A comprehensive review of various factors for application feasibility of natural fiber-reinforced polymer composites.pdf

Results in Materials 17 (2023) 100355
Available online 9 December 2022
2590-048X/© 2022 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
A comprehensive review of various factors for application feasibility of
natural fiber-reinforced polymer composites
Manoj Kumar Singh a,*
, Renu Tewari a
, Sunny Zafar a
, Sanjay Mavinkere Rangappa b
,
Suchart Siengchin b
a
Composites Design and Manufacturing Research Group, School of Engineering, Indian Institute of Technology Mandi, Mandi, Himachal Pradesh, India
b
Natural Composites Research Group Lab, Department of Materials and Production Engineering, The Sirindhorn International Thai-German Graduate School of
Engineering (TGGS), King Mongkut’s University of Technology North Bangkok (KMUTNB), Bangkok, Thailand
A R T I C L E I N F O
Keywords:
Natural fiber
Composite
Mechanical behavior
Machining
Tribology
Degradation
A B S T R A C T
Composites consisting of natural fibers and plastics are gaining huge consideration nowadays because of the
relentless increase in the use of non-renewable resources. Worldwide, researchers are trying to explore new
techniques to effectively utilize non-renewable resources for various purposes. Plastic materials represent non-
biodegradable resources that are excessively used nowadays and are sources of environmental issues. Natural
fibers are one of the best substitutes for manufacturing composite products. They are not only useful in reducing
carbon footprints but also in solving various degradation issues. Natural fiber-based polymer composites are in
huge demand because of their manufacturing cost, lightweight, availability, and environmentally friendly na
ture. Prior to the usage of natural fiber-reinforced polymer composites (NFRPCs) for different applications, their
behavioral study is required under different application aspects. Therefore, this paper aims to review the
different application aspects of NFRPCs such as machining, tribology, and environmental degradation. Addi
tionally, this review also presents a brief introduction to composites, their constituents, and their various ap
plications. This review study will provide useful information to researchers working on NFRPCs and exploring
the possible applications of NFRPCs under different working conditions.
1. Introduction
Nowadays, composites are one of the essential materials used in
different sectors because of their unique properties such as lightweight,
high specific strength, high modulus, etc. [1–6]. These composites
consist of different reinforcements that may be natural or synthetic. The
detailed classification of composites can be seen in Fig. 1. Among re
inforcements, natural fibers are one of the commonly demanded re
inforcements in the polymer composite industry due to increasing
environmental concerns [1,7–11]. Natural fibers are proven to be a
better substitute for synthetic fibers for manufacturing composites for
low-load applications and energy conservation applications [12–17].
Researchers are further exploring the use of natural fibers for
higher-load applications by reinforcing them with ceramic fillers and
synthetic fibers. The choice of materials is changed to the biological
origin and recyclable nature because of the increased burden on the
environment [18–22]. This shift in the usage of more environmentally
friendly materials will help in maintaining ecological balance by
reduction of non-degradable waste by reuse. Composites manufactured
using natural fibers have various benefits such as the availability of a
variety of fibers, biodegradability, reduced greenhouse gas emission,
increased job availability, reduced energy consumption, and reduced
cost [23–28]. Also, these natural fiber-reinforced composites have some
limitations i.e., moisture absorption, debonding, poor wettability, and
inadequate adhesion [29]. Some of the natural fibers abundantly
available in India are coconut coir, jute, hemp, flax, banana, kenaf,
bamboo, sisal, and flax [30–34]. The consumption of these natural fibers
in India is more than 400 million tons [35,36]. The predicted demand
for natural fibers is approximately 60% of the whole demand each year
alone in the US. This demand has been increasing from 10% to 22%
every year as predicted [37].
Natural fiber-reinforced composites (NFRPCs) have various appli
cations in different sectors such as infrastructure, sports equipment,
household applications, furniture, consumer goods, and automotive [1,
12,39–41]. Moreover, there are some advanced applications of these
natural fiber-reinforced polymer composites e.g., photovoltaic, optic,
* Corresponding author.
E-mail address: manojsingh.iitmandi@gmail.com (M.K. Singh).
Contents lists available at ScienceDirect
Results in Materials
journal homepage: www.sciencedirect.com/journal/results-in-materials
https://doi.org/10.1016/j.rinma.2022.100355
Received 3 September 2022; Received in revised form 13 November 2022; Accepted 7 December 2022

2
and electrical, which are being explored [42–45]. The two most
important factors behind renewed interest in natural fibers are envi
ronment and cost [46–52]. Natural fibers are preferred over synthetic
fibers because of their easy decomposition, lightweight, high specific
mechanical properties, less tool wear during processing, and less density
[53]. The motivation behind increased research on NFRPCs is due to
their recyclability, less environmental impact, and less associated en
ergy during processing [54]. The emanation of CO2 during the pro
cessing of natural fiber is lesser or negligible as compared to synthetic
fiber [55]. This is an enormously significant aspect that has placed these
fibers under the Kyoto protocol as non-greenhouse emitting material
[56]. Table 1 lists some important physical and mechanical properties of
the synthetic and non-synthetic fibers for comparative analysis.
Nowadays, bio-based polymers are also gaining attention [62–64].
Researchers throughout the world are exploring the possibility of using
bio-based polymers for composite manufacturing on a wider scale. At
present, the cost of biopolymers is higher than non-biodegradable
polymers due to their inherent processing complexities. Various prop
erties i.e., mechanical, thermal, dielectric, and tribological properties of
bio-based polymer composites are being explored [29,65–68]. Nowa
days, so many design techniques are used to select the proper polymers
and natural fibers as per the requirement of composite properties
[69–75]. The delamination factor of the composites is one of the
important properties that need to be considered by designers and in
dustry [76]. Artificial neural network is the best way to reduce the cost
of testing by storing and recalling the data of matrices, reinforcement,
and composites [77]. Mechanical properties and applications of some of
the prevalent polymers are presented in Table 2. Thermoplastic poly
mers melt on heating and are softened repeatedly under the influence of
heat. On the contrary, thermosetting polymers have excellent mechan
ical properties as compared to thermoplastic polymers but the limitation
of using thermosetting polymers is their non-reformability or remold
ability after curing. This is due to the formation of 3D cross-linked chains
upon heating which makes it difficult to break the bonding of molecules
present inside the polymer. With the increase in cross-linking effect of
thermosetting polymer, the thermal stability and rigidity also increase.
Thermosetting polymers are easily processed and molded in different
shapes due to their availability in liquid resin form at room temperature.
The resin provides better wettability to reinforced fibers.
2. Manufacturing of natural fiber-reinforced polymer
composites
Different raw materials such as fabrics, mats, fibers, resins, fillers,
prepregs, etc. can be used to manufacture natural fiber-reinforced
polymer composites (NFRPCs) [82]. For every individual NFRPC
manufacturing technique, different kinds of material systems, different
processing conditions, and different tools are required. Before using
natural fiber as reinforcement in the fabrication of NFRPCs, pretreat
ment after the extraction is required for enhancing the compatibility of
the fiber with the polymer matrices. Fig. 2 shows the extraction process
of one of the natural fibers. After the extraction process and chemical
treatment, NFRPCs can be manufactured using different techniques
shown in Fig. 3. Out of mentioned manufacturing techniques, hand
lay-up, spray-up, injection molding, compression molding, and resin
transfer molding are the most common techniques. Other techniques are
also used as per the product requirement that has some attributes such as
cost, property, quality, quantity, etc. Environmental concern is one of
the biggest concerns nowadays while manufacturing composites. Thus,
the selection of the manufacturing technique should be focused on the
cleaner technique i.e., microwave-assisted manufacturing [83].
The above mentioned manufacturing techniques are associated with
their advantages and drawbacks that depend on the processing condi
tions as well as on the constituent materials used for processing
composites. Composites manufactured using the above-mentioned
techniques require more time and energy which calls for a need to
explore other novel techniques which can reduce the time and energy
during composite processing. Recently, microwave-assisted composite
manufacturing has gained attention of researchers and industrialists to
resolve the issue of high processing time and energy [85–94].
Fig. 1. Detailed classification of composite materials [38].
M.K. Singh et al.

3
Microwave-assisted composite manufacturing can be used to
manufacture thermosets as well as thermoplastic-based composites.
Thermoset-based composites are cured using microwave energy since
1987 [95]. On the other hand, thermoplastic-based composites are
manufactured via microwave heating only since 2014 [96]. Nowadays,
microwave-assisted composite manufacturing technique is promising in
the field of polymer composites. The microwave-assisted technique can
be used as a compression molding technique (MACM) or resin transfer
molding (MARTM) [80,97]. This may be due to the less processing time
and cost as well as a sustainable route of NFRPCs manufacturing. The
MACM and MARTM encompass volumetric heating, and uniform prop
erties resulting in enhanced behavior [96]. The evolution and contri
bution of different composite manufacturing techniques can be seen in
Fig. 4.
Innumerable investigations were dedicated to the exploration of
natural fibers/fillers to develop natural fibers-based composites. Despite
economic and environmental advantages, these composites lack per
formance attributes designed for specific industrial applications [98].
Various studies have been done on the manufacturing and detailed
characterization of the NFRPCs, some of which are discussed in detail in
this study. These studies are helpful in exploring the feasibility of the
application of various natural fiber composites in different sectors.
Mechanical performance characterization of NFRPCs is one of the
baseline characterizations out of various characterizations [99–103].
With the reinforcement of natural fibers in the polymer up to a certain
percentage (20%–30%), the mechanical performance enhanced after
that it decreased [69,99,104–107]. This may be due to the improper
wetting of the fibers [74,108]. Also, the treated natural fibers showed
good interfacial bonding as compared to untreated fibers [69].
Ochi analyzed the mechanical and degradation behaviors of the
kenaf fiber and kenaf/PLA composites [109]. It was noticed that the
tensile and flexural properties increased linearly up to 50 wt% kenaf
fiber reinforcement. Thermal analysis showed that kenaf fiber treated at
160 ◦
C for 1 h did not affect it. From the biodegradability test, it was
noted that the weight of the composite decreased by 38% in four weeks.
Parab et al. chemically modified the coir pith for the adsorption of cobalt
(II) ions from an aqueous solution [110]. A significant change was
observed in the structure and properties of the chemically modified coir
fiber. It was also observed that chemically modified coir pith showed
enhanced adsorption capacity. Chin and Yousif studied the potential
effect of differently oriented kenaf fiber reinforcement on the tribolog
ical performance of kenaf fiber-reinforced epoxy composite [111]. It was
observed that the wear and frictional characteristics of the epoxy were
enhanced on reinforcement of kenaf fiber. Moreover, applied load and
sliding velocities have less effect on specific wear rates.
Cao et al. explored the effect of kenaf powder filling on recycled
high-density polyethylene (HDPE)/natural rubber (NR) composite
[112]. The author found an increment in the tensile modulus was found
with the decrement in the elongation at break and tensile strength of the
composite. But, with the addition of a compatibilizer (maleic anhydride
grafted polyethylene (MAPE)), an increase in the tensile properties of
Table 1
Important properties of various fibers.
Fibers Properties Reference
Density (g/
cm3
)
Tensile strength
(MPa)
Young’s
modulus (GPa)
Natural Fibers
Hemp 1.5 550–1110 58–70 [57]
Kenaf 1.4 223–930 14.5–53
Jute 1.3–1.5 393–800 10–55
Sisal 1.3–1.5 507–885 9.4–28
Flax 1.5 345–1830 27–80
Ramie 1.5 400–938 44–128
Harakeke 1.3 440–990 14–33 [58]
Alfa 1.4 188–308 18–25 [57]
Cotton 1.5–1.6 287–800 5.5–13
Silk 1.3 100–1500 5–25 [59]
Coir 1.2 131–220 4–6 [57]
Feather 0.9 100–203 3–10 [23]
Wool 1.3 50–315 2.3–5 [59]
Abaca 1.5 400–980 6.2–20 [57]
Bagasse 1.25 222–290 17–27.1
Bamboo 0.6–1.1 140–800 11–32
Banana 1.35 500 12
Curaua 1.4 87–1150 11.8–96
Henequene 1.2 430–570 10.1–16.3
Isora 1.2–1.3 500–600 –
Nettle – 650 38
Pineapple
leaf
0.8–1.6 180–1627 1.44–82.5
Piassava 1.4 134–143 1.07–4.59
Oil palm 0.7–1.55 80–248 0.5–3.2
Synthetic Fibers
Kevlar 1.44 3900 131 [7]
Aramid 1.45 3000–3200 62–67 [59]
E-glass 2.55 2100–3500 71
Carbon 1.40 4000 230–240
S-glass 2.55 4590 87 [60]
PET 1.367 510–691 6–11 [61]
Nylon 66 1.146 350–551 3–6.5
Nylon 6 1.136 450–701 2.5–3.4
PP 0.946 411 6.4
Table 2
Properties of thermoplastic and thermosetting polymers.
Polymers Density (g/cm3
) Modulus (GPa) Tensile strength (MPa) Applications Reference
Thermoplastic Polymers
Acrylonitrile
butadiene
styrene
1.0–1.2 1.0–2.6 22.1–49.0 Instrument board, outer panel, steering wheel, and dashboard [78]
Polyamides 1.11–1.17 1.3–4.2 50–90 Safety airbags in cars
Polystyrenes 0.013–1.18 0.303–3.35 17.9–60.7 Smoke detectors, plastic cutlery
Polypropylene 0.88–2.40 0.008–8.25 9–80 Packaging and automotive industry
Polyethylene 0.93–1.27 0.62–1.45 11–25 Packaging, pipes and fitting [79]
Polyether ether ketone 1.26–1.50 2.20–6.48 54.5–265 Bearing, piston parts, and pumps
Polyvinyl chloride 1.13–1.85 1.12–4.83 3.74–55.9 Wiring and cables
Poly-lactic acid 1.00–2.47 0.08–13.8 14–114 Disposable goods, medical begs
Thermosetting Polymers
Polyester 0.60–2.20 1.0–10 10–123 Car tires reinforcements, conveyor belts, and safety belts [78]
Vinyl Ester 1.03–1.95 3.72–94.5 30.3–993 Tanks, and vessels
Polyurethanes 1.21 2.62 36.5–55.2 Building insulation, and footwear
Melamine 1.50–1.81 5.0–13.3 53–76 Wiring, and cables [79]
Urea Formaldehyde 1.5 9 55 Disposable goods, and medical begs
Polyimides 1.60 27.6 241 Firefighters’ cloths, acoustic, and thermal insulation
Phenolic 1.3 4.43 50–55 Electronics, offshore water pipe, ballistics, aerospace, rail, and mass transit [80,81]
Epoxy 1.38 15–35 35–85 Coatings, electronic materials, adhesives, and matrices for FRPCs [80]
M.K. Singh et al.

4
the composite was observed. Additionally, the water uptake percentage
of the kenaf-reinforced MAPE composite was found to be more than that
of the composite without MAPE. Rozman et al. studied the effect of
mechanical interlocking on the kenaf/PP composite [113]. With the
reinforcement of non-woven kenaf fiber in the PP matrix, the tensile
properties of the composite increased with the increase in the kenaf fiber
loading. Further, there was an increase in the tensile properties of the
composite with the mechanical interlocking. With the increase in the
number of needles used for punching, there was a significant increase in
the tensile properties, however, no effect was seen on the water ab
sorption characteristics of the kenaf/PP composite.
Mokhtar et al. manufactured UHMWPE/HDPE blended kenaf, basalt,
and kenaf/basalt composites using compression molding [114]. It was
observed that the tensile strength of the UHMWPE/HDPE blend increased
by the reinforcement of fibers. The flexural and Charpy strengths showed
insignificant results, where the performance of the polymer was reduced
by 5% in absence of kenaf fiber. Hamma et al. studied the mechanical and
viscoelastic behavior of starch-grafted-polypropylene/kenaf fibers
composites [115]. Composites of different fiber loading and aspect ratio
were manufactured in compression molding setup. It was observed that
by reinforcement of kenaf fiber, the mechanical properties were
enhanced. Mahjoub et al. studied the physical and mechanical properties
of continuous unidirectional kenaf fiber-reinforced epoxy composite
[116]. The composites were manufactured using the hand layup tech
nique. A linear stress-strain curve of kenaf fiber-reinforced epoxy com
posite was obtained. The tensile strength and modulus exhibited an
increasing trend with kenaf fiber reinforcement up to 40%. Also, the
maximum elongation at break decreased due to the increase in kenaf fiber
loading. The variation in ultimate tensile strain was not affected by fiber
reinforcement variation.
Yong et al. studied the effect of kenaf fiber orientation on the tensile,
flexural, and impact properties of the sandwich polyester/kenaf com
posite [117]. The composites were fabricated using the hand lay-up
technique. Polyester: kenaf volume percent was 70:30. It was found
that the kenaf fiber in anisotropic position had better properties than the
fiber in perpendicular or isotropic direction. Islam et al. studied the
physical, mechanical, and biodegradable properties of hybrid kenaf/coir
polypropylene composites [6]. These fibers were treated with 2% NaOH
solution before use. It was observed that the biodegradability and water
absorption characteristics of the hybrid composites were enhanced on
hybridization. Whereas these properties were reduced with the addition
of methylcyclopentadienyl manganese tricarbonyl (MMT) in the hybrid
composites. The mechanical properties were enhanced after the rein
forcement of MMT in the hybrid composites. Sathishkumar et al. char
acterized hybrid composite of sisal/cotton fiber woven mat [118].
Composite with 40 wt% of sisal/cotton showed the highest mechanical
properties and vibration characteristics as compared to composite with
10 wt%, 20 wt%, 30 wt%, and 50 wt% reinforcement. This was due to
higher value of fiber/matrix interfacial strength.
Sanjay and Yogesh studied the effect of stacking sequence of jute,
kenaf, and glass fiber reinforced hybrid composite manufactured using
vacuum bagging method [119]. It was observed that the hybrid com
posites of kenaf and glass showed better tensile properties than jute glass
hybrid composites. Moreover, the mechanical properties depended on
the stacking sequence of high strength fibers.
Sadia et al. studied the effect of 6% NaOH treatment on the kenaf
fiber for the kenaf fiber-reinforced lightweight foamed concrete (KFLFC)
[120]. It was found that the treated kenaf fiber composite had better
interfacial bonding with the matrix which resulted in the higher
toughness of the treated composite than the untreated one. Also, the
Fig. 2. Process of extracting kenaf fibers, Adapted from [84].
Fig. 3. Classification of polymer composites manufacturing techniques.
M.K. Singh et al.

5
durability of the treated kenaf fiber composite increased due to the
reduction in the water absorption of the composites. Ku et al. studied the
effect of a compatibilizer ((3-aminopropyl) triethoxysilane) content on
the tensile, flexural, and impact properties of the PP/kenaf and PP/ke
naf/PU (PU-Polyurethane) composites [121]. It was observed that 1 wt
% of the compatibilizer mixed with the composite yielded the highest
tensile, flexural, and impact properties of both composites. Kumar et al.
studied the effect of 6 mm, 8 mm, and 12 mm of kenaf fiber reinforce
ment in polyester resin for polyester/kenaf composite [122]. It was
found that the 6 mm of kenaf fiber reinforced with polyester resin
exhibited the highest flexural and tensile properties. Verma and Shukla
studied the effect on the tensile, flexural, and dynamic mechanical
analysis (DMA) of 10 wt%, 20 wt%, and 30 wt% of reinforced kenaf fiber
in the HDPE/kenaf polymer composite [123]. It was found that the
maximum tensile properties and flexural strength of the composite were
at 10 wt% and 30 wt% of kenaf fiber reinforcement, respectively.
Valášek et al. studied the mechanical and tribological properties of
the alkali-treated white/brown coir epoxy composites [124]. It was
observed that the surface of the fiber got rough due to 6% alkali treat
ment, which improved interfacial interaction. The outcome of alkali
treatment was observed in terms of enhanced mechanical properties
(tensile strength). Azammi et al. studied the physical and damping
properties of kenaf fiber-reinforced rubber/polyurethane composites
[125]. These composites were manufactured in a hot press. Physical
properties were highest in the case of composite having a maximum
fraction of natural rubber. Whereas, composite with the highest amount
of polyurethane showed the highest damping properties at the highest
temperature (135 ◦
C). Nadzri et al. reviewed kenaf/glass hybrid com
posite for the application of low velocity impact [126]. It was observed
that the mechanical properties were mainly affected by fiber loading
percentage, fiber orientation, and chemical treatment of the fiber. It was
found that the optimum fiber loading was 30% and orientation was 90◦
,
which can resist impact strength and have better mechanical properties.
The properties of the natural fiber-reinforced polymer composites
depend on the fiber treatment of the reinforcement [127]. Some of the
significant studies on the manufacturing of NFRPCs are recorded in
Table 3.
3. Machining of NFRPCs for assembly application
The machining process is one of the important secondary operations
required to obtain a near neat shape product [135–137]. Most of the
product components required the assembly of parts before they can be
used. For assembly purposes, holes are required in most of the parts.
Thus, machining plays an important role at this stage of the
manufacturing process. The efficiency of the components mostly de
pends on their fastening techniques. Moreover, the fastening efficiency
depends on the quality of hole produced and fitness of the fastener in the
hole. Therefore, performing machining on composite materials without
affecting their mechanical properties is also a challenging part of the
industry. For the machining of polymer composites, some specific
machining processes can be used as shown in Fig. 5. The literature has
reported several limitations with conventional machining processes
[138]. The main reason behind this is due to the anisotropic and inho
mogeneous nature of composites which results in improper machining
and countless rejected components [139] (see Fig. 6).
Nowadays non-traditional machining is proven to be a better alter
native to traditional machining. Non-conventional machining has many
associated advantages such as absence of thrust force, absence of chatter
or vibration, minimum tool wear, and less frictional heat [138,140]. One
of the non-traditional machining processes is laser drilling. Laser drilling
utilizes thermal energy for the ablation of working material. Thermal
damage to the composite due to the heat-affected zone is one of the
major challenges. To achieve better hole quality, various process pa
rameters must be controlled during the machining of the composite. For
example, in most non-traditional machining processes standoff distance
should be optimum. Another important parameter is the feed rate or
speed of the nozzle. The consequences of improper selection of process
parameters are damaged hole quality and decreased mechanical
strength of the component. Therefore, parameter optimization of the
machining process for every individual work material is required.
Several investigations have been done by different researchers by
varying the process parameters for different materials. Some of the
significant related findings are reported in this review.
Li et al. performed machining on CFRP composites of different
thicknesses by using a diode-pumped solid-state UV laser [142]. Mini
mum HAZ was reported while machining with a short-pulsed laser. It
was also observed that the ablation heat accumulated into the composite
and was more in the case of chopped carbon fiber composite.
Hernandez-Castaneda et al. performed cutting of pine wood with the
help of ytterbium fiber laser and calculated its efficiency [143]. The
study established the optimum parameter for cutting pine wood using
single-jet gas and dual-jet gas conditions. Riveiro et al. performed cut
ting of a 3 mm thick CFRP sheet using a CO2 laser [144]. The effect of a
Fig. 4. Evolution and contribution of different composites manufacturing techniques, Adapted from [80].
M.K. Singh et al.

6
Table 3
Significant findings from manufacturing processes of NFRPCs.
Composites Manufacturing
process
Remarks Reference
Kenaf fiber-reinforced
PLA composite
Compression
molding
Linear incremental
behavior was obtained
for tensile and flexural
strength up to 50%
reinforcement. There
was no effect of heat up
to 160 ◦
C on kenaf
fiber.
[109]
epoxy composite
Vacuum-assisted
resin transfer
molding
With the reinforcement
of fiber, friction and
wear performance of
composite improved.
Fiber reinforcement
has a significant effect
during the wear test as
compared to sliding
velocities and normal
load.
[111]
Jute/betel nut fiber-
reinforced
polypropylene
composite
Compression
molding
Mechanical properties
increased after
reinforcing 10% bettle
nut into jute/
polypropylene
composite as compared
to neat polymer.
[128]
Kenaf powder
reinforced recycled
HDPE/natural
rubber composite
Compression
molding
The elastic modulus of
the composite
increased with fiber
reinforcement. Tensile
strength and strain
decreased. By adding
MAPE, tensile strength
increased. Water
uptake by composite
increased by adding
MAPE.
[112]
UHMWPE/HDPE
blended kenaf/
basalt composite
Compression
molding
The tensile strength of
the UHMWPE/HDPE
blend increased on
fiber reinforcement.
Insignificant
improvement in
flexural and impact
strength was observed
after fiber
reinforcement.
[114]
Starch-grafted-
polypropylene/
kenaf fiber
composite
Compression
molding
enhanced after kenaf
fiber reinforcement.
[115]
Sisal Fiber reinforced
polyester
composites
Hand lay-up
technique
Composite having
kenaf fiber in
anisotropic position
had better properties
than the fiber in a
perpendicular or
isotropic direction.
[117]
Sisal/cotton
reinforced epoxy
composite
Hand lay-up
technique
40 wt% reinforced
composite showed
better mechanical
properties as compared
to 10%, 20%, 30%, and
50%.
[118]
Jute/kenaf/E-glass
woven fabric epoxy
composite
Vacuum-assisted
resin transfer
molding
Better tensile strength
was observed in kenaf/
glass composite as
compared to jute/glass
composite. The
stacking of high
strength fiber showed
high mechanical
strength.
[119]
Table 3 (continued)
process
Remarks Reference
foam
Compression
molding
Better interfacial
bonding was observed
after the treatment of
kenaf fiber before
composite
manufacturing. The
durability of treated
fiber composite was
more.
[120]
natural rubber/
polyurethane
composite
Compression
molding
Composite with the
maximum percentage
of natural rubber
showed better physical
properties. Composite
with the highest
polyurethane showed
the highest damping
properties.
[125]
Glass fiber reinforced
nylon 66/
polystyrene and
LDPE composite
Microwave-
assisted heating
Welding of
thermoplastic-based
composites performed.
This process has the
potential to replace
thermosetting resins
with advanced
thermoplastic
composites.
[129]
Carbon fiber-epoxy
composite
Microwave-
assisted heating
A comparison of
flexural strength and
ILSS was done for
composites
manufactured through
microwave heating
and autoclave heating.
Composite
manufactured through
microwave heating
showed better
mechanical properties
as compared to
autoclave-heated
composites.
[130]
Eccobond/Bexloy
composite
Microwave-
assisted heating
The compressive
strength of the epoxy
resin increased.
Decrease in processing
temperature by
15–20 ◦
C. Processing
time decreased by two-
thirds.
[131]
Grewia optiva and
Nettle fiber/PLA
composites
Microwave-
assisted heating
The strength of the
joint was better in the
case of microwave-
processed composite as
compared to
adhesively joined
composites. For
microwave processing,
charcoal was used as a
susceptor.
[132]
Coconut fiber-epoxy
composite
Microwave-
assisted heating
The processing time in
the case of microwave
heating was less as
compared to
processing in the
convection oven.
[133]
Sisal and Grewia
optiva fibers-
reinforced
polypropylene and
ethylene-vinyl
acetate composites
Microwave-
assisted heating
The microwave curing
process was 83% faster
than the conventional
processing. Microwave
power of 900 W and
processing time of 570
s was used to prepare
the composites. Impact
strength depends upon
[96]
(continued on next page)
M.K. Singh et al.

7
pulsed and continuous wave was observed on cutting quality such as
kerf taper, HAZ, and kerf width. Process parameters such as laser power,
standoff distance, and speed were also optimized as per the requirement.
There was the same HAZ value obtained in the case of non-coaxial su
personic cutting heads and commercial conventional cutting heads.
Choudhury and Chuan performed laser cutting on the various thickness
of GFRP composites. It was evaluated that the material thickness and
cutting speed both affect the surface roughness positively [145]. The
surface quality of the hole and HAZ was improved while using a
double-pass laser beam. The surface roughness of the hole was in direct
proportion to speed, nozzle diameter, and material thickness. Nozzle
speed has the most significant effect on surface roughness. Takashaki
et al. examined the generation of the heat-affected zone and the quality
of kerf in carbon fiber-reinforced composites. High power pulsed fiber
laser was used to perform the drilling in the composite [146]. Nozzle
speed and standoff distance were the significant parameters to obtain
the high-quality hole.
Shunmugesh and Panneerselvam performed the Taguchi and Grey
relational analysis to optimize the process parameters while performing
the micro drilling on carbon fiber composites [147]. The output pa
rameters were affected by feed rate and spindle speed. High spindle
speed and low feed rate reduced the delamination factor, circularity, and
cylindricity. Jani et al. performed AWJM on the hybrid composite of
hemp and Kevlar. The neat composite was also manufactured to
compare the quality of the hole with the reinforced composite [148]. It
was observed that transverse speed has a higher influence than standoff
distance and flow rate on surface roughness, kerf width, and material
removal rate. It was also observed that fiber-reinforced composite
showed good surface quality with the absence of delamination or fiber
pullout. Jagdish et al. performed AWJM on green composites and opti
mized the process parameters using response surface methodology
[149]. A mathematical model was formulated to study the effect of
pressure, standoff distance, and nozzle speed on machining time and
surface roughness. It was observed that pressure and nozzle speed were
the significant parameters to influence the surface roughness. Whereas
nozzle speed was a significant parameter in the case of machining time.
Rao et al. accomplished the cutting of CFRP composite using laser and
optimized the process parameters using response surface methodology
[150]. The optimized input parameters selected were cutting speed, gas
flow rate, and laser power and the output parameters studied were HAZ,
kerf width, and kerf taper. The most influencing parameter was laser
power followed by cutting speed. It was found that kerf width increased
with the increased laser power and gas flow rate whereas it reduced with
cutting speed. Solati et al. studied the effect on tensile strength, surface
roughness, HAZ, and kerf taper angle on glass fiber composite drilled
through a laser [151]. Minimum surface roughness and high tensile
strength were observed by optimizing the laser drilling parameters.
Table 4 shows the findings of some of the significant machining work on
polymer composites.
4. Tribological behavior and applications of NFRPCs
Mechanical, tribological, chemical, thermal, etc. characterizations of
NFRPCs are required to check their feasibility for various applications.
In recent years, researchers are interested in studying the tribological
behavior of NFRPCs and have been investigating different routes to
improve the wear resistance of NFRPCs. As most of the mechanical parts
are failing due to tribological loading conditions [159,160]. Some of the
applications of NFRPCs composites could be upholstery, door paneling
elements, impellers, seals, cams, artificial prosthetic joints, bearings,
designer chairs, car interiors, windows, furniture, etc., which are
imperiled to abrasive wear [161,162]. Consequently, the tribological
evaluation of the NFRPCs composite is essential. Various investigations
have been done on the tribological analysis of NFRPCs, and some of
them are discussed further in detail.
Chand and Dwivedi fabricated sisal fiber-reinforced epoxy composite
and studied the wear behavior in three different orientations of sisal
fiber [163]. It was found that with the inclusion of sisal fiber in the
composite, abrasive wear decreased. Another interesting thing to note is
that the composite with normal fiber direction to the wear direction had
minimum wear. Composite with longitudinal fiber direction had
maximum wear. Singha and Thakur studied the mechanical and wear
Fig. 5. Classification of machining processes for polymer composites.
Table 3 (continued)
process
Remarks Reference
the power level setting
of the microwave
setup.
Basal fiber-reinforced
furan composite
Microwave-
assisted heating
The impact behavior of
the microwave-cured
and thermally cured
composite was
compared. The ILSS,
maximum load, and
penetration threshold
were improved for
microwave-cured
composite.
[134]
Jute, hemp, and linen
(flax) fiber-
reinforced
polypropylene
composites
Compression
molding
The tensile and flexural
properties of
composites were
enhanced by
modifying the natural
fiber surface with 1.5
wt % of hollow glass
microspheres (HGM).
Whereas, with the
addition of 3% HGM,
both properties
decreased. Impact
strength of the
composites increased
with increase in HGM.
[17]
M.K. Singh et al.

8
behavior of Grewia optiva-reinforced polymer composites [164]. It was
found that particle-reinforced composites had better mechanical and
wear performance as compared to short and long-range fiber compos
ites. Singha and Thakur fabricated and studied pine needle polymer
composites. The investigation was done on the mechanical and wear
properties of the fabricated composite [165]. It was observed that
composite with 30% reinforcement showed better mechanical and wear
properties. Mishra and Acharya conducted the abrasive wear
Table 4
Summary of machining processes of polymer composites.
Composite type Machining process Remarks Reference
Carbon fiber-reinforced epoxy
composite
UV laser Short-pulsed laser evidenced minimum HAZ. Composite with sliced carbon fiber showed more heat
accumulation.
[142]
Glass fiber-reinforced epoxy
composite
Conventional drilling The diameter of the drill and feed rate were the two most significant parameters on which the
delamination of glass fiber-reinforced epoxy composite depends.
[152]
composite
CO2 laser Non-coaxial supersonic head produced similar HAZ as in the case of commercial conventional head. [144]
composite
Conventional drilling With the increased cutting speed and feed rate, thrust force and torque increased. [153]
composite
CO2 laser Double pass laser beam produced better hole surface quality. [145]
composite
Pulsed fiber laser Scanning speed and hatching distance were the essential parameters during laser drilling. [146]
composite
Electro discharge
machining
Output parameters are affected by feed rate and spindle speed collectively. The delamination factor
was reduced with increased speed and decreased feed rate.
[147]
Hemp-Kevlar reinforced epoxy
composite
Abrasive water jet
machining
During machining, the most influential parameter was transverse speed. The quality of the surface
was better for filler-reinforced hybrid composite.
[148]
Sundi wood dust-reinforced epoxy
composite
Abrasive water jet
machining
Surface roughness was significantly dependent on the pressure and nozzle speed. The machining time
was mostly dependent on the nozzle speed.
[149]
Hybrid carbon/glass composite Conventional drilling Feed rate was the significant parameter for the damage due to delamination and associated surface
roughness. Spindle speed and tool geometry has less effect.
[154]
composite
Fiber laser The output parameters were affected by cutting speed and laser power. With increased gas flow rate
and laser power, kerf width increased. Whereas it reduced with the increased cutting speed.
[150]
composite
CO2 laser Minimum surface roughness and high tensile strength were obtained with optimum laser drilling
parameters.
[151]
Sisal fiber-reinforced polypropylene
and epoxy composite
Conventional drilling The drilling force decreased with an increase in spindle speed and increased with an increase in feed.
Damage during drilling was more in the case of sisal-epoxy composite as compared to sisal-
polypropylene composite.
[155]
Coir fiber-reinforced polyester
composite
Conventional drilling It was observed that the drill diameter of 6 mm and cutting speed of 600 rpm showed the minimum
effect on tool wear.
[156]
Hemp, glass, and their hybrid
polyester-based composites
Conventional drilling There was no effect of speed on the thrust force. Whereas it increased with increased feed rate. Glass
fiber-based composite showed minimum delamination factor as compared to hemp fiber-based
composite.
[157]
Cotton fiber reinforced-polyester
composite
Conventional drilling For minimum thrust force, low feed, low point angle, and high spindle speed were required. The drill
point angle has a great influence on the delamination factor at the exit.
[158]
Fig. 6. Comparison of the conventional and laser-drilled hole, Redrawn with permission [141].
M.K. Singh et al.

9
experiment on bagasse fiber composite in different directions [166]. An
increase in wear rate was observed with an increase in load and grit size.
Moreover, the wear rate of the composite with the parallel orientation of
fibers was greater than the anti-parallel and normal orientation com
posite. Singh et al. studied the tribological characteristics of
kenaf-reinforced polyurethane composite under wet conditions [167].
Different values of sliding distance applied load and fiber orientation
were considered during the study. Enhanced performance of wear was
observed when the fiber mats were exposed in the perpendicular di
rection of sliding distance. Moreover, the wetting medium helped in
lubrication and created a cooling effect. Nirmal et al. studied the ad
hesive wear performance of bamboo fiber-reinforced epoxy composite
[168]. They found that the anti-parallel composite showed excellent
wear resistance. Whereas minimum wear resistance was obtained in the
case of random orientation fiber composite.
Nirmal et al. studied the tribological behavior of kenaf fiber partic
ulate reinforced polymer composite and observed the specific wear rate
and friction coefficient at different sliding distances and normal load
[169]. The study revealed that composites reinforced with 20 wt% kenaf
particulates showed minimum wear rate and friction coefficient. Shu
himi et al. fabricated oil palm/epoxy and kenaf/epoxy composites and
compared their tribological characteristics under dry sliding conditions
[170]. The study was done in terms of fiber composition and
temperature. It was observed that at higher temperatures oil palm
fiber/epoxy composite showed a lower wear rate compared to the
kenaf/epoxy composite. Shanmugam et al. studied the wear behavior of
composite fabricated using palmyra palm leaf stalk fiber [171]. The
effect of fiber length and alkali treatment was done at different sliding
speeds and normal loads for a constant sliding distance. Decreased wear
loss and friction coefficient at a higher speed was observed during the
study. Parikh and Gohil investigated and predicted the wear behavior
of cotton fiber-reinforced polyester composites [172]. Graphite filler
of 0%, 3%, and 5% weight reinforcement was added to the
cotton-polyester composite to investigate its effect during the wear test.
It was found that at 5% graphite reinforcement, the wear resistance of
the composite was the highest among the composites. An artificial
neural network tool was used to predict the wear behavior of cotton
polymer composite at different parameters. Kumar et al. fabricated
the woven bast-leaf hybrid epoxy composite and studied the
physio-mechanical and sliding behavior of the composite [173]. They
found that the mechanical properties of the composite remained highest
at 6% hybrid fiber reinforcement. Moreover, the wear rate was mainly
affected by the fiber reinforcement percentage followed by sliding dis
tance, normal load, and sliding velocity. Rajini et al. studied the friction
and wear behavior of Cyperus pangorei fiber-reinforced polyester
composites [174]. The process parameters of the wear test were sliding
Fig. 7. Schematic of dry sliding wear mechanism of fiber-reinforced composite, Adapted from Ref. [179] with permission from the ASME.
M.K. Singh et al.

10
velocity and contact pressure. They found that the specific wear rate
increased with applied load. The coefficient of friction decreased at
increased contact pressure and decreased sliding velocity.
Bajpai et al. analyzed the tribological behavior of natural fiber-
reinforced PLA composite [159]. It was found that the average friction
coefficient of PLA was not affected after reinforcing the natural fiber.
But the maximum friction coefficient of the composite decreased after
reinforcement. Moreover, it was observed that the debonding of com
posites takes place during the wear test. Bajpai et al. explored the fric
tional and adhesive wear of natural fiber-reinforced polypropylene
composites [175]. They considered three types of natural fibers (i.e.,
nettle, Grewia optiva, and sisal) to study the wear mechanism at
different sliding speeds, loads, and sliding distances. It was found that
the wear performance of the polypropylene increased with the rein
forcement of the natural fiber. It was observed from surface morphology
that debonding of fiber and matrix was the principal mechanism behind
material removal from the composites. Moreover, the effect of the
normal applied load had a stronger effect on wear as compared to sliding
speed. Yallew et al. studied the sliding wear behavior of jute
fiber-reinforced polypropylene composites [176]. The effect of jute
reinforcement on the wear properties of composite was studied. It was
found that the friction coefficient was reduced up to 45% by the incor
poration of 40% jute fiber in the composite. Wear occurred due to
plowing, crack formation, and detachment fracturing of fiber and ma
trix. Nirmal et al. reviewed the tribological performance of natural
fiber-reinforced polymeric composites [177]. Different type of abrasive
wear mechanisms was discussed and their performance on wear, fric
tion, surface roughness, contact condition, temperature, and test
parameters was highlighted. A schematic of dry sliding wear mechanism
can be seen in Fig. 7 [178] (see Fig. 8).
Kumar and Anand performed dry sliding friction and wear tests of
ramie fiber-reinforced epoxy composites [180]. Different weight frac
tions of ramie fiber were considered to study the wear behavior at
variable sliding velocities and normal load. They found that the wear
rate decreased by up to 30% reinforcement beyond which it started
increasing. The coefficient of friction increased up to 25 N after that it
decreased for all the composites. Liu et al. investigated the effect of
silane treatment on the mechanical, tribological, and morphological
properties of the corn stalk fiber-reinforced polymer composites [181].
They found that the silane-treated fiber could not improve friction
performance, but the wear rate of the composite decreased. Moreover,
the friction coefficient of the composite increased at a working tem
perature of 100 ◦
C–150 ◦
C. That was rapidly reduced with increasing
temperature up to 350 ◦
C. The best wear resistance was obtained for the
composite having 5% corn stalk. Reddy et al. evaluated the mechanical
and wear performance of the different natural fiber-reinforced epoxy
composites [182]. They found that the wear performance of the com
posites increased with the incorporation of natural fibers. The friction
coefficient of the composites increased with increased sliding velocity.
Suresh et al. investigated the wear properties of composite manufac
tured using agricultural and industrial wastes [183]. They found that the
mechanical and wear properties of the composite increased by incor
porating bagasse, rice, coconut shell, and husk. Composite with 20% of
agricultural waste reinforcement exhibited low wear loss and friction
coefficient. The summary of the above-discussed literature is shown in
Table 5.
Fig. 8. Schematic mechanism of wetting and absorption of NFRPC placed in NaOH solution, Adapted from [193].
M.K. Singh et al.

11
5. Behavior of NFRPCs under different environments and its
applications
Most of the applications of NFRPCs are indoor applications. The
outdoor application of NFRPCs is limited due to their hydrophilicity.
The behavior of NFRPCs depends upon many factors, such as fiber vol
ume fraction, temperature, orientation of reinforcement, fiber nature
(permeable or impermeable), geometry of exposed surfaces, surface
protection, and diffusivity [184]. Therefore, proper treatment and
conditioning are required before and after the manufacturing of
NFRPCs. One of the outdoor applications of NFRPCs could be a liquid
storage tank. These liquid storage tanks may be exposed to various en
vironments and under different loads [185]. Therefore, the investigation
of these tanks in terms of physical, mechanical, structural, morpholog
ical, etc. is required. Various studies have been done on the environ
mental characterization of NFRPCs, and some of them are discussed in
detail further.
The degradation of wood flake composites under UV light and heat
was studied by Li [186]. After 205 days of conditioning, strength was
reduced by half of its value, and toughness was reduced by two-thirds.
Joseph et al. used UV radiation for the aging of sisal fiber composites
[184].
Water uptake was higher in the case of the higher fiber-loaded
composite due to its high cellulose content. Moreover, water uptake
was less in the case of chemically treated fiber-reinforced composites.
The tensile strength of composites was decreased with increased water
uptake, immersion time, and fiber loading. Dhakal et al. observed
hemp-reinforced composite and explored the mechanical properties of
hemp-reinforced composite conditioned in a water environment [187].
Composite consists of a higher volume fraction of hemp fiber showing
higher water absorption. Fickian and non-Fickian behavior of water
absorption in the composite was observed at room temperature and
elevated temperature of immersed water, respectively. Moreover, the
tensile and flexural properties decreased after wetting in both water
conditions. Yousif and Ku explored the use of coir fiber polymer com
posite for the manufacturing of liquid storage tanks [185]. Water ab
sorption followed by salt water was highest as compared to other liquids
due to its low viscosity. Bajpai et al. studied the tensile behavior of nettle
composite exposed under different environmental conditions for 512 h
Table 5
Summary of tribological performance of NFRPCs.
Composite type Wear type Working
medium
Remarks Reference
Sisal fiber-
reinforced epoxy
composite
Two-body
abrasive
wear
Dry Abrasive wear
decreased with
fiber incorporation.
Composite with
normal fiber
direction to the
wear direction has
minimum wear.
Composite with
longitudinal fiber
direction has
maximum wear.
[163]
Grewia optiva
reinforced
polymer
composites
Two-body
abrasive
wear
Dry Particle-reinforced
composite has
better wear
performance than
short and long-
fiber composites.
[164]
Pine needle
polymer
composites
Two-body
abrasive
wear
Dry 30% reinforced
composite showed
better mechanical
and wear
properties.
[165]
Bagasse fiber
composite
Two-body
abrasive
wear
Dry The abrasive wear
rate increased with
increased load and
grit size. The
parallel fiber-
oriented composite
showed higher
wear compared to
the anti-parallel
and normal-
oriented
composite.
[166]
Kenaf-reinforced
polyurethane
composite
Adhesive
wear
Wet Composites with
orthogonal
direction fiber
showed good
resistance to wear.
Lubrication and
cooling effect were
obtained with the
help of the wetting
medium.
[167]
Jute fiber-
reinforced
polypropylene
composite
Two-body
abrasive
wear
Dry The friction
coefficient of the
composite
decreased with the
incorporation of
natural fiber.
Wear occurs due to
plowing, crack
formation, and
detachment
fracturing of fiber
and matrix.
[176]
Nettle, Grewia
optiva, and sisal-
reinforced
polypropylene
composite
Adhesive
wear
Dry The wear
performance of
composites mainly
depends on
friction, surface
roughness, contact
condition,
temperature, and
test parameters.
[177]
Oil palm/epoxy
and kenaf/epoxy
composites
Two-body
abrasive
wear
Dry The wear rate of oil
palm fiber/epoxy
composite was
higher than kenaf/
epoxy composite
under higher
[170]
Table 5 (continued)
Composite type Wear type Working
medium
Remarks Reference
temperature
conditions.
Cyperus pangorei
fiber-reinforced
polyester
composites
Two-body
abrasive
wear
Dry Specific wear rate
increased with
applied load. The
coefficient of
friction decreased
at increased
contact pressure
and decreased
sliding velocity.
[174]
Prosopis juliflora,
Abuliton
indicum, and
Tapsi fiber-
reinforced epoxy
composites
Two-body
abrasive
wear
Dry Wear performance
increased by
incorporating
natural fibers.
At higher sliding
velocity friction
coefficient
increased.
[182]
Linen/jute-
reinforced
polyester
composite
Two-body
abrasive
wear
Dry The wear
performance of the
composite was
affected by the
percentage of fiber
reinforcement and
the direction of the
fiber also.
[178]
M.K. Singh et al.

12
[188]. It was found that the tensile strength of the composite was
reduced in all the cases. Maximum reduction in tensile strength was
shown in case of sunlight and river water. Whereas minimum reduction
was shown in case of soil. All the composites showed weight gain except
in case of sunlight. Maximum weight gain was in case of river water.
Saw et al. fabricated different combinations of composites by taking
jute and coir as reinforcement [189]. Water absorption behavior, me
chanical behavior, and morphological properties of the composites were
investigated. Composite having pure coir showed maximum water ab
sorption and swelling thickness as compared to pure jute composite.
From the water absorption and mechanical test, it was noted that the
dimensional stability of the hybrid composite having an outer layer of
jute was more. This may be due to the behavior of jute as a barrier to
water diffusion. Akil et al. studied the effect of environmental condi
tioning on the mechanical properties of jute/glass-reinforced polyester
composite [190]. Effect of water absorption until saturation (4076 h)
and temperature were studied. It was observed that composites tended
to deviate from Fickian behavior with increasing moisture resistance
due to the addition of glass fiber. The diffusion coefficient was depen
dent on fiber content and the stacking sequence. Under the aqueous
environment, tensile and flexural properties were reduced. Moreover, it
was observed that the hybrid composite showed superior mechanical
properties at higher temperatures. Mohammed et al. studied the effect of
weathering on the physical and mechanical properties of kenaf polyester
composite [191]. It was observed that the mechanical properties were
not retained after conditioning due to poor wettability and moisture
absorption by the fiber. Moreover, due to high moisture absorption
properties, the void formation took place at the interface of the fiber and
matrix. This reduced the mechanical properties. Yallew et al. fabricated
hemp, jute, and sisal-reinforced polypropylene composites and studied
their response to various conditioned environments [192]. All com
posites have shown weight gain as well as degradation after condition
ing. A reduction in tensile strength was obtained for all conditioned
composites. Maximum reduction was obtained in the case of 5%
NaOH-conditioned composite.
Manoj et al. fabricated kenaf/HDPE composite and placed it in
different environments such as deionized water, seawater, 5% NaOH,
vegetable oil, and diesel for six months [193]. It was observed from SEM
analysis that 5% NaOH solution has the highest environmental impact
on composite, whereas vegetable oil has the lowest impact (Fig. 9).
Daramola et al. explored the water uptake behavior of bamboo fiber/
HDPE composites [194]. Results revealed that, after 4% weight of
bamboo fiber addition, agglomeration took place, and the mechanical
properties decreased. Additionally, continuous absorption of water was
observed for up to 4 days, which stagnated afterward. Azammi et al.
studied the physical and damping properties of the kena
f/rubber/polyurethane composite [125]. It was observed that the com
posite consisting of treated kenaf fiber showed better resistance to water
absorption, thickness swelling, and damping factor. Fiore et al. treated
the jute and flax fiber with sodium bicarbonate and weathered the
composite in the marine environment [195]. It was observed that after
treatment the water absorption increased in the case of jute fiber.
Whereas, in the case of flax fiber, water absorption decreased. Treated
flax composite retained its flexural properties. Whereas, treated jute
composite showed reduced flexural properties. Sari et al. fabricated
different weight fractions of corn husk polyester composite and studied
the effect of water absorption on the mechanical properties of different
weight fraction composites [196]. Lesser water absorption by composite
consisting of lesser corn husk was observed. Best mechanical properties
were obtained for composite with 20% weight fraction of corn fiber. A
summary of major literature with significant remarks can be seen in
Table 6.
6. Conclusions
Natural fiber-reinforced polymer composites are applicable in a wide
range of applications from low load-bearing to medium load-bearing
structures. The properties of natural fibers are a significant challenge
for their proper usage in the fabrication of composites. The properties of
the natural fibers can be improved by the pre-treatment process which
plays a significant role in the prevention of debonding and failure.
Machining of the NFRPCs is an essential process for better finishing and
assembly applications. The quality of the machining majorly depends on
the machining parameters which makes it important to understand the
effect of different machining parameters. Some of the applications of
NFRPCs are related to adhesion and wear such as door panels, bicycle
paddles, chopping boards, etc. Most of the applications of NFRPCs are
indoors because of their permeability to moisture and environmental
degradation. Thus, these NRFPCs should be modified in such a way that
they can be used in a moisture environment also.
This review presents a detailed discussion of the mechanical
behavior, machining behavior, tribology behavior, and environmental
exposure behavior of NFRPCs. This review will work as a theoretical
groundwork for a researcher embarking in the research field of natural
Fig. 9. SEM micrographs of kenaf fiber-reinforced polymer composite placed in different environments for six months. Adapted from [193].
M.K. Singh et al.

13
fibers. From the literature, it can be concluded that nowadays NFRPCs
are used widely used in enormous applications and have some benefits
as well as limitations. One of the challenges is to increase the strength of
NFRPCs, which can be achieved by reinforcing ceramic particles or by
hybridizing them with synthetic fibers. Another major challenge is the
moisture absorption ability of NFRPCs, which can be addressed by the
careful coating of natural fibers.
Credit author contribution statement
Manoj Kumar Singh: Conceptualization, Methodology, Writing –
original draft, Data curation, Investigation; Renu Tewari: Writing –
review & editing; Sunny Zafar: Supervision, Writing – review & editing;
Sanjay Mavinkere Rangappa: Writing – review & editing; Suchart
Siengchin: Writing – review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Data availability
No data was used for the research described in the article.
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Physical, mechanical and biodegradable properties of kenaf/coir hybrid fiber
Table 6
Summary of environmental degradation of NFRPCs.
Composite type Environment Remarks Reference
Wood-HDPE
composite
Ultraviolet light,
oxidation, and heat
Initial strength
retained half and
toughness two-thirds
of initial values after
205 days of
conditioning.
Oxidative degradation
was not significant at
37 ◦
C but can degrade
at 67 ◦
C.
[186]
Sisal fiber-
reinforced
polypropylene
composites
Water and
ultraviolet radiation
Water absorption
increased with fiber
content and saturated
after some time.
Chemically modified
fiber composite
showed reduced water
absorption. Tensile
properties of UV-
exposed composites
decreased.
[184]
Hemp fiber-
reinforced
composite
Deionized water at
25 ◦
C and 100 ◦
C
Higher volume
fraction composite
showed higher water
absorption. Tensile
and flexural strength
decreased after
conditioning.
[187]
Coir fiber-
reinforced
polyester
composites
Brake oil, power
steering oil, diesel,
gasoline, engine oil,
salt water, and
water
Under wet conditions,
the tensile strength of
the composite
decreased. Tensile
strain increased under
wet conditions. The
wet composite showed
less interfacial
adhesion. Liquid
absorption was
inversely proportional
to the density of the
liquid.
[185]
Kenaf/pineapple
reinforced HDPE
composite
Water Better mechanical
properties were
obtained for the
composite having
kenaf fiber with a
higher aspect ratio.
Composite with a
higher fraction of
natural fiber showed
reduced strength after
wetting.
[197]
Nettle fiber-
reinforced
polypropylene
composite
River water, diesel,
freezing conditions,
sunlight (UV), and
soil
Tensile strength was
reduced in all cases.
River water and
sunlight were more
effective for
degradation.
Soil has a minimum
effect.
[188]
Jute/coir fiber-
reinforced
composite
Water Composite with pure
coir showed maximum
water absorption. Pure
jute composite showed
minimum water
absorption. Composite
with jute fiber at the
top layer showed
better properties.
[189]
Kenaf-reinforced
polyester
composite
Natural
environment
were not retained to
poor wettability and
moisture absorption
by the fiber. At high
[191]
Table 6 (continued)
Composite type Environment Remarks Reference
moisture absorption,
voids formed.
Jute, hemp, and
sisal-reinforced
polypropylene
composite
5% NaOH,
industrial liquid
waste, drinking
water, petrol, and
peanut oil
Weight gain and
degradation were
observed after
conditioning.
Maximum
degradation was
observed for 5%
NaOH-conditioned
composite.
[192]
Bamboo fiber-
reinforced HDPE
composite
Water Water is absorbed for
up to 4 days after that
stagnation occurred.
[194]
Corn hunk
reinforced
polyester
composite
Water Composite having
higher weight fraction
of corn husk showed
higher water
absorption. Composite
showed the best
mechanical properties
at 20 wt%.
[196]
M.K. Singh et al.

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