamid
[237]
a
250–400a
–
Reinforcement of polypropylene composite with palm leaf
fiber increased the weathering stability of the composite as
compared to composite without fiber, owing to the natural
antioxidant properties of lignin, which increased the
protection of the composite from UV radiation
Exposing the composite without any treatment to weathering
led to color fading, fibrillation, black spots, bulging, and
resin cracking, as well as a 22% reduction in the flexural
strength of the composite after 2 years
Bleaching reduced the effects of weathering
Alkalization and silane fiber modifications slowed the effects
of weathering
Diameter of a fiber bundle
Shrinkage
Rearranging and reorienting of resin molecules of
composites in the liquid phase result in shrinkage [1].
The shrinkage of natural fibers generates internal
stresses at the fiber/matrix interface and causes
damage and degradation of the initial properties of
the composite. The influential parameters on the
866
Other properties
Thermal properties
Fire resistance
The flammability behavior of natural fiber composites has become one of the most important issues in
recent years because of the use of natural fiber composites in different applications [1]. One of the
weaknesses of natural fiber composites is the loss of
strength and stiffness at high temperatures [257].
Thermal decomposition and combustion occur when
natural fiber composites are exposed to fire or any
other high-intensity heat sources [4]. The flammability of natural fiber composites is variable for different
types of natural fibers because they have different
chemical compositions and microstructures. By
decreasing the cellulose content of natural fiber,
increasing the crystallinity, and decreasing the polymerization of the composites, the fire resistance of the
3.5
Pure
5%, Untreated
5%, Treated
10%, Untreated
10%, Treated
Shrinkage (%)
3
2.5
2
1.5
1
0.5
0
0
30
60
90
120
150
180
150
180
Time (min)
(a)
6
Pure
5%, Untreated
5%, Treated
10%, Untreated
10%, Treated
5
Shrinkage (%)
volumetric shrinkage of composites are the density,
processing conditions, flow pattern, shape, residual
internal stress, fiber content, crystallinity, and nonuniform cooling rates [4]. The use of fibers in the
longitudinal direction, an increase in the fiber content, and a decrease in the residual internal stress,
non-uniform cooling rates, and the crystallinity of the
composites all decrease the shrinkage of natural fiber
composites [252–254]. Figure 29 shows the shrinkage
behavior of a random-oriented coir fiber/PLA composite manufactured by compression molding technique over time at 80 °C for the longitudinal and
transverse directions of the composite, in which
fibers were subjected to plasma treatment with 0%,
5%, and 10% fiber content by weight in the composite
[173]. A limited number of studies investigated the
shrinkage behavior of natural fiber composites. Santos et al. [255] reported that the shrinkage of a random-oriented bamboo fiber/HDPE composite
manufactured by injection molding technique was
reduced up to 58% by an increased fiber content in
the composite. Tan et al. [256] assessed the shrinkage
behavior of random-oriented coir fiber/polypropylene composites manufactured by injection molding
technique and found a 13% reduction in the volumetric shrinkage of the composite with a 40%
increase in the fiber content by weight.
J Mater Sci (2020) 55:829–892
4
3
2
1
0
0
30
60
90
120
Time (min)
(b)
Figure 29 Shrinkage behavior of random-oriented untreated and
plasma-treated-coir fiber/polylactic acid composite manufactured
by compression molding technique at 80 °C over time for
a longitudinal and b transverse directions of the composite.
natural fiber composites can be improved [257]. The
use of coatings and additives, such as ceramic, intumescent, silicone, phenolic, ablative, and glass mats,
also improves the fire resistance of natural fiber
composites. Heating intumescent materials beyond a
specific temperature results in the formation of the
cellular and charred surface by initiation of foaming
and expanding, which in turn can protect the
underlying surface from heat and flame. By adding
talc and nanoparticles as fillers to the natural fiber
composites, the fire resistance of the composites
increases by the formation of heat barriers. Several
researchers have studied the behavior of natural fiber
composites at high temperatures. Some studies
reported that natural fibers generally start to degrade
at approximately 240 °C [31, 258–260]. However,
lignin starts to degrade at approximately 200 °C and
hemicellulose and cellulose start to degrade at higher
temperatures [261]. Table 18 presents the decomposition temperature of major natural fibers, and
867
Table 18 Decomposition temperature of major natural fibers
[259, 262]
Fiber
Decomposition temperature (°C)
Bagasse
Bamboo
Cotton
Hemp
Jute
Kenaf
Rice straw
Rice husk
Pina
232
224
232
215
215
229
238
234
240
Table 19 presents glass transition and melting point
of major resins used in natural fiber composites.
Thermal conductivity
The use of heat-insulating materials is an appropriate
approach for reducing energy costs and improving
manufacturing efficiency in industries such as automotive, construction, and packaging [1]. Natural
fibers contain lumens, hollow portions filled with air.
As a result, the thermal conductivity of composites
decreases with an increased fiber content. By an
increased natural fiber volume fraction of the composites, the amount of air contained in the composite
increases, which results in heat insulation. Several
studies have been performed to investigate the thermal conductivity of natural fiber composites. Pujari
et al. [264] studied the thermal conductivity of random-oriented jute and banana fibers/epoxy
Table 19 Glass transition
temperature and melting point
of major resins [243, 263]
Thermal conductivity (w/m°k)
J Mater Sci (2020) 55:829–892
0.4
0.363
0.3
0.251 0.243 0.239
0.230 0.231
0.228
0.2
0.1
0
Epoxy
resin
Jute
30%
Banana
30%
Jute
40%
Banana
40%
Jute
60%
Banana
60%
Figure 30 Thermal conductivity of epoxy resin, and jute and
banana fiber/epoxy composites with different fiber contents.
composites manufactured by hand lay-up method
and reported that the thermal conductivities of the
jute and banana fiber composites were 0.231 and
0.228 W/m K in the - 20 °C to 300 °C range and at
the maximum fiber volume fraction, respectively.
Figure 30 shows the thermal conductivity of natural
fiber composites with different fiber contents [264].
Paul et al. [204] reported that an increase in fiber
loading decreased the thermal conductivity of a
random-oriented banana fiber/polypropylene composite manufactured by compression molding technique. The thermal conductivity of a randomoriented flax fiber/HDPE composite manufactured
by extrusion molding technique was assessed by Li
et al. [265]. They found that the thermal conductivity
of the composite reduced with increasing fiber content. Mangal et al. [266], Osugi et al. [267], Mounica
et al. [268], and Ramanaiah et al. [269] found the same
result for random-oriented pina fiber/phenolic
manufactured by compression molding technique,
unidirectional hemp fiber/epoxy manufactured by
Resin
Glass transition temperature (°C)
Melting point (°C)
Polypropylene
Low-density Polyethylene
High-density Polyethylene
Polystyrene
Nylon 6
Nylon 6,6
Polyester
Vinylester
Epoxy
Phenolic
Starch
PLA
PHA
0.9–1.55
- 120
- 80
100–135
40
50
60
104–121
70–167
170
60
58
2–15
160–176
105–116
120–140
–
222
250–269
250–300
–
–
–
110–115
150–162
160–175
868
J Mater Sci (2020) 55:829–892
Thermal analysis
Polylactic acid
Sisal fiber
1% Fiber/Composite
2% Fiber/Composite
3% Fiber/Composite
(a)
Polypropylene
Sisal fiber
1% Fiber/Composite
2% Fiber/Composite
3% Fiber/Composite
(b)
Figure 31 TGA curves of sisal fiber composites with a polylactic
acid and b polypropylene resin [272].
compression molding technique, unidirectional
bamboo fiber/polyester manufactured by hand layup method, and unidirectional fish tail palm fiber/
polyester composites manufactured by hand lay-up
method, respectively. Agarwal et al. [270] assessed
the influence of fiber treatment on the thermal conductivity of an oil palm fiber/phenolic composite.
They found that alkali and silane treatments
increased the thermal conductivity of the composite
more than acetylation treatment, meaning that
acetylation treatment was more suitable than alkali
and silane treatments for improving the heat-insulating characteristics of the composite.
Thermogravimetry analysis (TGA), differential scanning calorimetry (DSC), and differential thermal
analysis (DTA) were used to analyze the thermal
behavior of natural fiber composites. Feng et al. [271]
assessed the thermal behavior of random-oriented
kenaf fiber/polypropylene composite manufactured
by injection molding technique through DSC and
reported that maleated coupling agent reduced the
melting temperature of the composite. Joseph et al.
[87] used TGA and DSC to study the thermal
behavior of random-oriented sisal fiber/polypropylene composite. They found that incorporation of
sisal fiber with fiber weight fraction of 20% treated by
maleic anhydride-modified polypropylene, urethane
derivative of polypropylene glycol, and KMnO4,
respectively, caused 12%, 11%, and 9% increase in the
crystallization temperature of the composite compared to that with no fiber, which was because the
surface of the treated fibers acted as nucleating sites
for the crystallization of the polymer. Mofokeng et al.
[272] assessed the thermal stability of random-oriented sisal fiber/PLA and sisal fiber/polypropylene
composites manufactured by injection molding technique through TGA and their crystallization through
DSC. Figure 31 shows the TGA curves of composites
with fiber weight content of 1, 2, and 3%. They found
that the thermal stability of both polymers increased
with an increased fiber content, with a more significant improvement in the case of polypropylene. Figure 32 shows the DSC curves of composites. The DSC
results in the figure illustrated a significant influence
of the fibers on the cold crystallization and melting
behavior of PLA. They also reported that fiber content had a negligible influence on the melting characteristics of the polypropylene, but it had a
significant influence on the non-isothermal crystallization temperature range. In 2013, Azwa and Yousif
[273] studied the thermal stability of random-oriented untreated and 6% alkaline-treated kenaf fiber/
epoxy composite. Figure 33 shows TGA and DTA
curves presented in that study [273]. They reported
that the addition of the kenaf fiber into the epoxy
slightly improved both the charring and the thermal
stability of the composite. They also reported that
untreated kenaf fiber/epoxy composite started to lose
weight earlier compared to the other composites,
which was because of the higher moisture content of
the untreated fibers.
869
J Mater Sci (2020) 55:829–892
Polylactic acid
1% Fiber/Composite
2% Fiber/Composite
3% Fiber/Composite
Viscoelastic properties
Melting temperature
Glass transition temperature
(a)
Polypropylene
1% Fiber/Composite
2% Fiber/Composite
3% Fiber/Composite
Melting temperature
(b)
Crystallization temperature
Polypropylene
1% Fiber/Composite
2% Fiber/Composite
3% Fiber/Composite
(c)
Figure 32 DSC curves of sisal fiber composites with a polylactic
acid (heating curves), b polypropylene (heating curves), and
c polypropylene (cooling curves) resin [272].
Polymers are normally viscoelastic fluids that behave
as a viscous or elastic material depending on the rate
of flow or deformation [274]. Dynamic mechanical
analysis has been widely utilized to investigate the
viscoelastic properties (storage modulus, loss modulus, and damping factor) of composites [11]. Dynamic
mechanical analysis is a technique that a small
deformation is cyclically applied to a sample under
different temperatures, and the elastic and viscous
response of the sample is then monitored. In this
technique, a sinusoidal force is applied to the sample
at a set frequency and the stiffness and damping of
the sample are measured [275]. The in-phase component, out-of-phase component, and the ratio of the
loss of the storage are defined as storage modulus,
loss modulus, and damping factor, respectively.
Pothan et al. [275] investigated the influence of the
fiber content on the dynamic mechanical properties
of random-oriented banana fiber/polyester composite manufactured by compression molding technique.
They showed that an increase in the fiber volume
fraction up to 40% led to a decrease in the storage
modulus of the composite below the glass transition
temperature, but an increase in the storage modulus
above the glass transition temperature. They also
reported that, at 40% fiber content, the peak loss
modulus got broadened and peak damping factor
lowered, indicating the improved fiber/matrix
adhesion. Idicula et al. [276] assessed the viscoelastic
properties of random-oriented banana/sisal hybrid
fiber/polyester composite manufactured by compression molding technique and reported that the
storage modulus of the composite enhanced above
the glass transition temperature of the polymer with
an increase in the fiber volume fraction up to 40% and
then decreased. Pan et al. [277] studied the storage
modulus of random-oriented kenaf fiber/PLA composite manufactured by melt mixing technique and
reported that the storage modulus increased by 28%
by reinforcement of the composite with 30% kenaf
fiber by weight. Mofokeng et al. [272] investigated the
viscoelastic properties of random-oriented sisal
fiber/PLA and sisal fiber/polypropylene composites
manufactured by injection molding technique
through dynamic mechanical analysis. Figures 34
and 35 show the storage modulus (E0 ), loss modulus
(E00 ), and damping factor (tand) curves of composites
with PLA and polypropylene, respectively [272].
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Figure 33 a TGA and b DTA
curves of random-oriented
untreated and 6% alkalinetreated kenaf fiber/epoxy
composites [273].
They reported that the storage and loss modulus of
the PLA decreased with an increase in the fiber
content (shown in Fig. 34a, b). They also reported
that the cold crystallization of the PLA was around
110 °C and the existence of the fibers in the composite
led to an increase in the modulus between the cold
crystallization and melting of the polymer (shown in
Fig. 34b). The larger area under the a-relaxation peak
in Fig. 34c indicates that the molecular chains
exhibited a higher degree of mobility, therefore better
damping properties. The presence of fibers increased
the modulus and glass transition temperature of
polypropylene composite (shown in Fig. 35a, b),
which was because of the restriction in the segmental
motion. The existence of fibers had no impact on the
a-relaxation of the composite (shown in Fig. 35c),
which was related to the crystalline fraction of the
polypropylene. Mylsamy and Rajendran [278] performed dynamic mechanical analysis on untreated
and alkaline-treated unidirectional agave fiber/
epoxy composite manufactured by compression
molding technique and reported that the composite
with poor interfacial bonding tended to dissipate
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J Mater Sci (2020) 55:829–892
more energy compared to those with good interfacial
bonding.
Glass transition
temperature
Morphology
α-relaxation
region
Cold crystallization
region
Polylactic acid
1% Fiber/Composite
2% Fiber/Composite
3% Fiber/Composite
(a)
Glass transition
temperature
Polylactic acid
1% Fiber/Composite
2% Fiber/Composite
3% Fiber/Composite
(b)
Polylactic acid
1% Fiber/Composite
2% Fiber/Composite
3% Fiber/Composite
α-relaxation
peak
Cold crystallization
region
(c)
Figure 34 Dynamic mechanical analysis of random-oriented sisal
fiber/polylactic acid composite manufactured by injection molding
technique: a storage modulus, b loss modulus, c damping factor
[272].
Optical microscopy, SEM, and atomic force microscopy (AFM) are three main methods for morphological studies of composites. Optical microscopy is
ideal for general inspection purposes, and SEM provides detailed topographical and compositional
information of the composite by high magnification
view at micro-scale [277]. On the other hand, AFM is
a useful method to determine the surface roughness
of fibers at nanoscale by measuring the interatomic
forces between the sample surface and the measurement tip [10]. A large number of studies investigated
the morphology of natural fiber composites (e.g.,
[275, 277, 279]). Some of these studies are presented
in this review paper.
Pothan et al. [275] assessed the influence of the
fiber content on the morphology of random-oriented
banana fiber/polyester composite manufactured by
compression molding technique through SEM. Figure 36a–c shows SEM micrographs of composites
with fiber volume fractions of 10, 20, and 40%,
respectively. They reported that there was a good
fiber/matrix bonding in the composite with 40% fiber
content, whereas fiber/matrix debonding was evident in composites with 10% and 20% fiber content.
They also showed that there was no gap between the
fibers and matrix in 40% fiber composite owing to the
strong bonding. However, when the fiber concentration was lower, the packing of the fibers was not
efficient, leading to an easier failure of the bonding at
the interfacial region.
Pan et al. [277] studied the morphology of randomoriented kenaf fiber/PLA composite manufactured
by melt mixing technique using SEM. Figure 37
shows the SEM micrographs of kenaf fiber [277]. It
can be seen in the figure that the diameter of the
fibers ranged from 5 to 50 lm and their surface was
rough with the adhesion of some smaller fibers. This
was attributed to different sources and processing
history of the kenaf fibers. Figure 38 shows the SEM
micrographs of pure PLA and composite with 30%
kenaf fiber by weight [277]. The fractured surface of
the pure PLA was irregular and rough because of the
brittle behavior of the resin. It is also shown in the
figure that most fibers (indicated by ‘‘A’’) were tight
connected to the matrix, and a lot of fibers were
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Polypropylene
1% Fiber/Composite
2% Fiber/Composite
3% Fiber/Composite
(a)
Glass transition
temperature
Polypropylene
1% Fiber/Composite
2% Fiber/Composite
3% Fiber/Composite
(b)
Polypropylene
1% Fiber/Composite
2% Fiber/Composite
3% Fiber/Composite
α-relaxation
Glass transition
temperature
(c)
Figure 35 Dynamic mechanical analysis of random-oriented sisal
fiber/polypropylene composite manufactured by injection molding
technique: a storage modulus, b loss modulus, c damping factor
[272].
broken during the tensile testing. It can be seen in
Fig. 38 that there are some fibers in the composite
(indicated by ‘‘B’’) separated from the matrix during
the tensile deformation, indicating that the interface
interaction between the matrix and fibers further
improved. Aggregation of kenaf fibers (indicated by
‘‘C’’) and the presence of a small amount of air bubbles (indicated by ‘‘D’’) are also visible in the figure,
which was because of the rough surface of kenaf fiber
and high viscosity of the PLA.
Hebel et al. [279] investigated the morphology of
unidirectional bamboo fiber/epoxy composite manufactured by compression molding technique produced by a combination of heat and pressure
treatment through optical microscopy. Figure 39a–c
shows the optical micrographs of the fractured surface of the autoclave/compression molded composites produced at 100 °C with pressures of 15, 20, and
25 MPa [279]. It should be noted that the composite
produced by pressure of 20 MPa exhibited a higher
tensile strength than that by 15 and 25 MPa. They
reported that the surface of the composite at 20 MPa
pressure was rather smooth and homogenously
covered by the resin. At this pressure, the polymer
network formed a very thin layer that efficiently
interacted with the surface of the fiber and penetrated
into the fibers due to its viscosity. They also reported
that reducing the pressure to 15 MPa resulted in the
formation of large resin beads on the fiber surface,
which led to the reduction in the wetting and
homogenous coverage of the fibers. At high pressures
of 25 MPa, the epoxy formed larger crystal-like beads
than those at 15 MPa and infiltration inhibited by a
propagated carbonization of the fibers (as shown by
the darker color strip).
Lee et al. [280] used AFM to characterize the kraft
fiber treated by Trichoderma reesei enzyme. Figure 40 shows tapping mode AFM images of treated
kraft fiber and its cellulose aggregate fibrils. The
cellulose microfibril bundles are evident in Fig. 40a,
which can be a potential source for cellulose aggregate fibrils. Several structural imperfections such as
kinks and twists are evident in Fig. 40b that were
because of the conformability of fibers in the treatment process by the removal of the constraining
primary layer of the kraft fiber. George et al. [281]
used AFM to investigate the influence of the fiber
treatment on the nanostructure of the hemp fiber.
They reported that fiber treatment rendered the surface topography of the hemp fiber clean and exposed
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J Mater Sci (2020) 55:829–892
Figure 36 SEM micrographs
of random-oriented hemp
fiber/polyester composite with
fiber content of a 10%, b 20%,
and c 40% manufactured by
compression molding
technique [275].
Fiber/matrix
debonding
Fiber pull-out
Fiber/matrix
debonding
Matrix
cracking
(a)
(b)
(c)
Figure 37 SEM micrographs
of kenaf fiber with
magnification of a 9 100 and
b 9 500 in random-oriented
kenaf fiber/polylactic acid
composite manufactured by
melt mixing technique [277].
50μm
100μm
(a)
the individual fiber bundles. They also showed that
hemp fibers treated by 10% NaOH, xylanase enzyme,
and laccase enzyme had 250%, 218%, and 16% higher
surface adhesion forces than those with no treatment.
Spectroscopic characterization
Fourier transform infrared spectroscopy (FTIR) is an
effective technique to determine the chemical composition of the composite and functional groups
(b)
interacting within natural fibers, characterizing their
covalent bonding information [26]. Table 20 presents
the FTIR analysis peaks for different untreated natural fibers. Lu et al. [293] investigated the spectroscopic characteristics of the benzylated-treated sisal
fiber by FTIR and reported that peaks at 1250 cm-1
and 1363 cm-1 related to lignin and hemicellulose
partly removed by treatment of the sisal fiber.
Mofokeng et al. [272] used FTIR analysis to determine
the existence and type of interfacial interaction in
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Figure 38 SEM micrographs
of fractured surface of a pure
polylactic acid, and randomoriented kenaf fiber/polylactic
acid composite with
magnification of b 9 200,
c 9 500, and d 9 500
manufactured by melt mixing
technique [277].
D
C
50μm
(a)
100μm
(b)
A
B
50μm
(c)
random-oriented sisal fiber/PLA composite manufactured by injection molding technique (shown in
Fig. 41). Peaks at 1780–1680 cm-1, 3600–3000 cm-1,
and 1180 cm-1 corresponded to C=O, O–H, and C–
O–C stretches, respectively. They reported that the
O–H band of the composite became more pronounced and broader with an increase in the fiber
content, which was because of the existence of free
OH groups in the fiber. They also reported that a new
peak at 1650 cm-1 was created corresponded to OH,
which was oriented from bending of the unresolved
OH group of the absorbed water by cellulose. Alavudeen et al. [294] conducted FTIR analysis on the
alkaline- and sodium lauryl sulfate (SLS)-treated
random-oriented banana fiber/epoxy composite
manufactured by hand lay-up method. They reported
that fiber treatment removed most of the lignin and
hemicellulose components on the fibers, which
helped to improve the mechanical properties of the
composite. They also found that an increase in the
concentration of alkaline and SLS beyond 10%
50μm
(d)
damaged the fiber surface, leading to the poor fiber
adhesion and worse mechanical properties of the
composite. Faruk Hossen et al. [295] by FTIR analysis
on the propionic anhydride-treated random-oriented
jute fiber/polyethylene composite manufactured by
compression molding technique reported that the
band at 1641 cm-1 indicated absorbed water in
crystalline cellulose and the band disappeared upon
the fiber treatment. They also reported that the band
at 1512 cm-1 was because of the existence of the
aromatic rings in lignin.
Incorporation of nanoparticles in natural
fiber composites
Nanoparticles have been utilized as nanofiller materials to improve the properties of natural fiber composites [296]. Owing to the high surface area (higher
than 500 m2/g) and pore filling properties of
nanoparticles, the properties of natural fiber composites were enhanced by addition of a small amount
J Mater Sci (2020) 55:829–892
875
Figure 39 Optical
micrographs of the fracture
surface of
autoclave/compression molded
unidirectional bamboo fiber/
epoxy composite produced at
100 °C with pressure of
a 15 MPa, b 20 MPa, and
c 25 MPa [279].
(up to 5% by weight) of nanofillers in natural fiber
composites [297]. A few studies investigated the
properties of natural fiber composites containing
nanofillers. Table 21 presents a summary of the
existing studies of the influence of nanofillers on the
properties of natural fiber composites [298–302].
Application of natural fiber composites
Natural fiber composites have been utilized in the
automotive, construction, aircraft components, packaging, sporting equipment, electrical parts, and
biomedical industries owing to the eco-friendliness,
lightweight, good mechanical behavior, sound
attenuation, and vibration damping of them
[15, 17, 243, 303]. However, their low durability may
exclude them from being an alternative to glass fiberreinforced composites in wet areas such as in piping,
boats, and kayaks [243]. Henry Ford made the first
attempt to use natural fibers in the automotive
industry using flax and hemp fibers in 1941 [43].
Many automotive companies are now using natural
fiber composites made with polyester, polypropylene, flax, hemp, and sisal [304]. In structural applications, natural fiber composites have been utilized to
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J Mater Sci (2020) 55:829–892
Figure 40 Tapping mode
AFM images of a kraft fiber
treated by Trichoderma reesei
enzyme and b its cellulose
aggregate fibrils [280].
develop load-bearing elements, such as beam, roof,
wall, and pedestrian bridge [305, 306]. Jute fiber/
polypropylene composite has been used in primary
structural applications, such as indoor elements in
housing [1, 28], temporary low-cost outdoor housing
for defense and rehabilitation [29, 30], and transportation [307]. Yousif and Ku [308] investigated the
behavior of a liquid storage tank made with coir
fiber/polyester composite and reported that the
interfacial adhesion strength between coir fiber and
polyester resin soaked in water and diesel oil was
107 MPa. They suggested the potential application of
the coir fiber/polyester composite in design and
manufacture of the body of liquid storage tanks.
Natural fiber composites have also been used in
functional applications of protective medical apparels, such as baby diapers, feminine hygiene products,
and adult incontinence items, plasters, bike frames,
golf stick, tennis rackets, and helmets [309].
According to the reports by MarketsandMarkets
Inc. [310] and Grand View Research Inc. [311], the
natural fiber composite market was valued at $16.5
billion in 2016 and is projected to reach $46.3 billion
by 2025, because of the governmental regulations for
the use of environmentally friendly products in
industry. The global natural fiber composite market
share by the end-use industries in 2016 was 35%,
27%, 9%, 13%, and 16% for transportation, building
and construction, consumer goods, electrical and
electronics, and other applications, respectively [311].
Based on this report and recent reports by Dicker
et al. [243] and Gurunathan et al. [3], the automotive
and construction industries were the biggest market
for natural fiber composites in 2016–2017 to manufacture seat backs, headliners, truck liners, door
panels, decking, railing, window frames, dash
boards, and frames. Because of the rapid growth of
the use of natural fiber composites in the automotive
market, high-density glass fibers are being replaced
with low-density natural fiber composites; lightweight vehicles reduce the amount of the fuel
required, which in turn leads to a reduction of CO2
emission [3]. The study by Yang et al. [312] revealed
that a 25% reduction in the weight of an automobile
would result in a 220 billion pound reduction in CO2
emission per annum. Germany is a leader in the use
of natural fiber composites in automotive industry.
The automotive companies in Germany, such as
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Table 20 FTIR peaks of different natural fibers (in cm-1)
Reference
Fiber
OH
stretching
C–H
stretching
C=O
CH2
stretching symmetric
bending
C–O
stretching
C-vibration of
acetyl group
in lignin and
hemicellulose
bBending
vibration of C– glycosidic
linkages
H and C–O
groups of
aromatic ring in
polysaccharides
C–OH
bending
Mwaikambo
and Ansell
[207]
Keshk et al.
[282]
Bilba et al.
[283]
Spinace et al.
[284]
Saha et al.
[285]
Sawpan et al.
[286]
De Rosa
et al. [287]
Fiore et al.
[288]
Arrakhiz
et al. [289]
Reddy et al.
[290]
Neto et al.
[291]
Al–Maadeed
et al. [292]
Sisal
3447
–
1737
1384
–
–
–
–
Kenaf
3397
2902–2918 –
1416
–
1036
–
–
Banana
3600–3100 2910
1416
–
1036
–
–
Curaua
3200–3500 2750–2900 1740
–
1240
–
–
–
Jute
3200–3600 2910
1739
1430
1294
–
–
–
Hemp
3410
1732
1425
1247
–
892
–
Okra
3100–3600 2854–2925 1734
1430
1243–1384
1320–1370
894
598
2916
–
Artichoke 3342
2854–2923 1737
1422
–
1318–1372
892
589
Coir
3330
2910
1420
1235
–
–
–
Borassus
3435
2851–2920 1746
–
1254
1058
893
–
Pina
3450
2900
1730
–
1260
1050
–
–
Palm leaf
3300
2946
1735
–
–
–
–
–
1725
Mercedes, BMW, Audi, and Volkswagen, have taken
the initiative to use natural fiber composite for interior and exterior applications because natural fiber
composite components are lightweight, impact
resistant, nontoxic, and no-sharp edged fracture in
the case of a crash.
Asia Pacific (especially China, India, South Korea,
Thailand, and Indonesia) is expected to have the
fastest-growing market for natural fiber composites
during the next 5 years owing to the increasing
demand from the transportation, building, and construction [310]. The European Union’s ban on nonrecyclable plastic is expected to drive the market in
the Europe region. Based on the report by Global
Biocomposite Market in 2018 [313], some of the current leading companies in the global natural fiber
composite market are FlexForm Technologies (USA),
Tecnaro GmbH (Germany), Trex Company Inc.
(USA), Fiberon LLC (USA), Meshlin Composites ZRT
(Hungary), UPM (Finland), Jelu-Werk J. Ehrler
GmbH & Co. KG (Germany), Green Bay Decking
(USA), Universal Forest Products Inc. (USA), Toray
Industries Inc. (Japan), and Owens Corning (USA).
Table 22 summarizes the applications of natural fiber
composites based on different properties.
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Life-cycle assessment of natural fiber
composites
Figure 41 FTIR spectra of random-oriented sisal fiber/polylactic
acid composite manufactured by injection molding technique
[272].
Figure 42 shows the life cycle of a natural fiber
composite [19]. The fully environmental superiority
of natural fiber composites than that of synthetic
composites is still questionable due to the relative
excessive processing requirements of natural fiber
composites. Therefore, careful life-cycle assessment
(LCA) is necessary before natural fiber composite is
used commercially [4]. LCA is part of ISO14000 series
of Environmental Management System (EMS) and
determines the overall potential impact of natural
fiber composites on the environment throughout the
whole life cycle from extraction of raw materials to
end-of-life stage. A number of LCA studies have been
conducted to date on natural fiber composites. The
first LCA study on natural fiber composites was
performed by Wotzel et al. [314] on hemp fiber/
epoxy composite and acrylonitrile butadiene styrene
(ABS) for production of automotive side-panel component of passenger cars through cradle-to-grave lifecycle approach. They reported that the cumulative
Table 21 Summary of existing studies on the effect of nanofiller on the properties of natural fiber composites
Reference
Composite
Apparent
fiber
diameter
(lm)
Manufacturing
method
Observation
Huang and
Netravali
[298]
Unidirectional flax
fiber/soy protein
composite
237a
Resin transfer
molding
Sen and
Kumar
[299]
Random-oriented coir
fiber/epoxy composite
–
Compression
molding
13%, 4%, 8%, 24%, and 17% increase in tensile strength,
tensile strain, tensile modulus, flexural strength, and
flexural modulus of composite with fiber volume fraction
of 48% incorporating 5% nanoclay, respectively
The smoke density and limiting oxygen index (as fire
retardancy properties), respectively, 28% decreased and
26% increased when the composite contained 1.6% coir
nanofiller
19%, 27%, and 34% decrease in water absorption
incorporating 1%, 3%, and 5% nanoclay (by weight),
respectively. 7%, 3%, and 23% enhancement in the flexural
strength, modulus, and toughness incorporating 5%
nanoclay, respectively
163% increase in flexural strength incorporating 3%
nanoclay. 180% increase in flexural modulus incorporating
5% nanoclay
60%, 17%, and 12% increase in tensile strength, flexural
strength, and damage index of the composite incorporating
5% nanosilica, respectively
Alamri and Random-oriented recycled 200a
Low [300]
cellulose fiber/epoxy
composite
Lim et al.
[301]
Ashik et al.
[302]
a
Random-oriented Napier
grass fiber/epoxy
composite
Cross-plied jute fiber/
epoxy composite
Diameter of a fiber bundle
Compression
molding
–
Resin infusion
–
Compression
molding
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J Mater Sci (2020) 55:829–892
Table 22 Application areas of natural fiber composites based on their properties [13, 243]
Properties
Suitable application
High strength and low
weight
Renewability
Transportation (such as aircraft, automobile), mobile electronics, sport equipment, construction and building,
storage devices (such as grain storage silos, bio-gas containers)
Short-life-span products such as plastic cutlery, packaging, toys, and electronic products of computers, phones,
faxes, radios, stereos, CD players
Toys, hobbyist-built items, consumer-handled items
Medical devices and implants
Competitive consumer products such as window and door frames, furniture, railroad sleepers, automotive
panels, gardening items, shelves, noise insulting panels, and packaging
Dry-use products
Short-life-span products, limited exposure to harsh environments
Non-toxicity
Biocompatible
Low cost
High water absorption
Weak durability
energy demand of hemp fiber/epoxy composite was
45% lower than that of ABS. Table 23 presents the
summary of the LCA studies on natural fiber composites. The existing LCA studies revealed that natural fiber composites have superior environmental
performance than composites with synthetic fiber or
resin. However, cradle-to-gate LCA studies of flax, as
an agrochemical-based material, and glass by Dissanayake et al. [321, 322] reported that flax sliver is
comparable in energy terms to glass fiber mat, and
continuous glass fiber reinforcement appears to be
Figure 42 Life cycle of a
typical natural fiber-reinforced
composite [19].
superior from an environmental energy point of view
to spun flax yarn.
Conclusions
The properties of easy accessibility, low density, low
price, good thermal and acoustic insulation, ecofriendliness, recyclability, renewability, and satisfactory mechanical properties make natural fiber composites an attractive alternative to carbon, glass, and
other synthetic fiber composites. Natural fiber composites are currently being used in the construction
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Table 23 Summary of some of the studies on life-cycle assessment (LCA) of natural fiber composites
References Composite
Application
Life-cycle approach Results
Side-panel
component for
passenger car
(Audi A3)
Bus body
component
Cradle-to-grave
Cumulative energy demand of hemp fiber/
epoxy composite was 45% lower than that of
ABS
Cradle-to-grave
Composite with PTP had 65% and 39% lower
energy demand and global warming potential
than those with polyester, respectively
Curaua fiber/polypropylene composite had
75% and 66% lower climate change and
ozone depletion potential than those of glass
fiber/polypropylene, respectively
Hemp fiber/PTP composite had 54% and 44%
lower global warming and climate change
potential than those of glass fiber/polyester,
respectively
Sugarcane bagasse fiber/polypropylene had
20% and 25% lower energy demand and
global warming impact than those of talc/
polypropylene, respectively
Kenaf fiber/soy-based resin composite had
58%, 13%, 97%, and 22% lower global
warming, ozone depletion, hedgehog cancer,
and ecotoxicity potential than those of glass
fiber/polyester composite, respectively
Hybrid glass-hemp fiber/epoxy composite had
18%, 19%, 20%, and 20% lower global
energy requirement, global warming
potential, ecotoxicity, and life cycle cost than
those of glass fiber/epoxy composite,
respectively
Wotzel
et al.
[314]
Hemp fiber/epoxy versus ABS
Mussig
et al.
[315]
Hemp fiber/Triglycerides and
polycarbon acid anhydrides
(PTP) versus hemp fiber/
polyester
Curaua fiber/polypropylene
versus glass fiber/
polypropylene
Automotive
internal
component
Cradle-to-grave
Schmehl
et al.
[317]
Hemp fiber/PTP versus glass
fiber/polyester
Bus body
component
Cradle-to-grave
Luz et al.
[318]
Sugarcane bagasse fiber/
polypropylene versus talc/
polypropylene
Cradle-to-grave
Wang
et al.
[319]
Kenaf fiber/soy-based resin
versus glass fiber/polyester
Vehicle interior
aesthetic
covering
component
Sheet molding
compound
material
LaRosa
et al.
[320]
Hybrid glass-hemp fiber/epoxy
versus glass fiber/epoxy
Zah et al.
[316]
Cradle-to-gate
Elbow fittings for Cradle-to-grave
the seawater
cooling pipeline
and building, packaging, transportation, military,
sporting equipment, and medical industries in an
effort to reduce greenhouse gas emissions, and it is
anticipated that this type of composite will be
extensively used in other applications in the near
future. However, their drawbacks, such as poor
interfacial adhesion between the natural fibers and
the matrix, moisture absorption, poor fire resistance,
low impact strength, and low durability, have at least
partly prevented them from being considered a reliable alternative for conventional composites. This
paper has presented an up-to-date and all-encompassing review on the properties of natural fiber
composites. It has been shown that the use of modification techniques can lead to improved physical,
mechanical, durability-related, and time-dependent
properties of natural fiber composites, which points
to the feasibility of using these composites as an
alternative material in a wide range of applications.
The review presented here will help researchers to
better understand various properties of natural fiber
composites to facilitate the development of new
green materials with improved performance. Based
on this review, further investigations are recommended to investigate the influence of matrix modification and fiber length on the properties of natural
fiber composites.
Compliance with ethical standards
Conflict of interest Both authors declare that they
have no conflict of interest.
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J Mater Sci (2020) 55:829–892
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