Journal of Minerals & Materials Characterization & Engineering, Vol. 11, No.4, pp.385-416, 2012 Printed in the USA. All rights reserved
Review of Green Polymer Nanocomposites
Samson O. Adeosun1, G. I. Lawal1, Sambo A. Balogun1 and
Emmanuel I. Akpan2,*
1 Department of Metallurgical and Materials Engineering, University of Lagos, Lagos,
2 Department of Materials and Production Engineering, Ambrose Alli University,
Ekpoma, Nigeria
*Corresponding Author: emma_ ei
Recently, attention has been drawn to the use of bio-reinforced composites in
automotive, construction, packaging and medical applications due to increased concern
for environmental sustainability. Green polymer nanocomposites show unique properties
of combining the advantages of natural fillers and organic polymers. Plant fibers are
found suitable to reinforce polymers. They have relatively high strength and stiffness,
low cost of acquisition, low density and produce low CO2 emission. They are also
biodegradable and are annually renewable compared to other fibrous materials.
Organic polymers on the other hand, are desirable because they are either recyclable or
biodegradable without causing environmental hazards. This paper reviews current
research efforts, techniques of production, trends, challenges and prospects in the field
of green nanocomposites.
Keywor d s: bio-reinforced composites; organic polymers; plant fibers; mechanical
properties; environmental friendly
386 Adeosun Sunday. O. , Lawal G. I Vol.11, No.4
Research efforts are geared to the development and application of environmentally
friendly and sustainable bio-reinforced composites for use in automotive, construction,
packaging and medical fields. It has been reported [1,2] t hat addition of nanoparticles to
base polymers confer improved properties that make them usable in automotive,
construction and medical areas. Properties which have been shown to improve
substantially are mechanical properties (e.g., strength, elastic modulus and dimensional
stability), thermomechanical properties and permeability (e.g., gases, water and
hydrocarbons). Others are thermal stability and heat distortion temperature, flame
retardancy and smoke emissions, chemical resistance, surface appearance, physical
weight and electrical conductivity. Nanocomposite is a class of composites in which the
dimensions of the reinforcing material are in the order of nanometers. Because of this
nanometer size characteristic, nanocomposites possess superior properties than the
conventional composites due to maximizing the interfacial adhesion.
In the past, major interest has been in the use of synthetic materials such as aliphatic
polyesters, aliphatic-aromatic polyesters, polyvinyl alcohols, polyesteramides,
polystyrene, nanoclays, glass and carbon fibers and carbon nanotubes etc for the
production of nanocomposites. The use of these materials, however, present great
challenges [3-5]. These include shortage of the organic compounds due to declining oil
and gas resources, increasing oil and gas prices. Other effects include environmental
concerns for their degradation or incineration and global warming, uneconomical costs
and cross contaminations in their recycling and consumer toxicity risks [6-10]. These
concerns gave birth to the quest for materials that can overcome these challenges and
maintain the required properties for the various applications.
The use of polymer composites from renewable sources has advantages over synthetic
sources, particularly as a solution to the environmental problems generated by plastic
waste [11]. They offer alternatives to maintaining sustainable development of economic
and ecologically attra cti ve t echn ol o g y [12] . Green comp os it es i s t od a y widel y resea rch e d
because of the need for innovations in the development of materials from biodegradable
Vol.11, No .4 Review of Green Polymer Nanocomposites 387
polymers, preservation of fossil based raw materials, complete biological degradability
and reduction in the volume of carbon dioxide release into the atmosphere. Application
of agricultural resources (wastes and products) for the production of green materials are
some of the reasons why green composites have attracted tremendous research
interests [12]. The use of these biocomposites is expected to improve manufacturing
speed and recycling with enhanced environmental compatibility [11].
Materials are said to be ‘green’ when they are biodegradable and renewable. The major
attractions about green composites are that they are environmentally friendly, fully
degradable and sustainable in every way. At the end of their service life they can be
easily disposed of or composted without harming the environment. The challenge of
gre en composite involves basically the challenge of obtaining ‘green’ polymers that are
used as matrix for the production of the composites. P olymer is said to be green when it
possesses environmentally favorable properties such as renewability and degradability.
Biodegradation implies degradation of a polymer in natural environment that includes
changes in the chemical structure, loss of mechanical and structural properties and
changing into other compounds that are beneficial to the environment [6,11]. Polymers
from natural sources (such as starch, lignin, cellulose acetate, poly-lactic acid (PLA),
polyhydroxylalkanoates (PHA), polyhydroxylbutyrate (PHB), etc.) and some synthetic
sources (aliphatic and aromatic polyesters, polyvinyl alcohol, modified polyolefins etc.)
that are degradable are classified as biopolymers [11,13]. However, those from synthetic
sources are not renewable and therefore do not conform wholly to the concept of
renewability and degradability. A number of natural and other degradable polymers h ave
been used in green composites and some of these are discussed below.
2.1. Thermoplastic Starch Based Composites
Earlier researches of thermoplastic starch based composites focused on the use of
plasticized starch as matrix for green nanocomposites. De Carvalho et al. [14] first
reported the use of thermoplastic starch for the production of composites by melt
388 Adeosun Sunday. O. , Lawal G. I Vol.11, No.4
intercalation in twin screw extruder. The composites were prepared with regular
cornstarch plasticized with glycerin and reinforced with hydrated kaolin. The study
recorded significant increase in the tensile strength from 5 to 7.5 MPa for the composite
with 50 phr clay composition. The modulus of elasticity increased from 120 to 290 MPa
while the tensile strain at break decreased from 30 to 14%. The maximum value for the
modulus of elasticity observed and the tensile strength corresponded to the maximum
quantity of clay that was incorporated in the matrix . Above which the authors noted that
increase in the amount of filler increases the fragility of the composite.
Pandey and Singh [15] varied the sequence of addition of plasticizers to determine the
effect of plasticizers on the mechanical and structural properties of resulting composites
using solution method. Thermal stability, mechanical properties and water absorption
studies were conducted to measure the material properties whereas FT-IR spectroscopy
was used to study the microdomain structure of composites. It was deduced that the
sequence of addition of components (starch/plasticizer (glycerol)/clay) had a significant
effect on the nature of composites formed and consequently the properties were altered
with no well established sequence to determine appropriate process method for the
composites. The filler dispersion was found to become highly heterogeneous and the
more brittle when starch was plasticized before filling with clay. The modulus of
composites increased significantly for all compositions relative to the unfilled matrix,
irrespective of the method of preparation. The authors posited that improvement in all
the properties of the resulting composites could be achieved by better dispersion of the
clay filler and not necessarily on varying the sequence of addition of plasticizers.
Subsequently, Guan and Hanna [16] modified the starch with acetate and produced
biocomposites with cellulose fibers. Starch acetates with degrees of substitution (DS) of
1.68 and 2.3 prepared from 70% am ylose corn starch were extruded with 10%, 20% and
30% (w/w) cellulose and 20% (w/w) ethanol in a twin screw extruder at 150, 160 and
170 °C barrel temperatures and 170, 200 and 230 rpm screw speeds. Results of X-ray
diffraction showed losses in the crystallinity of starch and cellulose. FTIR spectra
revealed that functional groups and chemical bonds were maintained after extrusion. The
Melting temperatures of the composites were observed to change significantly when
higher DS starch acetate were used. Experimental variables such as cellulose content,
Vol.11, No .4 Review of Green Polymer Nanocomposites 389
barrel temperature and screw speed have profound effect on thermal, physical and
mechanical properties of extruded foams. For the extruded starch foams, thermal
properties were strongly related to blend composition, barrel temperature and screw
speed. Tg and Tm of the extruded foams decreased as cellulose content increased. The
author n oted that as barrel t emperature and screw speed incre ased, Tg and Tm in creased
initially and then decreased when barrel temperature and screw speed were higher than
160 °C and 200 rpm respectively. The Authors posited that Barrel temperature serve as
an index of the thermal energy applied to the blends whereas screw speed indication of
the mechanical energy applied to the blends.
Further study by Kumar and Singh [17] was done through starch modification by photo-
induced cross linking. Composite films were made from the aqueous dispersions of
starch with microcrystalline cellulose using glycerol as plasticizer and irradiated under
ultraviolet (UV) light using sodium benzoate as photo-sensitizer via casting. Young’s
modulus of composites reinforced with 5, 10 and 15 wt % which were irradiated for 30
mins improved by 72.41%, 42.5% and 32% respectively, when compared to control
The elon gatio n (%) v alues were foun d to d ecreas e wit h increas in g cellu lose fi ber cont ent
and time of photo-irradiation. Other researches on the use of thermoplastic starch without
further modification (i.e., changes in experimental conditions) include the work of
Lu et al. [18], Ma et al . [19], Svagan [20], Famá et al. [21], Kaushik et al.[22], Liu et al.
[23], Guimarães et al. [24] and Kaith et al.[25]. These studies reported significant
increase in tensile and thermal properties of thermoplastic starch when reinforced with
2.2. Poly Lactic Acid Based Composites
Ogata et al. [26] in their study prepared nanocomposites using poly lactic acid (PLA).
PLA/ organically modified clay (OMLS) blends were made by dissolving the pol ymer in
hot chloroform in the presence of dimethyl distearyl ammonium modified MMT
(2C18MMT). X-ray diffraction results of PLA/MMT show that the silicate layers
390 Adeosun Sunday. O. , Lawal G. I Vol.11, No.4
forming the clay could not be intercalated in the PLA/MMT blends when prepared by the
solvent-cast method. The clay existed in the form of tactoids with several stacked silicate
monolayers. The formation of geometrical structures in the blends was attributed to the
tactoids, which led to the formation of superstructures in the thickness of the blended
film. This could lead to a structural feature that promotes increase in the Young's
modulus of the hybrid. A series of nanocomposites using PLA were reported by Sinha et
al. [27-32], Yamada et al. [33], Maiti et al. [34], Paul et al. [35], Lee et al. [36] and
Chang et al. [37]. These studies revealed that nanocomposites prepared with PLA had
improved mechanical, thermal and impact properties. Bondeson and
Oksman [38] used commercial PLA as the matrix with cellulose whiskers treated with
anionic surfactant (5, 10 and 20 wt%) as reinforcement. The compounded materials were
extruded in three steps with pelletizing between the first and the second steps. Extruded
nanocomposites were compression moulded and characterized. The tensile strength and
elongation at break decreased for this composite compared to PLA. This lack of
improvement in the mechanical properties of the composite was attributed to poor
adhesion between the fiber and the matrix which is directly related to the nature of the
fiber and the type of mechanical treatment given.
Studies on the thermal, mechanical and morphological properties of PLA-based
composites have been reported by Lee et al. [39]. Composites were prepared by melt
compounding and injection molding. Thermal degradation, thermal transition,
morphological, and mechanical properties of the composites were evaluated. Tensile
modulus of the composite increased from 62.5 to 169.5% contrary to the earlier studies
where no improvement was recorded. More recently, nanocomposites of PLA with a
compatibilizer and cellulose fibrils have been developed by Qu, et al. [40]. Bleached
wood pulp was used as the fiber and commercial grade PLA as the matrix. A chemo-
mechanical method was used to prepare cellulose nanofibrils dispersed uniformly in an
organic solvent. Poly ethylene gl ycol (PEG) was added to the matrix as a compatibiliz er
to improve the interfacial bonding/adhesion between the matrix and the fiber. The
composites were obtained by solvent casting methods using
N,N-Dimethylacetamide (DMAC) and characterized PLA reinforced with cellulose
nanofibrils resulted in no improvement in tensile strength (30 MPa compared with pure
Vol.11, No .4 Review of Green Polymer Nanocomposites 391
PLA) and percent elongation (2.5% compared with pure PLA) of the composites. The
authors attributed this to the poor interfacial bonding between cellulose nanofibrils and
the PLA marix. Addition of PEG to the blend of PLA resulted in significant
improvement in tensile strength (28.2%) and percentage elongation (25%) compared to
pure PLA. The authors posited that PEG covers the surface of the cellulose nanofibrils
and act not only as a plasticizer for PLA to improve its elongation, but also as a
compatibilizer between the hydrophobic PLA and the hydrophilic cellulose nanofibrils. It
was also evident that PEG also prevents the aggregation of the nanofibrils, so that the
cellulose nanofibrils disperse in the PLA matrix homogeneously to form a network
structure. It was also noted by the authors that the optimum composition of cellulose
nanofibils to obtain the best properties was 3 % above which tensile strength and
percentage elongation decreased. The FT-IR analysis shows that PEG improved the
intermolecular interaction, which is based on the existence of intermolecular hydrogen
bonding among PLA, PEG, and cellulose nanofibrils.
2.3. Cellulose Based Composites
Cellulose from agricultural products have been identified as a source of biopolymer that
can replace petroleum polymers. Green nanocomposites have been successfully produced
from cellulose acetate (CA), triethyl citrate (TEC) plasticizer and organically modified
clay via melt compounding [41]. The cellulosic plastic with 80 wt% pure cellulose
acetate and 20 wt% triethyl citrate plasticizer was used as the polymer matrix for
nanocomposite production. Mechanical properties of the composites were determined
and correlated with observations from X-ray diffraction and transmission electron
microscopy. Results show that Cellulosic plastic-based nanocomposites containing 5 and
10 wt% organoclay have better exfoliated and intercalated structure than those of 15 wt%
organoclay. Tensile strength and modulus of cellulosic plastic reinforced with 10 wt%
organoclay improved by 75 and 180% respectively. Thermal stability of the cellulosic
plastic also increased.
Recently, a cellulose based nanocomposite material has been investigated as a flexible
humidity and temperature sensor [42]. Cellulose was obtained from cotton pulp via acid
392 Adeosun Sunday. O. , Lawal G. I Vol.11, No.4
hydrolysis using a solution of Lithium chloride and N,N-dimeth ylacetami de. Nanos caled
polypyrrole was used as the second component of the nanocomposite. Nanocomposites
were prepared by polymerization induced adsorption process. Nanoscaled polypyrrole
were coated on the surface of the cellulose and its influence on the cellulose membrane
was characterized by Atomic force microscopy. The authors focused basically on the
sensing ability and not the structural integrity of the nanocomposite material. However,
the analysis revealed that there was successful deposition of the polypyrrole nanolayer
onto the cellulose surface. Because the material was not analyzed on structural basis it
could not determined if cellulose served as the reinforcement or the polypyrrole.
An active antimicrobial packaging material has been developed using methyl cellulose
(MC) as the base material with montomorillionite (MMT) as reinforcement [43]. MC and
MMT were industrially prepared and mixed with Carvacrol (CRV) to form
nanocomposites as shown in Figure 1 and the resulting nanocomposite characterized.
Figure 1. MC/CRV/MMT nanocomposite film preparation procedure.
TEM showed that there were both intercalated and exfoliated nanocomposite structures
with exfoliated structures being more prevalent in nanocomposites with low
concentration of MMT. Thermal stability of nanocomposite was found to increase with
increase in concentration of MMT. The authors posited that the little thermal degradation
shown by the nanocomposite is due to the structural degradation of MC at higher
Vol.11, No .4 Review of Green Polymer Nanocomposites 393
temperatures. The study did not assess the nanocmposite structurally. Further works in
this area includes the works of Zimmermann et al. [44] and Zadagan et al. [45] who
evaluated the use of cellulose based nanocomposite with hydroxylapatite for medical
applications. Both authors did not analyze their nanocomposite materials structurally
providing no basis of comparison with other nanocomposite materials of interest.
2.4. Plant Oil Based Composites
Bio-based nano composites have been developed f rom epoxidized so ybean oi l, diglycidyl
ether of bisphenol-A (DGEBA) and organically modified montmorillonite (OMM) [46].
Soy-based epoxy resin (30/70 wt%) was mixed with different organophilic
montmorillonite clay (as reinforcement) concentrations (1 wt%, 3 wt%, 5 wt% and 7 wt
%) to form nanocomposites using a mechanical stirrer. The tensile strengths of 1 wt%, 3
wt%, 5 wt% and 7 wt% OMM reinforced soy-based epox y nanoco mposit es increased by
2%, 7%, 12.6% and 23.3% respectively. Impact strength and flexural properties of the
nanocomposites show significant improvement. A maximum increase of 25% was
recorded for flexural strength of the nanocomposites.
Nanocomposites reinforced by exfoliated clay were found to exhibit higher storage
moduli. The SEM images of the composites reveal relatively uniform dispersion of
organoclay layers in addition to the good interface of the material. The authors argue that
improvement of properties is due to proper exfoliation of the nanocomposite. The
improvement of both toughness and impact strength in the case of the exfoliated
nanocomposites was attributed to the plasticizing effect of the long aliphatic chains in the
soy-based epoxy polymer matrix and reinforcement effect of the clay nano-layers.The
composite produced is not completely from green source because of the use of
organophillic montorillonite clay as reinforcement.
Azeredo et al. [47] had earlier reported the use of mango puree-based edible films for
nanocomposites. Nanocomposite were developed by adding cellulose nanofibers (CNF)
in different concentrations (up to 36 g/100 g) as nanoreinforcement to mango puree
based edible films for packaging applications. The study revealed that mango puree
394 Adeosun Sunday. O. , Lawal G. I Vol.11, No.4
edible films can be used as matrix material for biocomposites as the tensile strength
increased from 4.06 MPa to 8.09 MPa and the Young’s modulus from 19.85 MPa to
322.05 MPa. This improvement in properties was attributed to the fact that the
nanofibers seem to have presented good interactions with the mango puree. They were
found to disperse in the matrix without significant agglomeration. The authors suggested
that the interactions may have occurred between cellulose and neutral sugar side chains
of the mango pectin.
Soy based polyurethane can be used as a matrix for the production of bio-based
nanocomposites [48]. The polyurethane modified with Halloysite Nanotubes (HNT)
nanoparticles was used to manufacture E-glass reinforced composites using low cost
vacuum assisted resin transfer molding process and characterized for ultimate
compressive strength, flexural strength, flexural modul us and interlaminar shear strength
(ILSS). Results show that interlaminar shear strength increased with increase in
percentage of HNT used (from 14.28MPa for 0 wt% HNT to 21.28MPa for 2.4 wt%
HNT). Ultimate compr es sive str ength, flexur al mo dulus and flexur al str ength di d not sho w
appreciable changes. 0.8 wt% HNT gave the best combination of properties with 6 percent,
28 percent, and 82 percent increase in flexural strength, flexural modulus and ILSS
respect i vely.
2.5. PolymerPolymer Blends Based Composites
The blending of two or more polymers to achieve a polymer that is biodegradable has
also draw n res earch i nte rest . Thes e pol ymers have been tested for their de gradability and
mechanical properties and thus recommended for use as degradable polymers for
composite applications. Such polymer blend Starch/PLA blends, poly butylenes
succinate/cellulose acetate blends, starch/modified polyester blends,
polycarprolactone/poly vinyl alchohol blends and thermoplastic starch/polyesteramide
blends [49-54] has been reported.
Recent use of binary and ternary blends of polylactide (PLA), polycaprolactone (PCL)
and thermoplastic starch (TPS) as composites has been reported by
Vol.11, No .4 Review of Green Polymer Nanocomposites 395
Sarazin et al. [55]. The blends were prepared using a one-step extrusion process. The
transition temperatures for TPS/PLA and TPS/PLA and PCL blends were found to be
approximately −57 °C for blends with 36% glycerol. Increasing the concentration of TPS
in PLA resulted in a corresponding increase in strain at break with decrease in elastic
modulus and maximum strength. Natural rubber has also been blended with starch to
form nanocomposites [56]. SEM images of the fractured surface of the starch/natural
rubber/clay nanocomposite (prepared with unmodified natural rubber latex) loading with
3 wt% clay (see Figure 2a) revealed that both thermoplastic starch matrix (gray areas) and
dispersed rubber phase (white areas) are clearly separated indicating lack of interfacial
adhesion. On the other hand, the use of modified natural rubber (see Figure 2b.) resulted in
a finer dispersion and improved interfacial adhesion.
(a) (b )
Figure 2. SEM micrographs of starch/natural rubber/clay nanocomposites with
(a) unmodified natural rubber, and (b) modified natural rubber.
2.6. Other Biopolymer Based Composites
Other biopolymers used in nanocomposites synthesized are Polyhydroxyl butyrate
(PHB)a natural occurring polyester produced by numerous bacteria in nature,
Gelatin—a biopolymer obtained by thermal denaturation of collagen isolated from
animal skin and bones with dilute acid and Chitosan—a natural polymer found in
exoskeletons of crustaceans and insects and in the cell wall of fungi and micro organisms
[57-60]. Mechanical and water vapor barrier properties of chitosan based
nanocomposites were improved by addition of cellulose nanofiber whereas chitosan
396 Adeosun Sunday. O. , Lawal G. I Vol.11, No.4
filled with hydroxyapatite, tricalcium phosphate, carbon fiber and montori llionite did not
show substantial improvement in mechanical properties.
The challenge of green composites extends to the quest of reinforcements that are green
and renewable. Cellulose (which gives plant their structural rigidity) is the most
abundant renewable fiber in nature. Moreover, they can be produced from agricultural
waste whi ch m akes it m ore eco nomi call y viable th an an y oth er sou rce o f fib ers cu rrently
in use. Cellulose fiber has offers an outstanding balance of excellent mechanical
properties, low density, safer handling and working conditions compared to synthetic
fibers [20,13]. Efforts, in the last few years have been focused on the exploitation of
natural fibers as load bearing constituents in composite materials. Table 1 shows the list
of natural fibers that have been extracted and their properties. These fibers have been
applied as reinforcement for composites and nanocomposites.
Table 1. List of natural fibers and their properties.
Fiber source
Alkaline boiling
modified with
carbomet hylcel l
Bark of
cotton stal k s
NaOH treatment
Velvet Leaf
Vol.11, No .4 Review of Green Polymer Nanocomposites 397
Switch grass
Simple alkaline
treatmen t
Switch grass
Simple alkaline
treatmen t
Kena f
treatmen t
treatmen t
Hop stem
treatmen t
treatmen t
NS: Not specified
The use of natural fiber in the production of composites in the early 90’s was largely
seen in the use of steam exploded hemp fiber as reinforcement in polypropylene-based
composites [76]. Hemp fibers were first purified by steam explos ion in a flash hydrolysis
laboratory pilot unit. It was then compounded directly with polypropylene and after
surface treatment with propylene maleic anhydride co-polymer. Microstructural and
tensile properties were studied with a SEM and Instron tensile testing machine
respectively. The steam exploded fiber was had a cellulose content of 55%. Slight
increase in strength was noticed in the treated fiber reinforced composite at a higher filler
content, strength of untreated fiber reinforced composites decreased in because of poor
adhesion between fiber and the matrix. The percentage elongation generally decreased
with increase in fiber content, while tensile modulus increased with percentage of fiber in
the polymer. Treated fiber reinforced composites had the highest modulus. A similar
398 Adeosun Sunday. O. , Lawal G. I Vol.11, No.4
study on bamboo fiber [77] show that 20% of fiber in propylene matrix led to 12.5 and
10 % increase in flexural and tensile strengths respectively.
The use sisal, kenaf, hemp, jute and coir fibers as reinforcement in polypropylene based
composites were investigated by [64]. The fibers were reinforced in polypropylene by
compression moulding using a film stacking method. Tensile testing was on Instron
tensile test machine. The elastic modulus, failure strain and tensile strength were
calculat ed from the stress s train curv e. Flexural tests were al so perfo rmed with the same
machine using three point bending techniqu e. Impact tests were performed on an Instron
charpy impact testing machine. Results show that hemp fiber composites displayed the
highest tensile strength (52 MPa) while coir has the lowest (about 10 MPa ). The strength
of kenaf, sisal and jute composites was approximately 30 MPa. Hemp composites were
showed the best flexural strength (54 MPa). This compares well with glass mat
composites (60 MPa). The tensile modulus of coir composites was 1.3 GPa while that of
hemp and kenaf composit es was app roxim ately 6. 8 GPa. M echanic al prop erties of natu ral
fiber composites tested compared favour ably with corresponding glass mat composites and
could be used in many applications that require high load bearing capabilities.
A similar study was conducted on the use of chemically modified banana fiber reinforced
in polyester resin [61]. Previously extracted cellulose fibers from banana plant were
collected and treated with two types of silanes (A174 and A151) and used to make
composites with polyester as the matrix material (40% fiber content). Mechanical
properties of the fiber and composites were determined with a dynamic mechanical
analyzer. Scanning electron microscope was used to examine the microstructure of both
modified and fractured composites. The storage modulus of the composites was found to
be temperature dependent with a sharp decrease between 80 and 90 followed by a
modulus plateau at higher temperatures. Treatment of fiber led to improved mechanical
The study on the potential of wheat straw fibers prepared by mechanical and chemical
processes as reinforcing additives for thermoplastics was carried out by Panthapulakkal
et al. [73]. Composites of polypropylene filled with 30% wheat straw fibers were
Vol.11, No .4 Review of Green Polymer Nanocomposites 399
prepared and their mechanical properties evaluated. The fibers were processed
chemically using NaOH pulping and mechanically using a laboratory scale refiner.
Composites with mechanically prepared wheat straw fibers yielded 29% and 49%
improvement in tensile and flexural strength whereas those with chemically processed
fibers ex hibi ted 12% and 49% en hancement. It is expect ed that th e chemicall y processed
fiber would exhibit higher strength and modulus because they were more homogeneous,
stronger and longer than the mechanically processed fibers. This the authors attributed to
poor dispersion of the cellulose rich chemically processed fibers, which resulted in a
strong interaction between the hydrophilic hydroxyl groups of cellulose. The poor
dispersion was attributed to the production process. It is the position of the authors that
full potential of the reinforcement could be achieved by proper dispersion of the
chemically treated fibers but did not specify the process that would lead to proper
dispersion.In another study, a series of environmentally friendly glycerol plasticized
starch (PS) biocomposites were prepared using 040 wt% ramie cellulose nanocrystalites
(RN) as fillers [18]. The ramie cellulose nanocrystalites, with lengths of 538.5 ± 125.3
nm and diameters of 85.4 ± 25.3 nm on average, w ere prepared f rom ram ie fi bers b y acid
hydrolysis. Different fractions of RN were used to form composites with the glycerol
plasticized starch. The morphology, thermal behavior and mechanical properties of the
resulting composites were investigated by scanning electron microscopy, differential
scanning calorimetry, thermogravimetric analysis, dynamic mechanical thermal analysis
and tensile testing. Results reveal that there are synergistic interactions between fillers
and between filler and PS matrix and these plays a key role in reinforcing the
composites. Tensile strength and Young’s modulus of the PS/RN composites,
conditioned at 50% relative humidity, increased from 2.8 MPa to 6.9 MPa and from 56
MPa to 48 0 MP a res pect iv el y as RN co nt ent in cr e ases from 0–40 wt% but the elongation
at break decreased from 94.2% to 13.6%.
Green composites using fibers as a structural material have been developed [78].
Polylactic acid (PLA), L-polylactide acid (PLLA), poly 3-hydroxylbutyrate (PHB),
polycaprolactone and starch thermoplastic, poly butylene succianate (PBS) and poly
butylene adipate-co-terephtalate (PBAT) were used as matrix. The composites were
produced using a film stacking technique. Test samples were cut out from the composites
to study their mechanical properties under tensile loading conditions. These tensile
400 Adeosun Sunday. O. , Lawal G. I Vol.11, No.4
properties were then compared to those of similar polypropylene flax composites.
Young’s modulus of a PLLA/flax fiber composite increased from 3321 MPa (neat
polymer) to 9519 MPa (30% flax f iber vol ume fr actio n) and t he ult imat e st ress in creased
from 60 to 99 MPa. The strain to failure reduced from 2.4% to 1.5%. With the same fiber
volume fraction (30%) Young’s modulus for the PLLA composite was 9519 ± 481 MPa.
A study of the synthesis and mechanical properties of new series of green composites
involving Hibiscus sabdariffa fiber as a reinforcing material in urea–formaldehyde (UF)
resin based polymer matrix has been reported b y Singha and Thakur [79]. Reinforcement
of the resin with the fiber was accomplished at three different levels: particle size, short
fiber and long fiber by employing optimized resin. Static mechanical properties (tensile,
compressive and wear properties) of randomly oriented intimately mixed Hibiscus
sabdariffa fiber reinforced polymer composites were investigated as a function of fiber
loading using a computerized Universal Testing Machine. Scanning electron microscopy
was used to examine the morphology of the resulting composites. It was observed that
composites with particle reinforcement show higher tensile properties, followed by short
fiber and long fiber. The load bearing capacities are 332.8 N at extension 2.2 mm, 307.6
N at extension 2.23 mm and 286.1 N at 2.28 mm extension respectively for particle,
short and long fiber reinforced composites. Similar trend was observed for compressive
strengths and wear resistances. SEM images show uniform mixing in particle reinforced
composite as compared to short and long fiber reinforcement. Thus Hibiscus sabdariffa
fiber has immense scope in the production of natural fiber reinforced polymer
composites for a variety of industrial applications.
The development of novel nanocomposites using cellulose nanofibers (CNF) in different
concentrations (up to 36 g/100 g) as nano-reinforcement in mango puree-based edible
films for packaging applications have been reported [47]. The effect of CNF was studied
in terms of tensile properties, water vapor permeability, and glass transition temperature
(Tg) of the nanocomposite films using standard methods. CNF increased tensile strength
from 4.06 to 8.09 MPa, and Young’s modulus from 19.85 to 322.05 MPa, especially at
higher concentrations, which suggest the formation of a fibrillar network within the
matrix. The water vapor permeability significantly decreased when CNF was
Vol.11, No .4 Review of Green Polymer Nanocomposites 401
incorporated at loadings of at least 10 g/100g. The effect of the filler on glass transition
temperature was low but significant.
Kenaf fiber bundles having 50–150 µm diameters were used to make unidirectional
composites materials with emulsion type polylactic acid resin moulded at 180 °C [80].
The resulting composite was characterized for structural, thermal and mechanical
properties using standard methods. The tensile strength of the composite decreased when
kept at 1 for 60 minutes. The unidirectional fiber reinforced composite has the highest
tensile (223 MPa) and flexural strengths (254 MPa) at 70% fiber content. Tensile and
flexural strengths and elastic moduli of the composites increased linearly up to 50%.
Recently, natural cellulose obtained from the bark of cotton stalks have been used to
develop composites [63]. Cellulose fibers w ere extracted from cotton stalk s using simple
alkaline extraction. The reinforcing fibers were carded (separated) and compression
molded with polypropylene matrix to form composites and then tested for tensile
properties on an Instron tensile tester. Polypropylene composites reinforced with the
fibers had a flexural strength of 12.4 MPa, an offset yield of 22.5 MPa, stiffness of 1.4
N/mm, modulus of elasticity of 502 MPa, tensile strength of 15.7 MPa, tensile modulus
of 806 MPa and an impact resistance of 0.3 J. The fiber had a single cell length of 2.1
mm and a width of 9.7 µm.
Reddy [81] went further to investigate natural cellulose fibers from milkweed stem using
simple alkaline treatment. The amount of cellulose in the fibers was determined as the
acid detergent fiber according to AOAC method. The physical structure of the fibers in
terms of percentage crystallinity, shape and position of the cellulose peaks were observed
using an X-ray diffractometer. An SEM with variable pressure was used to observe the
morphological features of the fibers. Tensile properties in terms of breaking tenacity,
percentage breaking elongation and Young’s modulus were determined using Instron
tensile testing machine. Results obtained showed that milkweed stem yielded 75.4%
cellulose of 0.9 mm average length single cells and diameter of 13.0 µm. Fibers obtained
had crystallinity of 39%, and breaking elongation of 4.7%. A modulus of 122 g/denier
(15.8 GPa) was also reported for the fibers. The milkweed stem fibers properties are
402 Adeosun Sunday. O. , Lawal G. I Vol.11, No.4
found similar to that of common fibers like cotton and linen and are suitable for textile,
composite, automotive and other fibrous applications.
The influence of wheat bran content as fillers in biodegradable composites based on
cassava starch containing glycerol and potassium sorbate was examined by Famá et al.
[21]. Composite Films were produced by casting using three di fferent frac tions of w heat
bran fiber: 1.5 mg, 13.5 mg and 27.1 mg/g of matrix. Mechanical properties of the
resulting composites were evaluated using dynamic and quasi static test. Physical
properties were also investigated. The study revealed that the mechanical properties of
starch-wheat bran composites were improved when the content of the fiber was
increased. The storage modulus and hardening of the films increased but maintained high
deformation to rupture (beyond 70%). This reinforcing effect was attributed, to the high
storage modulus of wheat bran due to its water-insoluble fiber content. On the other hand
the composites showed a decrease in the moisture content, with increase i n bran content.
Thus increase in the wheat bran content produces an improvement in water vapor barrier
Kaushik et al. [22] recently reported research on green nanocomposite using natural
polymers and agro waste cellulose fibers. The novel nanofibril/thermoplastic starch
(TPS) based nanocomposites were made with nanofibers from wheat straw and
plast icized m aize star ch. The cellulose nanofibrils were ext racted using ste am explosion,
acidic treatment and high shear mechanical treatment and dispersed in thermoplastic
starch (TPS) using a high shear mixer in varying proportions. The resulting composite
was analyzed for mor phology, thermal and mechanical properti es using standard m ethods.
The nanocomposite showed the highest tensile modulus (about 220 MPa) at 15% fiber
content and the highest yield strength (about 6.5 MPa) at that same fiber content. A
scanning electron microscope image of 10% nanofiber in thermoplastic starch showed
dispersed nanofibers in the matrix (see Figure 3).
Vol.11, No .4 Review of Green Polymer Nanocomposites 403
Figure 3. SEM image of starch cellulose nanofibril nanocomposites with 10%
cellulose nanofibrils.
Polylactic acid and cellulose fibrils nanocomposites have been developed by Qu, et al.
[40] . Bleached wood pul p was used as the fiber and commercial grade pol ylact ic acid as
the matrix. A chemo-mechanical method was used to prepare cellulose nanofibrils which
were dispersed uniformly in an organic solvent. Polyethylene glycol (PEG) was added to
the matrix as a compatibilizer to improve the interfacial bonding between the matrix and
the fiber. The composites were obtained by solvent casting methods from N, N-
Dimethylacetamide (DMAC) and characterized. After adding cellulose nanofibrils to the
PLA matrix, the composite has tensile strength of 30 MPa with 2.5% elongation which is
lower than that of pure PLA. Addition of PEG to the blend of PLA and cellulose
nanofibrils resulted in a significant improvement in tensile strength and % elongation of
the composites. The tensile strength and the elongation (of PLA + PEG + nanofibrils)
increased by 28.2% and 25%, respectively, compared with pure PLA, and increased by
56.7% and 60% compared with the PLA/cellulose nanofibrils. The FT-IR analysis
showed an improvement in the intermolecular interaction, which is based on the
existence of intermolecular hydrogen bonding among PLA, PEG, and cellulose
Starch composites reinforced by bamboo cellulosic crystals were produced by Liu et al.
[23]. Bamboo cellulose crystals (BCCs) were prepared by Combined HNO3KClO3
404 Adeosun Sunday. O. , Lawal G. I Vol.11, No.4
treatment and sulphuric acid hydrolysis and used to reinforce glycerol plasticized s tarch.
The structure and morphology of BCCs were investigated using X-ray diffraction,
electron microscopy, and solid-state 13C NMR while tensile properties of the composites
were det ermined us ing In stron Tens ile machi ne. Res ults sh ow that BCC s were of t ypical
cellulose I structure (50-100 nm in diameter). The morphology of BCCs depends on its
concentration in the suspension. The elongation at break increased as BCC content
decreased . It was also observed that incorporation of a significant quantity of cellulose
crystals cause the resulting composites to exhibit a high relaxation peak.Finally, BCCs at
the optimal 8% loading level exhibited a higher reinforcing efficiency for plasticized
starch plastic than at any other loading level.
Guimarães, et al. [24] have reported the use of corn starch with banana and sugarcane
fibers for the production of green composites. Commercial grade starch (about 28%
amylose) was obtained from corn while fibers were obtained from banana pseudo stem
and sugarcane bagasse via chemo mechanical method. Composites were fabricated by
mixing laminates of starch and fibers in a ball mill. X-ray diffractometer was used to
determine the crystallinity of starch and its composites. Tensile test was performed on an
Instron tensile testing machine to obtain stress-strain curves. The resulting composites
had good thermal stability, while fractographic studies of 70 wt% starch and 30%
glycerol matrix revealed cracks between smooth and rough surface and dimples in rough
regions, suggesting the sample was ductile. The Young’s modulus increased by 186%,
294% and 201% over the matrix for banana fiber contents of 20%, 25% and 35%,
respectively, in the starch + glycerol matrix. The ultimate tensile strength (UTS)
remained relatively constant. The yield strength (YS) increased by about 129%, 141%
and 133% for 20, 25 and 35 wt% fiber contents, while the elongation (%) decreased
about fivefold for 20 wt%, sixfold for 25 wt% and sevenfold for 35 wt% fiber content.
The researchers attributed the remarkable change in yield strength and Young’s modulus
to the compatibility (both chemical and structural) between the reinforcements (cellulose
chains) and the starch-glycerol matrix. They also posited that Improvement in Young’s
modulus can also be attributed to the deplastification of starch caused by the partition of
glycerol between the constituents of the co mposites (matrix and fibers).
Vol.11, No .4 Review of Green Polymer Nanocomposites 405
Many methods have been employed for the production of nanocomposites in an attempt
to achieve optimum dispersion of fibers in the matrix.
Extrusion followed by injection molding has been used [82,83]. Although the effect of
the method on the properties of the resulting composite has not been reported, it is
believed that the process method leads to better exfoliation of fibers in the matrix. Melt
extrusion is another method frequently been used [56,16]. Majdzadeh [56] noted that in
the use of this method, extrusion speed has a significant effect on the thermal properties,
specific energy requirement, radial expansion ratio and compressibility of polymer
foams. Another common method is film stacking which involves compressing a stack of
polymer film and fiber for a period of time. It has been reported that this moulding
method has a profound impact on the tensile properties of the composites [78]. Some
other workers have used melt compounding followed b y compression moulding [84,76],
injection moulding [85] solution casting after gelatinization [21,23], direct melting and
solidification [86] and one step in-situ intercalative solution polymerization [87]. The
one step in-situ polymerization method involves the dispersion of nanofillers in
monomer(s), followed by bulk or solution polymerization. The nanofillers are always
modified by functional groups to increase the interaction between the polymers and the
nanofillers, or to get a good dispersion in the polymer matrix. This method has been
reported to have many advantages, such as ease of handling and better performance of
the final products [88,89].
The greatest challenge in the making of nanocomposites over the past decade has been
the compatibility of the hydrophobic (water repelling) polymer matrix and hydrophilic
(water absorbing) fibers which result in non-uniform dispersion of fibers within the
matrix and poor mechanical properties [13,90]. To improve the affinity and adhesion
between fibers and thermoplastic matrices in composites production, additives such as
chemical coupling agents or compatibilizers (Maleated polyethylene (MAPE),
406 Adeosun Sunday. O. , Lawal G. I Vol.11, No.4
carboxylated polyethylene (CAPE), titanium derived mixture (TDM), Maleic anhydride
polypropylene (MAPP), corono discharge), calendaring, stretching, thermo treatment,
reaction with methanolmelamine, isocyanates, triazine, silane and mercerization of the
matrix have been employed [11,39,40,74,84,86,91-93] with no significant result. A better
understanding of the molecular structure and interfacial interaction between the matrix
and the fibers and the relationship between the structure and property would be a major
breakthrough in this area of research.
Another challenge of interest is the quest for true green polymers with good mechanical
properties to be used as matrix material. Biopolymers such as starch have poor water
resist ance, inferior tensile properties and are highly brittle due to their large particle size
which necessitates the use of plasticizers such as glycerol in the presence of heat and
pressure [49,94]. No significant result has been recorded apart from ductility imparted by
the plasticizers. Moreover, it is noted that the use of plasticizers in starch leads to the
presence of residual sugar in the matrix which impairs its adhesion properties to natural
fillers [85]. On the other hand PLA have unique properties such as high mechanical
strength, low toxicity and good barrier properties [6]. However, PLA composites are
limited in applications due to its low glass transition temperature, weak thermal stability,
low ductility and toughness and low modulus above the glass transition temperature [95].
Generally, modifiers have been used to improve stiffness at elevated temperatures,
reduce cost and increase the degradation rate [6]. PLA is the most promising of all the
biopolymers currently in use and require much attention. Other biopolymer sources such
as cellulose, gelatin, chitosan and plant-based oi ls are more scar ce sources and involve a
more tedious and costly production process.
In nanotechnology it is necessar y to separate filler particles into the right shape and layer
structure. To achieve maximum properties these particles need to be very thin (one
nanometer) and very wide (500 nanometers). Achieving this is an enormous task and
requires sophisticated machines such as high pressure homogenizers and inline
dispersers. Although recent technology has made this possible, it has been observed that
efforts to attain nanosize particles produce wide size ranges result which gives rise to
inconsistencies [22,43]. In addition, particle orientation has effect on the tensile
Vol.11, No .4 Review of Green Polymer Nanocomposites 407
properties of nanocomposite materials. It is difficult to take the orientation of nanosize
fibers into consideration but in macromechanics it has been observed that t he orientation
of the fibers have an overiding effect on the mechanical properties of the composite
material. How to achieve proper orientation of particles in the matrix is already being
examined [96]. Besides there is a possibility of reaggregation - where the particles are
lumped together. This is a function of the processing method and its prevention must be
taken into consideration when processing the nanocomposites.
Nanocomposites have been used in several applications such as mirror housings on
various vehicle types, door handles, door panels, trunk liners, instrument panels, parcel
shelves, head rests, roofs, upholstery and engine covers and intake manifolds and timing
belt covers. Other applications currently being considered include impellers and blades
for vacuum cleaners, power tool housings, mower hoods and covers for portable
electronic equipment such as mobile phones, pagers et c [1,11]. Its excellent barrier
properties, chemical resistance and surface appearance make it an excellent material for
packaging applications such as in beer and carbonated drinks bottles and paperboard for
fruit juice and dairy. Nanocomposites also have future in aerospace applications because
of their light weight.
Natural fiber reinforced composites using biodegradable polymer as matrix are
considered as most environmentally friendly material. The making of novel polymers
with inherent environmentally favorable properties such as renewability and
degradability, a series of interesting polymers have been realized through multiple
research activities ranging from thermoplastic starch and its blends, PLA and its
modifications, cellulose, gelatin, chitosan, etc. Natural fiber being preferred over
synthetic fibers for environmental reasons has also been synthesized from agricultural
sources such as kenaf, jute, hemp, flax, banana, bamboo, sisal and coconut coir, etc.
There is thus a wide range of possible applications of nanocomposites. However, the
408 Adeosun Sunday. O. , Lawal G. I Vol.11, No.4
many problems of poor adhesion of matrix and fiber, difficulty of fiber orientation, the
achievem ent of nanos cale sizes and the evolution of truly green polymers, which will be
environmentally friendly and renewable, must first be solved.
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