Natural fibre reinforced polylactic acid composites: A review

Abstract

Biopolymer-based composites have attracted the atten- tion of researchers and industries due to their eco- friendliness and environmental sustainability, as well as their suitability for a number of applications. Biocompo- sites containing natural fibers and biopolymers would be the ideal choice in the development of biodegradable materials for different applications. Polylactic acid (PLA) is an environmentally interesting biopolymer, which also has exclusive qualities, such as good transparency and processability, glossy appearance, and high rigidity, although it has some shortcomings as well, for example, its brittleness and high rate of crystallization. PLA-based natural fiber composites are entirely bio-based materials with promising biodegradability and mechanical proper- ties. Several research studies have been carried out on PLA and its composites to explore their potential to sub- stitute petroleum-based products, but until now there is no comprehensive review with up-to-date research data available in the literature. The aim of this review is to highlight the trends in the research and development of PLA and PLA-based natural fiber composites over the past few years. This review article covers current research efforts on the synthesis and biodegradation of PLA, its properties, trends, challenges and prospects in the field of PLA and its composites. PLA-based compo- sites are moderately abundant; and further research and development is needed for cost reduction and broader utilization.

Full text

Natural Fiber Reinforced Polylactic Acid Composites:
A Review
Ramengmawii Siakeng,
1
Mohammad Jawaid ,
1
Hidayah Ariffin,
2
S. M. Sapuan,
1
Mohammad Asim,
1
Naheed Saba
1
1
Laboratory of Biocomposite Technology, Institute of Tropical Forestry and Forest Products (INTROP),
Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia
2
Laboratory of Biopolymer and Derivatives, Institute of Tropical Forestry and Forest Products (INTROP),
Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia
Biopolymer-based composites have attracted the atten-
tion of researchers and industries due to their eco-
friendliness and environmental sustainability, as well as
their suitability for a number of applications. Biocompo-
sites containing natural fibers and biopolymers would be
the ideal choice in the development of biodegradable
materials for different applications. Polylactic acid (PLA)
is an environmentally interesting biopolymer, which also
has exclusive qualities, such as good transparency and
processability, glossy appearance, and high rigidity,
although it has some shortcomings as well, for example,
its brittleness and high rate of crystallization. PLA-based
natural fiber composites are entirely bio-based materials
with promising biodegradability and mechanical proper-
ties. Several research studies have been carried out on
PLA and its composites to explore their potential to sub-
stitute petroleum-based products, but until now there is
no comprehensive review with up-to-date research data
available in the literature. The aim of this review is to
highlight the trends in the research and development of
PLA and PLA-based natural fiber composites over the
past few years. This review article covers current
research efforts on the synthesis and biodegradation of
PLA, its properties, trends, challenges and prospects in
the field of PLA and its composites. PLA-based compo-
sites are moderately abundant; and further research and
development is needed for cost reduction and broader
utilization. POLYM. COMPOS., 00:000–000, 2018. V
C2018 Soci-
ety of Plastics Engineers
INTRODUCTION
Biopolymers are considered as potential alternatives to
conventional fuel-based polymers since the latter have
triggered current environmental concerns in terms of
environmental contamination, greenhouse gas emissions
and the diminution of fossil resources [1]. In this regard,
polylactic acid (PLA) has been the frontrunner among
many biopolymers due to its outstanding physical and
mechanical properties, renewability, biodegradability and
relatively good availability [2, 3]. PLA is one of the most
extensively researched and utilized biopolymers, it has
received wide attention for conventional consumption,
such as in packaging materials, production of fibers and,
lately, in composites for a variety of practical and
mechanical applications. PLA has a key position on the
market of eco-friendly polymers, and represents one of
the most promising candidates for future developments
[4].There is a rising trend to use natural fibers as rein-
forcements in polymer composites due to their flexibility
during processing, highly defined solidity, easy accessibil-
ity, biodegradability, low cost (on a volumetric basis) and
eco-friendliness [5]. Rising environmental and energy
concerns about depleting non-renewable fossil fuels have
further paved the way towards natural and environmen-
tally friendly polymers as alternatives to petroleum-based
resources [6].
Natural fiber-based polymer composites offer various
noteworthy advantages over conventional synthetic ones
in terms of biodegradability, eco-friendliness, cost, avail-
ability, low density, and so forth [7]. Natural fiber rein-
forced polymer composites are gaining more and more
recognition and further acceptance in food packaging, in
automobile, railway coach and aeroplane interiors, as well
as in storage devices [8], in building and structural appli-
cations [9]. A number of bio-based natural polymers are
being explored and studied for diverse applications
[10–12]. Scientific and technological progress, along with
consumer demands and outlook, continues to raise the
demand for resources, which leads to major issues of
material accessibility and environmental sustainability [5].
Correspondence to: M. Jawaid; e-mail: jawaid_md@yahoo.co.in
Contract grant sponsor: Universiti Putra Malaysia and Ministry of
Higher Education of Malaysia-HiCoE; contract grant numbers: 6369108,
9520200.
DOI 10.1002/pc.24747
Published online in Wiley Online Library (wileyonlinelibrary.com).
V
C2018 Society of Plastics Engineers
POLYMER COMPOSITES—2018

There is an escalating interest in research and develop-
ment of biocomposites materials, referred to as “green”
composites [13],which is triggered by the strong world-
wide demand for creating a civilization with circulating
resources [14]. In the case of biocomposites, a variety of
natural fibers are being incorporated into biopolymer,
such as PLA [15] and starch, to reinforce natural poly-
meric matrix materials and improve their overall proper-
ties [7].
Over the last few years, natural fiber composites have
been undergoing a notable transformation and turned out
to be increasingly adequate as regards their application
and their environmental impact, while fabrication pro-
cesses have been intensively researched and subsequently
applied [5].
The idea of using biopolymers as matrices for natural
fiber biocomposites is gaining constantly and increasing
approval day by day [5]. The current developments in
emerging natural-based polymers are amazing from the
technological point of view, as well as considering the
advances that have been made, and they reflect the rapid
expansion of biopolymers on the market [16]. The world-
wide biopolymer production capacity is likely to increase
from 2.33 million metric tons in 2013 to 3.45million met-
ric tons expected in 2020, and the focal product in terms
of production volumes will be PLA and starch-based plas-
tics [17].
Now, PLA has become a feasible material with a wide
range of applications, such as packaging, automotive, and
biomedical applications [18]. PLA is an environmentally
and commercially interesting biopolymer as it has many
exclusive qualities, such as good transparency, glossy
appearance, high rigidity, and good processability. On the
other hand, there are some limitations, especially its
inherent brittleness and poor toughness (<10% elongation
at break), slow degradation rate (hydrolysis of backbone
ester groups), which hinder its extensive application [19,
20]. PLA-based packaging materials are unlikely to face
market-based cost fluctuations, as these are produced by
natural resources [21]. A lot of efforts have been made to
develop fully biodegradable composite materials with the
combination of PLA and natural plant fibers [2]. Given
that both PLA and natural fibers come from renewable
resources and these are biodegradable and bio-
compostable, natural fiber/PLA composites are green and
eco-friendly materials that can also be recycled [6].
Completely biodegradable composite materials have
strong advantages due to their reduced processing costs
and reduced costs of waste disposal after service (natural
fiber reinforced composites can be disposed of simply by
landfill or incineration). Furthermore, biopolymers are
compatible with a variety of processing techniques, such
as injection molding, extrusion, compression molding,
and so forth, which can be used for composite fabrication
[2, 22].
Research efforts have been directed towards the
mechanical and thermal properties of the PLA polymer,
as it serves as a better alternative to available conven-
tional polymers [23]. Among the available thermoplastic
polymers, PLA is a naturally sourced one, whose global
production is estimated to exceed 140,000 tonnes per
annum. The production of PLA requires a low amount of
energy, which, in turn, leads to the release of less green-
house gases [24]. Industry sectors, specifically, automo-
tive interior manufacturers, have also realized the
significance of biocomposites and made a shift towards
the use of PLA, thus reducing the reliance on petroleum-
based polymers [14, 25, 26].
Life cycle assessment of PLA shows that it is a better
choice to substitute current interior component materials,
as its supply chain requires less transportation and thus
emits less greenhouse gases [18, 27, 28]. PLA-based com-
posites have a good opportunity to do well in the market
due to their biodegradability, renewability and less CO
2
emission. Moreover, by proper marketing, PLA will help
customers to realize that high performance products can
be made from renewable resources, which are healthier
for the environment [21].
NATURAL FIBERS
Natural fibers are potential alternative materials for the
composite industry, owing to their flexibility, eco-friendly
nature, low cost, renewability, local availability, as com-
pared with synthetic fibers [29]. In general, natural fibers
are considered as renewable and sustainable, but they are,
in fact, neither, as the plants from which natural fibers
are extracted are renewable and sustainable, but not the
fibers themselves [5]. Increasing research effort have
been made in recent years to enhance the utilization of
plant-based natural fibers, such as coconut coir, pineapple
leaf fiber, kenaf, bamboo fiber, and so forth, due to envi-
ronmental and sustainability concerns [6, 7]. Moreover,
polymer composites containing natural fibers have come
into the focus of manufacturing industries as well [30].
Table 1 shows annual production of natural fibers and
sources [31].
Fiber Source and Fiber Types
The plants that are used to produce natural fibers are
divided into primary and secondary, depending on their
utilization. Primary plants are those that are grown partic-
ularly for their fibers, such as cotton, jute, java-kapok,
hemp, kenaf, and sisal, whereas secondary plants are
those in which the fibers are produced as a by-product,
such as banana, coconut coir, pineapple, oil palm [32].
Six basic types of available natural fibers are classified
as follows: bast fibers (flax, jute, hemp, kenaf, and
ramie), leaf fibers (agave, abaca, and pineapple), fruit and
seed fibers (coir, cotton, and kapok), core fibers (kenaf,
hemp, and jute), grass and reed fibers (bamboo, elephant
grass, wheat, corn, and rice) and all the rest (wood and
2 POLYMER COMPOSITES—2018 DOI 10.1002/pc

roots). These different types of fibers are presented in the
flowchart in Fig. 1.
Structure and Properties
About all natural fibers, excluding cotton, are mainly
composed of cellulose, hemicelluloses, lignin, waxes, and
several water-soluble compounds.The quantity of cellu-
lose in a given fiber defines itsstrength and stiffness,
which are provided by the hydrogen bonds and other link-
ages present in the cellulose [33]. Numerous factors have
an effect on the overall properties of fibers, starting from
different production stages up to ultimate processing [34].
Many properties of natural fibers, particularly mechanical
properties, depend on the kind of cellulose present in the
fibers, because mechanical properties depend upon the
microfibril angles, the degree of polymerization of the
cellulose, and total cellulose content in the fibers [35–39].
Thus, fiber selection will mainly depend on its
strength, in accordance with the strength requirements for
different uses. Table 2 enlists the physical and mechanical
properties of various natural fibers. Density, microfibril
angle, young’s modulus, and fiber elongation are some of
the chief variables that establish the overall properties of
the fibers [31] . In addition to these, the properties of nat-
ural fibers depend on the ecotype, maturity and location
FIG. 1. Classification and sub-classification of fiber [31, 137].
TABLE 1. Annual production of natural fibers and sources [39].
Fibers
World production
(10
3
Tons) Origin Fiber
World production
(10
3
Tons) Origin
Abaca 70 Stem Hemp 215 Stem
Bamboo 10,000 Stem Jute 2,500 Stem
Banana 200 stem Kenaf 770 Stem
Broom Abundant Whole plant Linseed Abundant Fruit
Coir/coconut 100 Fruit pineapple Abundant Leaf
Cotton 18,500 Stem Carua - Leaf
Elephant Grass Abundant Stem Nettles Abundant Stem
Flax 810 Stem Oil Palm Fruit Abundant Fruit bunch
Palm Abundant Stem/fruit Rice Straw Abundant Straw
Ramie 100 Stem Sisal 380 Stem
Roselle 250 Stem Sun Hemp 70 Stem
Rice Husk Abundant Fruit/grain Wheat Straw Abundant Straw
Sugarcane bagasse 75,000 Stem Wood 1,750,000 Stem
DOI 10.1002/pc POLYMER COMPOSITES—2018 3

of the plant, as well as on the fiber extraction process.
Natural fibers, such as coir, kenaf, hemp, pineapple leaf
fibers, and flax, are incorporated into composites and
have become promising alternatives to synthetic fibers in
many specific applications [40].
By comparison, synthetic fibers have better mechanical
and physical properties than natural fibers [39]. Plant
fibers, which are lignocellulosic in nature, are made up of
cellulose, hemicelluloses, lignin, pectin, and waxy sub-
stances. Cellulose consists of many fibrils, which associ-
ate due to hydrogen bonds, providing strength, and
inflexibility [41]. The chemical composition of natural
fibers, such as coir, banana, pineapple leaf, sisal, Palmyra,
Sun hemp, and others, is shown in Table 3. The composi-
tion depends on the fiber origin, as well as on the envi-
ronmental and soil conditions during plant growth [42].
Treatment of Natural Fibers
A lot of treatments have been developed in recent
years to improve interfacial bonding and achieve better
values for mechanical and water resistance properties of
natural fibers [42, 43]. The most serious drawback of
these natural fibers is their hydrophilic nature, which
causes a fragile interfacial bonding between fiber and
matrix in polymer composites [30]. Various physical
impurities and the hydroxyl groups present on the fiber
surface determine the fate of natural fibers as enforcement
materials [44]. Several research studies have been carried
out on the surface modification and treatments of natural
fibers to achieve the desired qualities [45, 46]. However,
the mechanical performance of a composite material also
depends on the orientation and nature of fibers and
TABLE 2. Physical and mechanical properties of fibers [39].
Fibers
Density
(g/cm3)
Elongation
(%)
Tensile strength
(MPa)
Moisture
absorption
Young’s
modulus (GPa)
Cotton 1.5–1.6 3.0–10.0 287–597 8–25 5.5–12.6
Jute 1.3–1.46 1.5–1.8 393–800 12 10–30
Flax 1.4–1.5 1.2–3.2 345–1,500 7 27.6–80
Hemp 1.48 1.6 550–900 8 70
Ramie 1.5 2.0–3.8 220–938 12–17 44–128
Sisal 1.33–1.5 2.0–14 400–700 11 9.0–38.0
Coir 1.2 15.0–30.0 175–220 10 4.0–6.0
Softwood 1.5 _ 1000 _ 40.0
E–glass 2.5 2.5–3.0 2,000–3,500 _ 70.0
S–glass 2.5 2.8 4,570 _ 86.0
Aramide(normal) 1.4 3.3–3.7 3,000–3,150 _ 63.0–67.0
Carbon (standard) 1.4 1.4–1.8 4,000 _ 230.0–40.0
TABLE 3. Chemical composition of natural fibers.
Fibers
Cellu-lose
(wt%)
Hemi-cellulose
(wt%)
Lignin
(wt%)
Pectin
(wt%)
Ash
(wt%)
Waxes
(wt%)
Moisture
content (%)
Micro-fibrillar
Angle (deg) References
Abaca 56–63 20–25 7–9 - 3 3 5–10 - [31, 49, 139]
Bagasse 55.2 16.8 25.3 - 1.5-5 - 8.8 - [5]
Bamboo 73.83 12.49 10.15 0.37 - - 3.16 - [31, 140]
Banana 60–65 6–8 5–10 - 9.6 - 10–15 11 [49, 141]
Coir 32–43 0.15–0.25 40–45 - 2.7–10.2 - 10–12 30–39 [5, 31, 49]
Cotton 82.7 5.7 - - - - 1 20–30 [31, 49]
Curaua 73.6 9.9 7.5 - - - - - [5]
Date-palm 30.3–33.5 59.5 27–31.2 – 3.9–9.6 - - - [142]
Elephant Grass 45.6 - 17.7 - 5 - - - [143]
Flax 71 18.6–20.6 2.2 2.3 - 1.7 8–12 5–10 [5, 39, 49]
Hemp 68 15 10 1 0.8 0.8 6.2–12 2–6.2 [5, 39, 49]
Henequen 77.6 4–8 13.1 - - - - - [39]
Jute 61–71 14–20 12–13 - 0.8 0.5 12.5–13.7 8 [5, 49]
Kenaf 45–57 21.5 8–13 3–5 2–5 - - - [39, 49]
Oilpalm 65 - 29 - 2.4 - - 46 [5]
Pine-apple 70–80 18.8 12.7 1.1-1.2 0.9-1.2 3.2–4.2 11.8 8–15 [39]
Ramie 68.6–76.2 13–16 0.6–0.7 1.9 - 0.3 7.5–17 7.5 [39]
Rice husk 35–45 19–25 20 - - 14–17 - - [5]
Rice straw 41–57 33 8–19 - 14–20 8–35 6.5 - [5]
Sisal 65 12 9.9 10 0.6–1 2 10–22 10–22 [5, 49]
Sugar palm 53.41 7.45 24.92 - 4.27 - 8.7 - [144]
Wheat straw 38–45 15–31 12–20 - 6.8 - 10 - [5]
4 POLYMER COMPOSITES—2018 DOI 10.1002/pc

matrix, while the fibers-matrix bonding plays a very
important role as well [47]. The weak bonding between
fibers adversely affects the mechanical strength of fiber-
board [44, 48]. The main setback of natural fiber compo-
sites lies in the hydrophilic nature of the natural fiber and
the hydrophobic nature of the polymer matrix [48]. The
innate incompatibility between these two phases results in
weak bonding at the interface of the composite. Chemical,
physical, or biological treatments of natural fiber helps
remove the impurities from the fiber surface and can
reduce its hydrophilicity, while increasing fiber/matrix
compatibility [42, 49, 50], as illustrated in Table 4.
POLYLACTIC ACID
Growing environmental awareness and public concerns
have led to the hunt for biodegradable plastics as an alter-
native to conventional plastics [51–53]. Biodegradable
polymers are of different types based on their production
or origin, as can be seen in Fig. 2. Biodegradable plastics,
such as PLA, cellulose esters, starch plastics, aliphatic
polyesters/co-polyesters and poly(-caprolactone), are
now emerging as prospective replacements for non-
biodegradable conventional plastics in various applica-
tions [23, 54, 55]. PLA has been recognized since 1845,
but was never commercialized until the early 90s [56].
PLA is an aliphatic polyester with lactic acid (LA) as its
basic constitutional unit. PLA or polylactide [PLA:
(_CH(CH3)_CO_O_)n] is the most extensively studied and
promising biopolymer with the potential to replace conven-
tional petroleum-based polymers [57, 58]. PLA is a renew-
able, biodegradable, recyclable and compostable polymer
that exhibits outstanding processing ability [59]. PLA has
several industrial applications, such as in packaging [60]
and textiles, biomedical [58] and structural applications
[61, 62] as well as usage in automotive manufacturing
industries due to its distinctive properties [56].
Synthesis
The PLA polymer can be synthesized from LA by
direct polymerization (solution-polycondensation and
melt-polycondensation) reaction or ring-opening polymer-
ization (ROP) of the lactide monomer. Nonetheless, due
to environmental concerns and intrinsic disadvantages of
the usual synthesis methodology, other synthesis methods,
such as biosynthesis of PLA by enzymatic means, as well
as new solutions incorporating non-toxic catalysts, such
as Magnesium(Mg), Calcium (Ca), Zinc (Zn), and so
forth, are being developed [63–65].
Direct Polymerization. As LA monomer has both
hydroxyl (–OH) and carboxyl (–COOH) groups, which
are necessary for polymerization, the self-condensation
reaction can take place directly. There are two types of
direct polymerization. The first one is solution polycon-
densation, in which an organic solvent capable of
dissolving the LA without disturbing the reaction is added,
and then the mixture is refluxed with elimination of the
water generated in the polycondensation process, which is
advantageous to get a high molecular weight, but is suscep-
tible to impurities from the solvent and a variety of side
reactions, such as racemization and trans-esterification. It
also consumes huge volumes of organic solvents, which
can harm the environment. The second type is melt poly-
condensation, which can proceed without any organic sol-
vent. This technique can lower the cost of production
significantly due to the simplified processes; however,
major problems, such as high reaction temperature and
low-molecular weight PLA, still need to be solved. These
given one-step polymerization processes are comparatively
economical and simple to control, but the polymer pro-
duced from these reactions generally has an inadequately
low-molecular weight [58, 63, 65].
Ring-Opening Polymerization. ROP is an important
successful method to produce high molecular weight
PLA, in which PLA is obtained by the use of a catalyst
with the monomer under vacuum ambience. Controlling
the ratio and sequence of D- and L-LA units is possible
in the final polymer by controlling the temperatures and
time in combination with the type of catalyst used and its
concentration in the process [63, 66].
Bioprocess/Biosynthesis. This is a chain-extension pro-
cess, where carboxyl- or hydroxyl-terminated low-molec-
ular weight PLA from direct polymerization is linked
together through a chain extender, such as hexamethylene
diisocyanate, which is a bi-functional complex carrying
highly reactive functional groups. The biosynthesis
method of PLA is illustratedin Fig. 3 and the properties
of the produced PLA can be affected by this procedure to
some extent [58, 67].
New Approaches. To resolve the impending pollution
problems caused by heavy metal catalysts in direct and
ROP, several non-toxic catalysts derived from magne-
sium, zinc, alkali metals and aluminum [67] have been
developed and tested for ROP of lactides for PLA synthe-
sis. Table 5 highlights comparison of different types of
synthesis of PLA, with regard to their advantages and
disadvantages.
Properties
PLA is a commercially and environmentally appealing
biopolymer, as it has numerous distinctive characteristics,
such as good transparency, high rigidity, glossy appear-
ance and good processability, regardless of some of its
limitations, particularly, its inherent brittleness and poor
toughness (<10% elongation at break), time-consuming
degradation rate (hydrolysis of backbone ester groups),
which encumber its widespread application [19, 20]. PLA
is a thermoplastic amorphous or semi-crystalline polymer,
DOI 10.1002/pc POLYMER COMPOSITES—2018 5

TABLE 4. Chemical, Physical, and Biological treatments of natural fibers.
Treatment Effects/Results References
Chemical treatments
Alkali treatment (Mercerization) It reduces fiber diameter by breaking the fiber bundle, thereby increases the surface
are of fiber which results in good adhesion with the matrix and improves mechani-
cal and thermal behaviors of the composite.
[48], [145], [146]
Silane treatment It is one of the most effective coupling agents for natural fibers surface modification.
It is a multifunctional molecule which deposit on the fiber surface which makes bet-
ter linkage with the matrix through a siloxane bridge. It improves the fiber matrix
adhesion and stabilizes the composite properties
[48], [49], [103], [147]
Acetylation treatment This treatment is known as esterification process for plasticizing natural fibers. The
reaction of Acetyl group (CH3CO) with the hydroxyl groups (–OH) reduce the
hydrophilicity of natural fiber and improves dimensional stability of the composites
[42], [49]
Benzoylation treatment. Benzoyl chloride is used as an agent to decrease hydrophilic nature of the natural
fibers and improve its compatibility with the matrix, which therefore enhance the
thermal stability and strength of the composite
[42], [148]
Peroxide treatment. It improves the interfacial adhesion, thermal stability and reduces the moisture absorp-
tion of fiber and matrix.
[83], [149], [150]
Maleated coupling agents. It gives proficient interaction with the functional surface of the fiber and matrix which
reduces the melting temperature and lowers stiffness of the fibers.
[150], [151]
Sodium chlorite (NaClO
2
)
treatment.
It is used for bleaching of fibers in acid solution. NaClO
2
is acidified and releases
choleric acid (HClO
2
), which undergoes an oxidation reaction and forms chlorine
dioxide (ClO
2
). ClO
2
reacts with lignin constituents and remove it from the fiber
thus improving the adhesive properties of the fiber.
[152], [153]
Acrylation and acrylonitrile
grafting.
Acrylic acid reacts with the hydroxyl groups of the fiber and provides more free radi-
cal of reactive cellulose macro-radicals which helps to create good interfacial bond-
ing. It reduces hydroxyl groups from the fiber and improves moisture resistance.
[42], [154]
Calcium hydroxide
Ca(OH)
2
treatment
It helps in degradation of the amorphous materials present in the fiber structure. It is
connected with increasing the crystallinity index of cellulose and improved thermal
stability
[44], [155]
Isocyanate treatment: Act as a coupling agent in fiber surface modification and helps in better moisture
resistance mechanism of the fiber and provides better bonding with the matrix to
improve the composite properties.
[50], [127], [150]
Triazine treatment. Triazine reacts with the hydroxyl groups of cellulose and lignin in the natural fibers
that improves its moisture resistance properties.
[156], [157]
Stearic acid treatment Stearic acid in ethyl alcohol solution is used for treating natural fiber surfaces to facil-
itate better interfacial bonding of fiber and matrix.
[27], [158].
Permanganate treatment Potassium permanganate (KMnO4) in acetone solution is used for treating fiber sur-
face for enhancing interfacial adhesion between natural fibers and matrix by reduc-
tion of hydrophilicity of the fiber and increase thermal stability of the fiber
[42], [158–160]].
Fatty acid derivate (Oleoyl
chloride) treatment.
Fatty acid derivative act as a coupling agent to alter natural fiber surface to improves
its adhesion with the matrix and wettability properties of natural fiber in
composites.
[157], [161], [162]].
Physical treatments
Plasma treatment It offers a unique approach of modifying the physio-chemical structures fiber surfaces
without altering the bulk structures and characteristics of the composites. Plasma
has been regarded as a clean and dry method for fiber treatments and it resulted in
increased surface roughness, which facilitates better dispersion within the matrices.
[49], [163]
Corona discharge It is a green technique for modification of fibers and the subsequent reinforcement
into composites that tend to show significant improvements in mechanical
properties.
[49], [164]
Steam explosion Steam explosion involves heating of natural fiber materials at high temperatures and
pressures followed by mechanical disruption of the pre-treated material by violent
discharge (explosion) into a collecting tank. It results in fiber properties improve-
ments which include smoother surface, reduced stiffness, improved bending proper-
ties and dispersion in the matrices.
[49], [165], [166]
Biological treatments
Enzymatic treatment Enzyme treatments have been shown to be an effective method of treating natural
fibers to improve the interface between the fibers and the matrix, leading to
improved mechanical properties of the composites. However, it is expensive and
limited to pilot scale only.
[50], [167], [168]
Fungal treatment It removes non-cellulosic components and lignin from the fiber surface by the action
of specific enzymes as well as increases hemicelluloses solubility and thus reduce
hydrophobic tendency of the fiber.
[42], [50]
6 POLYMER COMPOSITES—2018 DOI 10.1002/pc

according to its stereo-chemical structure and thermal
record [68], which has the potential to substitute tradi-
tional polymers, such as polyethylene terephthalate,
polystyrene (PS), or polycarbonate for a variety of appli-
cations [54, 59]. Selected physico-chemical properties of
PLA are shown in Table 6. The homopolymer of LA is a
white powder material at room temperature with a melt-
ing point (T
m
) of about 1758C and glass transition (T
g
)of
about 558C. PLA is a colourless, glossy, high modulus
thermoplastic polymer with properties very similar to
those of PS [67]. However, the melt rheology of PLA is
different from that of other commodity plastics, because
of its chemical stability [68].
Biodegradation of PLA
The biodegradation of plastics/polymers is an intricate
process with multiple steps [69] and PLA is entirely
biodegradable. The degradation of the PLA polymer
occurs by hydrolysis to LA, which is converted into
water and carbon monoxide by microorganisms. Biodeg-
radation occurs within two weeks by composting
together with other biomass, such as compost soil, and
the material can entirely disappear within 3–4 weeks. In
the early degradation stage, PLA is broken down into
LA monomers, where the ester bonds are cleaved hydro-
lytically [70]. This degradation stage into smaller frag-
ments is a rate limiting step in the processes. When the
molecular weight is below 20,000 g/mol, PLA becomes
water soluble [71]. The subsequent metabolic activity
after uptake by microorganisms breaks the polymers
down into metabolic end products, such as water and
carbon dioxide, and a portion of the carbon is converted
into biomass [72]. Ecological degradation might be an
equally suitable name for the overall mechanism of PLA
biodegradation, due to the simultaneous action of abiotic
and biotic processes. Composting of PLA works effi-
ciently under adequate conditions due to the mutual
effects of hydrolysis and microbial activity [2]. These
microorganisms release extracellular enzymes, which can
cause cleavage of the PLA chains, which helps in the
degradation process [71].
The biodegradation mechanism of PLA polymers is
displayed in Fig. 4a and b. The biodegradation of PLA
has been the subject of many studies and it has been
found to depend on a variety of factors, such as tempera-
ture, time, impurities, and residual catalyst concentration,
but mainly its molecular weight. The catalysts used in the
process reduce the degradation temperature and increase
the rate of PLA biodegradation [67, 73–75].
NATURAL FIBER/PLA COMPOSITES
The increased utilization of petroleum-based polymer
composites has amplified the “burden” on the environ-
ment. Growing environmental public awareness has
forced manufacturers and industries to look for more eco-
friendly materials for their products. For example, compo-
sites based on natural fibers with polypropylene as a
matrix material are very common for automotive applica-
tions these days [38]. Scarce research is known to have
been dedicated to the study of composites with renewable
polymer matrices, which are produced from plants, such
as PLA, cellulose esters, polyhydroxyalkonoates, and so
forth. For this reason, the research and development of
biopolymer-based natural fiber composites has become an
area of huge interest in materials science and technology
studies from both technical and environmental perspec-
tives [76].
Biodegradable polymers, for example, PLA, starch and
polyhydoxyalkaoates, can be reinforced with natural fibers
to fabricate environmentally favorable and biodegradable
composites [77]. Biocomposites are biodegradable, sustain-
able, and renewable, helping reduce the dependency on
exhaustible petroleum resources, as well as the environ-
mental burden [6]. The good mechanical properties and
outstanding barrier capability of PLA can be exploited to
produce biomaterials suitable for a variety of applications
[24]. The disadvantages of PLA, such as its inherent
FIG. 2. Classification of Biopolymer according to origin and produc-
tion [24]. [Color figure can be viewed at wileyonlinelibrary.com]
FIG. 3. Biosynthesis of PLA [67]. [Color figure can be viewed at
wileyonlinelibrary.com]
DOI 10.1002/pc POLYMER COMPOSITES—2018 7

brittleness [78], water sensitivity and low-impact strength
[79], can be duly enhanced by the addition of fibers and/or
fillers, which is a convenient way to improve the overall
properties of the PLA polymer [18, 80].
Properties of Natural Fiber/PLA Composites
There has been a keen interest in natural fiber reinforced
PLA composites in recent years. The realization of a bio-
composites, a completely biodegradable material, is essen-
tially possible by combining a biodegradable matrix with
natural fiber reinforcement [38]. Natural fiber-based PLA
composites present the opportunity of achieving several
desired properties, such as material stiffness, degradability,
and strength. Research on natural fiber-based PLA compo-
sites revealed that they have certain advantages, such as
good processability, high-specific strength, compostability,
high-toughness, renewability and recyclability [58].
Physical Properties. Physical properties include den-
sity, moisture/water absorption, void content, ageing, and
thickness swelling, which are commonly used for quality
control purposes and to measure the degradation of the
composite materials. The effect of moisture content in
natural fibers on semi-crystalline grade PLA degradation
and on the composite’s mechanical performance was
investigated using fibers such as cotton, flax, and ramie,
containing 6–9 mass% moisture before drying and 0.2–
0.4 mass% after drying, by intrinsic viscosity and melt
flow index analyses, which revealed that the effect of
moisture in the fibers had a parallel and small effect on
PLA degradation [81]. The durability of such composites
becomes a new main concern due to the intrinsic moisture
absorption property of natural fibers and the biodegrad-
ability of the PLA polymer in the environment. Tawakkal
et al. [82] prepared kenaf-derived cellulose (KDC) filled
PLA composites and found out that the composite con-
taining the highest quantity of KDC (60 wt.%) was
denser, compared with the pure PLA, and had a moisture
uptake of 12%, which is conspicuously low for a bio-
composites system. However, most research studies these
days are generally focused on fabricating composites and
improving their mechanical properties [22]. Sawpan et al.
[83] investigated surface modification of hemp fibers as a
means of improving the interfacial shear strength (IFSS)
of hemp fiber reinforced PLA and unsaturated polyester
composites. Hemp fibers were treated with sodium
hydroxide, acetic anhydride, maleic anhydride and silane,
and it has been found that the IFSS of the PLA/hemp
fiber samples increased after the fiber treatment. The
higher IFSS could be explained by enhanced bonding of
the PLA matrix with the treated fibers, as well as by the
increased PLA transcrystallinity. A number of the studies
investigating the physical properties of natural fiber-based
PLA composites are tabulated in Table 7.
Mechanical Properties. Mechanical properties area
subject of high interest as they belong to the most
TABLE 5. Comparison of different PLA synthesis methods.
Synthesis methods Advantages Disadvantages
Solution polycondensation One-step, economical and easy to control High Impurities, side reactions, pollution, low-
molecular weight PLA
Melt polycondensation One-step, economical and easy to control High reaction temperature, sensitivity to reaction
conditions, low-molecular weight PLA
ROP High molecular weight PLA production Requires strict purity of the lactide monomer, rel-
atively high cost
New solutions (new catalysts, polymeri-
zation conditions, etc)
Efficient, non-toxic, no pollution, high molecular
weight PLA, etc
Under development
Bioprocess/Biosynthesis One-step, efficient, non-toxic, no pollution, low cost. Slow process and Under development
TABLE 6. Selected physio-chemical properties of PLA [58, 67, 136].
Properties
PDLLA
poly(D,L-lactic acid)
PDLA
Poly(D-lactic acid)
PLLA
poly(L-lactic acid)
Structure Amorphous Crystalline Hemicrystalline
Melting temperature (T
m
)/8C 120–170 120–150 173–178
Glass transition temperature (T
g
)/8C 43–53 40–60 55–80
Density (g/cm
3
) 1.25 1.248 1.290
Decomposition temperature/8C 185–200 200 200
Elongation at break/(% Variable 20–30 20–30
Breaking strength/(g/d Variable 4–5 5–6
Half-life in 378C normal saline 2–3 months 4–6 months 4–6 months
Solubility PLLA solvents and acetone, ethyl lactate, tetrahydrofuran, ethyl
acetate, dimethylsulfoxide, N,N xylene and dimethylformamide.
Chloroform, Furan,
Dioxaneanddioxolane
8 POLYMER COMPOSITES—2018 DOI 10.1002/pc

important factors in composite characterization. The
fibers used as fillers in PLA composites provide entirely
natural materials with enhanced mechanical properties
[84]. The effect of different fibers on the behavior of the
resulting composite material has been studied through
various characterization techniques. The factors consid-
ered in mechanical characterization are fiber volume/
weight fraction, stacking sequence of the fiber layers,
methods of processing, treatment of fibers and the effect
of environmental conditions [85]. To enhance the
mechanical properties, natural fibers, such as hemp [86]
ramie [87], kenaf [88, 89], rice straw [19], abaca, wood,
coir [89], jute, sisal [80], bamboo [90, 91], rice husk [86],
oil palm [92], and flax [80], have been used to reinforce
PLA.
The investigation of mechanical properties has been
mainly focused on tensile strength, tensile modulus and
elongation at break of natural fiber-based PLA compo-
sites, and less on other mechanical properties, such as
fracture and impact. Shih et al. [93] fabricated biocompo-
sites from recycled disposable chopsticks and a PLA
matrix by the melt-mixing process. Mechanical test
results showed that the tensile strength of the composites
noticeably increased with the fiber content, attaining 115
MPa in the composites reinforced with 40% fibers, which
was about three times higher than that of the pure PLA.
Islam et al. [94] conducted tests on alkali-treated indus-
trial hemp fiber (30 wt.%) reinforced PLA composites
and found a Young’s modulus of 10.9 GPa and tensile
strength of 82.9 MPa. Kim and Netravali [95] developed
alkali-treated sisal fiber reinforced soy protein resin-based
biocomposites and concluded that the treatment improved
the fracture stress and stiffness by 12.2 and 36.2%,
respectively, in the sisal fiber, while the fracture strain
and toughness decreased. Increased Young’s modulus
(3.73 10.247 GPa) and decreased tensile strength
(37.2 12.0 MPa) were also observed by Gregorova et al.
[96] for PLA/spruce wood flour (40 wt%) biocomposites
with various surface treatments of the wood flour. Silane
treatment was found most efficient in improving the inter-
facial adhesion between the PLA matrix and the spruce
wood flour. Qin et al. [19] studied the morphological and
mechanical properties of polybutyl acrylate (PBA) modi-
fied rice straw fiber (RSF) reinforced PLA biocomposites
and found a good interfacial adhesion between PLA and
RSF and good dispersion of RSF in the polymer by scan-
ning electron microscopy. However, poor interfacial adhe-
sion between PLA and RSF was observed when the PBA
FIG. 4. (a) Flowchart of biodegradation mechanism of biodegradable polymers [69] (b) Acid-catalyzed
hydrolysis of PLA [138].
TABLE 7. Recent works on physical characterization of PLA/natural fibers composites.
Composites Fiber % Physical characterization References
PLA/jute 40 Moisture absorption, ageing behavior in hygrothermal environment. [22]
PLA/hemp 30 Influence of accelerated ageing on physico-mechanical properties of composite [110]
PLA/hemp 30 Water absorption,Influence of hygrothermal ageing on physico-mechanical properties [109]
PLA/KDC 0–60 Density, water absorption, [82]
PLA/bamboo fabric 51 Density, Moisture content [169]
PLA/ramie/flax/cotton 0–50 Effect of water content of natural fibers on PLA Degradation [81]
DOI 10.1002/pc POLYMER COMPOSITES—2018 9

content was increased. This was confirmed by the tensile
tests, which showed the tensile strength of the PLA/RSF
biocomposites rose notably to 6 MPa and then rapidly
decreased when the content of PBA exceeded 7.98 wt. %
in the biocomposites. They concluded that the addition of
PBA to PLA led to a decrease in the tensile strength of
the biocomposites, while the elongation at break was
somewhat increased. Yu et al. [87] developed surface
treated ramie fiber reinforced PLA biocomposites and
found that PLA-based composites had better tensile
strength than the neat PLA. Mercerization of fibers (alkali
and silane) improved the tensile strength and strain of the
composites, and the maximum strength was 64.24 MPa
(the composite treated by alkali) due to good interfacial
adhesion between the ramie fibers and the PLA matrix.
The mechanical properties of the studied biocomposites
are given in Table 8, as reported by different researchers.
Thermal Properties. Natural fiber composites are rec-
ognized to provide good thermal, low-density, low-cost,
non-abrasive processing materials. Knowledge of the ther-
mal behavior of mixtures based on fiber materials and
polymers is very important from the processing point of
view [97]. It has been reported in the literature that natu-
ral fibers are less thermally stable than synthetic fibers
[98]. Mohamad et al. [99] obtained improved T
50
decom-
position temperature with the addition of oil palm micro-
crystalline cellulose (MCC) to PLA, and revealed an
increase in the heat resistance of the composites by TGA.
On the other hand, Yussuf et al. [84], in their comparative
study of PLA/kenaf and PLA/rice husk, found reduced
thermal stability with the addition of kenaf and rice husk
fibers in the composites, as compared with the neat PLA.
The decrement was most prominent in the PLA-rice husk
composite. Thermal properties of composites can be stud-
ied using differential scanning calorimetry (DSC),
dynamic mechanical analysis (DMA), thermo-gravimetric
analysis (TGA), and thermo-mechanical analysis (TMA).
Available literature shows that less attention has been
paid to the TMA study of thermal properties, therefore
TMA characterization should be carried out in the future
to enhance understanding of the influence of incorporat-
ing natural fiber into PLA composites. Recent works on
the thermal properties of PLA/natural fiber composites by
different researchers are outlined in Table 9.
Flame Retardancy. The flammability of natural fibers
is determined by their chemical composition. Thus, higher
flammability is mainly induced by higher cellulose con-
tent, while higher lignin content results in higher char for-
mation. The presence of ash or silica enhances the fire
resistance of natural fibers as a whole, and high crystal-
linity and lower polymerization improve the fire resis-
tance in the case of fiber microstructure [100, 101].
Flammability is a very important parameter, often limit-
ing the application of a composite to a given area [102].
A polymer behaves poorly and becomes very unsafe
when exposed to high temperature. To improve this con-
dition, fibers are incorporated into it, but the improvement
is limited in the case of natural fibers because they are
more flammable compared with synthetic fibers. Various
methods can be used to boost the durability of natural
fiber composites against thermal degradation [103]. The
most widely used means to obtain flame retardancy in
natural fiber composites is the incorporation of fire retard-
ants (FRs).To improve the fire retardancy of natural fiber/
PLA composites, FRs such as ammonium polyphosphate
(APP) or nanoclay can be added to the polymer during
the melting process [104]. FRs for biocomposites should
be temperature-resistant to avoid disintegration during
processing and should not contain any halogen or any
other compounds that produce toxic gases [105] and can
be hazardous to human health.
The incorporation of organic FRs in biocomposites can
produce toxic products after thermal decay and combus-
tion, which has brought into focus inorganic compounds,
such as metallic hydroxide additives, as FRs for environ-
mental and health safety reasons [106]. According to
Stark et al. [104], the most effective substances to pro-
duce FRs are compounds containing bromine, chlorine, or
phosphorous, or two of these elements. Nonetheless, it
must be noted that the addition of FRs into composites
decreased the outdoor durability/strength, interfacial adhe-
sion, orientation and dispersion of fillers, thus an optimal
blend ratio shall be employed to attain a balance between
composites fire retardancy and durability [22, 105, 107,
108]. Some examples of FRs used in PLA biocomposites
are given in Table 10.
Biodegradability. The resilience or stability of natural
fiber composites upon exposure to UV-light is of particu-
lar concern, as UV radiation can cause decay or modifica-
tion of surface chemistry in the composites, commonly
known as photodegradation [109]. Natural fiber/polymer
composites exposed to direct sunlight are subjected to
photoradiation, which disrupts the chemical bonds in
organic polymers, causing yellowing, color fading, and
deterioration of many other physical and mechanical
properties, the deterioration being higher under wet condi-
tions, especially involving high humidity and microbial
activity [84, 92, 97]. After the weathering/degradation
period, the strength of the composites decreased because
of the degradation of both fibers and polymers. Ageing or
degradation in natural fibers occurs due to absorption of
ultraviolet radiation by the lignin present in the fiber, the
development of quinoid structures, Norrish reactions and
reactions of photo-yellowing that occur in lignin [103,
110, 111]. Figure 5 shows a step-by step flowchart of the
UV degradation process of natural fiber-based polymer
composites.
The biodegradability of natural fiber-based polymeric
composites has been studied using different methods,
such as natural weathering, accelerated weathering, soil
burial [112] in normal garden soils or composts,
10 POLYMER COMPOSITES—2018 DOI 10.1002/pc

TABLE 8. Mechanical properties of natural fiber-reinforced PLA composites.
Fibers
Fiber
(%) Processing
Tensile
strength
(MPa)
Young’s
modulus
(GPa)
Flexural
strength
(MPa)
Flexural
modulus
(GPa)
Impact
strength
(KJ/m
2
) References
Chopped recycled newspa-
per cellulose fiber
30 Injection molding 67.9 60.5 5.3 60.4 106.2 61.8 5.4 23.5 60.4 J/m [89]
Coconut 0.5 Extrusion 1
compression molding
67.99 63.75 2.37 60.15 102.9 61.3 81.37 61.23
KJ/mm
2
[89]
Cordenka 25 Injection molding 108 4.2 8.5 [89]
Cotton Compression molding 4.12 62 4.24 60.64 28.7 64.4 [89]
Man-made cellulose 30 Injection molding 92 8.032 152 7.89 7.9 [89]
Man-made cellulose fibers Injection molding 92 64.7 5.860.15 11.25 [89]
Abaca 30 - 74 5.85 124 6.51 5.3 [89]
Recycled cellulose fibers 30 Injection molding 82.6 63.8 6.2 60.1 21J/m [89]
Flax (Random orientation) 30 Compression molding 53 8.3 _ _ [80]
Flax(Random orientation) 30 Injection molding 54.15 6.31 _ 11.13 [80]
Flax(Random orientation) 25 Film -stacking 81.3 8.85 _ _ [80]
Cellulose Nanowhiskers
freeze dried
5 Chloroform solution casting 37.23 63.2 1.23 60.2 [89]
Hemp(Random orientation) 30 Injection molding 75 7.9 _ _ [80]
Hemp (multi-directional) 35 Compression molding 85 12.7 _ 7.5 [80]
Hemp (multi-directional) 40 Compression molding 44.63 7.39 90 _ [80]
Corn stover 1wheatstraw
1soy stalk
(10%110%110%)
30 Extrusion 1injection
molding
58 5.55 80 6.9 23 J/m [89]
Jute (Uni-directional) 50 Compression molding 152 5.3 174 _ [80]
Jute (multi-directional) 40 Film-stacking 93.5 _ _ 14.3 [80]
Jute (Random orientation) 30 Compression molding 48 _ 101 8.3 [80]
Rice hull/CA (CA:PLA-
grafted Maleic anhydride)
26.7 61.4 2.76 60.11 28.8 63.14 1.63 60.09 48.7 64.16 [89]
Kenaf (multi-directional) 40 Compression molding 82 7.6 126 14 [80]
Kenaf 70 Hot pressing 223 32 254 35.5 9.5 61.3 [89]
Kenaf (Uni-directional) 35 Compression molding 131 15 160 _ [80]
Kenaf(Random orientation) 30 Compression molding 32 4.5 _ _ [80]
Kenaf 20 Injection molding 90 61.99 4.5 60.34 34 62.98 J/m [89]
Ramie(Random orientation) 30 Compression molding 52.5 - 170 10 [80]
Ramie(Random orientation) 30 Compression molding 52 - 105 9.3 [80]
Sisal(Random orientation) 30 Injection molding - - 97 3.3 [80]
Sisal 30 - 23.3 60.3 3.5 60.07 [89]
Sugarbeet pulp 1polymeric
diphenyl methane
diisocyanate(2%)
30 Extrussion 1injection
molding
61.1 5 3.25 [89]
Wood flour 30 Injection molding 58.28 61.83 6.22 60.54 35.96 [89]
DOI 10.1002/pc POLYMER COMPOSITES—2018 11

degradation by moisture or chemicals and microorgan-
isms, and so forth. Natural weathering conditions or the
environment in which the material is used influenced the
natural ageing or decaying process. The long-term perfor-
mance of composites subjected to environmental exposure
is evaluated by real time observations for a period of sev-
eral years [113]. A 2 years natural weathering study on
jute/phenolic biocomposites was conducted by exposure
to natural ageing, which showed polymer cracking, black
spots, bulging, fibrillation, and over 50% tensile strength
decline [100]. Accelerated ageing is conducted in ageing
chambers that replicate natural environmental conditions
and the destructive effects of prolonged outdoor exposure
by exposing the composite samples to ultraviolet radia-
tion, moisture and temperature in a controlled manner,
which is a much faster and more convenient alternative
method, compared with natural ageing, and is reproduc-
ible as well. Accelerated weathering tests have been per-
formed by Umar et al. [114] on a kenaf/HDPE composite
for testing its durability. As a result, micro-cracking of
the surface and reduced tensile property were observed,
which allowed the authors to conclude that the biodegrad-
ability of the composite was enhanced by the presence of
natural fibers. This is supported by Yu Dong et al. [115],
who investigated the degradation behavior of coir fiber
reinforced PLA biocomposites. They noted that the bio-
composites degraded much faster than neat PLA, record-
ing the maximum weight loss of 34.9% in treated coir
fiber reinforced biocomposites, compared with 18% in
PLA, after 18-day burial, due to the hydrophilic nature of
coir fibers. Similar work has been done on ramie, flax,
cotton [81]; hemp [109]; kenaf, rice husk [84]; and other
fibers, revealing that the biodegradability is enhanced by
increasing the fiber content of the composites. PLA is
easily processable, biocompatible, and biodegradable in a
natural environment, like garden soil, compost, and aque-
ous solution [116–118]. The most important qualities of
PLA/NF composites are satisfying biocompatibility and
biodegradability [119]. Biodegradability tests of PLA/NF
composites have been carried out by several researchers,
some of the works are listed in Table 11.
Issues/Challenges in Natural Fiber-Based PLA
Composites
With the exception of some applications, PLA has so
far failed to step forward on markets because of its inade-
quate properties or high production cost [74]. Biopoly-
mers must meet the required standards to justify their use
over synthetic polymers, and in order to expand their
potential market. There are still a number of challenges to
overcome in realizing the potential of PLA, regardless of
the efforts made by researchers and scientists [2, 17,
120]. Some of the challenges consist in the homogeniza-
tion of the fiber properties, the complete understanding of
crystallization and degree of polymerization, fiber/matrix
adhesion, flame retardancy, moisture repellence, and bio-
degradability properties, to name just a few [24].
The main concern of the PLA-based natural fiber com-
posite is the price of this polymer [24, 74, 80, 89].
According to a cost investigation, the base manufacturing
cost of LA is estimated to reach 0.55 US$/kg in the near
future [121]. The manufacturing cost of the LA monomer
is now targeted to <0.8 US$/kg on an industrial scale,
because the selling price of PLA should decrease almost
by half from its present price of 2.2 US$/kg [2, 121]. One
of the foremost technical challenges in the general accep-
tance of biopolymers consists in the difficulty to achieve
physical and mechanical properties comparable to con-
ventional synthetic polymers, while maintaining its biode-
gradability [21, 58, 122].
TABLE 9. Thermal characterization of PLA/NF composites.
Composites Fiber % Characterization References
PLA/oil palm MCC 1,3,5 TGA [99]
PLA/kenaf, PLA/rice husk 20 TGA [84]
PLA/ramie 30 DMA, DSC, TGA [87]
PLA/ramie/PEG (poly ethylene glycol) 20 DMA, DSC [170]
PLA/RSF 7, 8, 9 DSC, TGA [19]
PLA/MCC 5 DSC, TGA [97]
PLA/bamboo fabric 51 TGA [169]
PLA/coir 5–30 DSC, TGA [115]
PLA/kenaf/thymol 40,10 TGA [171]
PLA/banana fiber 10, 20, 30, 40 DSC,TGA [172]
PLA/banana fiber/nanoclay 10–40 DMA,DSC [173]
PLA/Bamboo fiber/wood fibers/Coir fibers 10 DMA,DSC [170]
TABLE 10. FRs used in PLA/NF composites.
Composites FR References
PLA/kenaf APP [102]
PLA/hemp sepiolitenanoclay (Sep) and
multiwalled nanotubes
[124]
PLA/starch Microencapsulated ammo-
nium polyphosphate
[174]
PLA/ramie Ammonium polyphosphate [175]
PLA/coconut/jute Diammonium phosphate [101]
12 POLYMER COMPOSITES—2018 DOI 10.1002/pc

An inherent drawback of PLA is that it is very stiff
with low-impact strength and brittleness [2, 24, 58, 63],
which has been a major bottleneck of PLA-based biocom-
posites, hindering extensive commercial applications [25,
77, 123]. Various approaches, such as plasticization,
block copolymerization, amalgamation with hard poly-
mers and rubber toughening; have been used to improve
the toughness of the brittle PLA polymer, which, in turn,
leads to substantial decreases in the strength and modulus
of the toughened PLA. Consequently, a PLA-based mate-
rial having good stiffness-toughness stability, along with
high bio-based polylactide content, is still obscure
[2].Other core limitations of PLA that hamper its wider
industrial application are its limited gas barrier property
and poor thermal resistance [100, 123], which prevent its
absolute access to industrial sectors, such as automobiles
FIG. 5. UV degradation of natural fiber/polymer composite and its components [103]. [Color figure can be
viewed at wileyonlinelibrary.com]
TABLE 11. Some of the recent Biodegradability Tests carried out in PLA/NF Composites.
Composites Biodegradability Tests Duration (Days) References
PLA/Flax Soil Burial test in a flower pot containing farmland soil
having pH 7.5–7.7 by maintaining high relative
humidity (98%) by daily sprinkling water.
20, 30, 40, 50, 60, 70, 80, 90 [111]
PLA/ramie/PEG (5, 10, 15%) (PEG) Hydrolytic Degradation: immersion of samples in Alka-
line solution at 378C in humidified incubator
50, 100 [170]
PLA/kenaf (20%) Soil Burial test in normal garden soil without enzyme
activity. Temperature 308C, Humidity 80%
10, 30, 90 [84]
PLA/Kenaf Enzymatic Degradation Test by Pleurotus ostreatus in
at 258C
30,60,90.180 [176]
PLA/rice husk (20%) Soil Burial test in normal garden soil without enzyme
activity. Temperature 308C, Humidity 80%
10, 30, 90 [84]
PLA/banana fiber Bacterial Degradation by Burkholderiacepacia media
incubation at 25 628C and 50 65% relative
humidity
_ [172]
PLA/coir (5–30%) Soil Burial test in Compost soil with high humidity and
microbial activity
18 [115]
PLA/Hemp Accelerated weathering: 1 h UV irradiation followed by
1-min spray with deionized water and subsequent
2 h condensation in 508C temperature
250, 500, 700, 1,000 h [109]
DOI 10.1002/pc POLYMER COMPOSITES—2018 13

or packaging. However, it has been found that some of
these drawbacks can be surmounted to some extent by the
addition of nanoparticles [27, 124].
There is an ongoing dispute, with extensive political and
industrial implications, on the use of agricultural commodi-
ties for producing raw materials for biopolymers [125,
126]. Should we use agricultural crops for biopolymers
when people are starving is a question of debate, which dis-
closes that while keeping in mind the proper distribution of
land, logistics and financial assets, we should encourage
the utilization of biopolymers to maintain sustainability [2,
74]. This can be achieved by biocomposites and nanocom-
posites with efficient incorporation of natural fibers and
agricultural wastes.
Continuing essential studies, together with the progress
in large scale applications and the development of com-
mercial products, will guarantee a viable future for PLA
and natural fiber composites [17, 74, 125].
Application of Natural Fiber-Reinforced PLA Composites
As the demand for eco-friendly and renewable materi-
als continues to rise [26, 127, 128], PLA and its compo-
sites are receiving increasing attention. PLA-based
composites and blends have found major green applica-
tions [2, 77, 129]. Even though these biopolymers are
emerging as alternatives to existing non-renewable fossil
fuel-based plastics, the current low-level production [58,
67] and high price [24, 80] limit their extensive applica-
tions. However, the demand for biopolymers has been
amplified during the past decade due to their improved
applications and environmental concerns [6, 25, 127].
PLA has been widely used in various areas [18, 57], such
as packaging, medical, upholstery, textile and automotive
interiors, due to its light weight, good biocompatibility,
biodegradability and some mechanical properties. The
simplest and most eco-friendly way to improve the
mechanical and thermal properties of PLA is the addition
of natural fibers or filler materials. Both synthetic and
natural fibers, as well as nanoparticles are used as rein-
forcement in PLA-based composites to improve their
overall properties [83, 94, 130–133].
The application of PLA has been extended to other
commodity areas, especially in the composite industry,
ever since its production cost has been lowered by new
technologies [2, 17, 134]. Natural fiber/PLA composites
are being widely used in a wide range of applications
[77, 100, 103, 135], such as packaging, medicines, tex-
tiles, automobiles, industrial, infrastructure, building, fur-
niture, and other commercial markets as listed before.
Various automobile parts, for example, dashboards, door
panels, package trays, headliners and some interior parts,
are being developed using these natural fiber-based bio-
polymers [24, 89, 135]. Industrial markets for biodegrad-
able polymers are likely to increase considerably in the
near future [17, 21, 25, 53, 120]. Some of the uses of
PLA are showcased in Figure 6a–f. [177–182].
CONCLUSIONS AND FUTURE PROSPECTS
Increasing demands for renewable and recyclable
materials, the alarming energy crisis, environmental regu-
lations, and public concerns about plastic dumps and pol-
lution have spurred efforts to develop biodegradable
composites. PLA and natural fibers offer a potential alter-
native to the conventional fibers and synthetic polymers,
which are difficult to recycle and not sustainable. Natural
fiber reinforced PLA composites appear to have a bright
outlook for a broad range of applications. These biocom-
posite materials with diverse attractive properties may
FIG. 6. (a) PLA-3D Printing thread [177] (b) 3-D printed soap dish from colored PLA [178] (c) Biodegradable
PLA cups used at restaurant [179] (d) PLA-bioabsorbable implants [180] (e) Tea bags made of PLA [181] (f)
Mulch film made of PLA-blend “bio-flex” [182]. [Color figure can be viewed at wileyonlinelibrary.com]
14 POLYMER COMPOSITES—2018 DOI 10.1002/pc

soon be competitive enough to replace the existing fossil
fuel-based synthetic materials. However, the attempts to
develop fully biodegradable composites from natural
fibers and biopolymers that can fully replace synthetic
fibers and polymer-based composites have had limited
success so far. Extensive research studies have been done
to understand the behavior of PLA and natural fiber-
based PLA composites and their properties under various
processing conditions. Such studies have mostly resulted
in similar findings, confirming previous results that natu-
ral fibers and biopolymers complement each other in
many ways, while some research studies displayed a
decrease in properties. The increasing availability of bio-
polymers, the unique properties of natural fibers and the
environmentally friendly nature of biodegradable plastic
products, compared with synthetic polymers and fiber
products, justify further studies on developing PLA-based
natural fiber composites. However, for further improve-
ments of PLA-based natural fiber composites, a paradigm
shift in the synthesis and production of PLA for cost
reduction will be necessary to successfully replace syn-
thetic polymer composites with fully biodegradable
composites.
It is expected that, in the future, PLA-based natural
fiber composites will show equivalent mechanical, func-
tional, and biodegradability properties to those of syn-
thetic composites. Future development trends in PLA and
its composites are as follows: first, low-cost production,
which can win general acceptance. Considering market
demands, mass production of biocomposites and increas-
ing development of cheaper biopolymers, the cost will
probably decrease. Second, current and future research
should examine the fabrication and improvement of PLA-
based composites with different types, ratios, and forms
of natural fibers for multifunctional applications. Finally,
a proper database should be prepared on fibers and bio-
composites due to the complex and diverse nature of nat-
ural fibers. Fully biodegradable composites with great
multifunctional properties are realizable in the near
future. Fiber modification techniques, such as surface
treatments with chemicals including alkali, silane, cou-
pling agents, and so forth, can improve the fiber surface
properties and fiber/matrix interface, which can provide
better biocomposites to match various requirements. Fur-
thermore, the incorporation of nanocellulose and/or nano-
clay into biocomposites, which can amplify the various
functional properties, and the investigations of the tribo-
logical properties of PLA-based natural fiber composites
would be prime areas of interest in future research
studies.
ACKNOWLEDGMENT
The authors are grateful to UPM for providing Putra grant
GP- IPS/2017/9520200. Authors are also thankful to Minis-
try of Higher Education for supporting first author studies
through CSFP-2015 scholarship fund and providing HICoE
grant No: 6369108 to INTROP, Universiti Putra Malaysia.
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18 POLYMER COMPOSITES—2018 DOI 10.1002/pc