DSpace at VNU: Maleated Natural Rubber as a Coupling Agent for Recycled High Density Polyethylene Natural Rubber Kenaf Powder Biocomposites

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Maleated Natural Rubber as a Coupling Agent for
Recycled High Density Polyethylene/Natural Rubber/
Kenaf Powder Biocomposites
a

a

a

b

Xuan Viet Cao , Hanafi Ismail , Azura A. Rashid , Tsutomu Takeichi & Thao Vo-Huu

c

a

School of Materials & Mineral Resources Engineering, Engineering Campus, Universiti Sains
Malaysia , Malaysia
b

Department of Environmental and Life Sciences , Toyohashi University of Technology ,

Japan
c

Department of Polymer Materials, Faculty of Materials Technology , Ho Chi Minh University
of Technology , Vietnam
Published online: 27 Jun 2012.

To cite this article: Xuan Viet Cao , Hanafi Ismail , Azura A. Rashid , Tsutomu Takeichi & Thao Vo-Huu (2012) Maleated Natural
Rubber as a Coupling Agent for Recycled High Density Polyethylene/Natural Rubber/Kenaf Powder Biocomposites, PolymerPlastics Technology and Engineering, 51:9, 904-910, DOI: 10.1080/03602559.2012.671425
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Polymer-Plastics Technology and Engineering, 51: 904–910, 2012
Copyright # Taylor & Francis Group, LLC
ISSN: 0360-2559 print=1525-6111 online
DOI: 10.1080/03602559.2012.671425

Maleated Natural Rubber as a Coupling Agent for Recycled
High Density Polyethylene/Natural Rubber/Kenaf Powder
Biocomposites
Xuan Viet Cao1, Hanafi Ismail1, Azura A. Rashid1, Tsutomu Takeichi2, and
Thao Vo-Huu3

Downloaded by [Temple University Libraries] at 02:47 15 November 2014

1

School of Materials & Mineral Resources Engineering, Engineering Campus, Universiti Sains
Malaysia, Malaysia
2
Department of Environmental and Life Sciences, Toyohashi University of Technology, Japan
3
Department of Polymer Materials, Faculty of Materials Technology, Ho Chi Minh University of
Technology, Vietnam

Kenaf powder (KP) was incorporated into recycled high density
polyethylene (rHDPE)/natural rubber (NR) blend using an internal
mixer at 165 C and rotor speed of 50 rpm. The tensile strength and
elongation at break of the composites decreased, while the tensile
modulus increased with increasing filler loading. The water absorption was found to increase as the filler content increased. The maleic
anhydride grafted natural rubber was prepared and used to enhance
the composites performance. The addition of MANR as a coupling
agent improved the tensile properties of rHDPE/NR/KP biocomposites. The water absorption was also reduced with the addition of
MANR.
Keywords Biocomposites; Kenaf powder; Maleated natural
rubber; Naturalrubber; Recycled high density

polyethylene

INTRODUCTION
The development of polymer composites using recycled
or recyclable polymers and natural organic fillers is very
actively pursued due to threats of uncertain petroleum supply in the near future and environmental concerns. This
class of composites indicated as biocomposite, which shows
various benefits and good properties inherited from its constituents. Fillers (bio-fibers or powders) used in polymer
composites mainly include banana, sisal, hemp, jute, pineapple, bamboo, cotton, coconut, rice husk, and kenaf.
These fillers offer several advantages such as large quantity,
annual renewability, low cost, light weight, competitive
specific mechanical properties, reduced energy consumption, and environmental friendliness[1–3].

Address correspondence to Hanafi Ismail, Polymer Division,
School of Material and Mineral Resources Engineering,
Engineering Campus, Universiti Sains Malaysia, 14300 Nibong,
Tebal, Penang, Malaysia. E-mail: hanafi@eng.usm.my

Kenaf is gaining a lot of attention in the composite
industry, since they can be applied as filler in polymer composites. It is widely planted in Malaysia and was found to
be the most suitable crop for commercial-scale production
due to the climate in Malaysia[4]. Kenaf stem is composed
of two distinct fibers, bast and core. The average stem composition is 35% bark and 65% woody core by weight. The
bark contains a long fiber called bast fiber, whereas the
woody core contains short core fibers[5]. The abundance
of kenaf core combined with the ease of its processability
is an attractive characteristic, could make it a desirable
substitute for synthetic fillers that is a potentially toxic.
Due to the difference in the composition of recycled
plastics, the performance of composites from recycled plastics is expected to be different from those of the corresponding virgin plastics. Some work has been carried out

on natural fiber reinforced of recycled PE[6–10]. However,
work done on natural fiber filled recycled PE=natural rubber (NR) blend is still very limited. In this study, the potential of using kenaf core and recycled HDPE=NR blend for
making biocomposites was examined.
The main problem that has prevented a more utilization
of natural fiber in TPE composites is the lack of good
adhesion between the hydrophilic fillers and hydrophobic
matrixes. This results in poor mechanical properties of final
products. It was found that the interfacial adhesion can be
improved by using coupling agents.It is well known that
maleated coupling agents have been widely used for various single polymer composites (both plastic and rubber
composites)[11–15].
However, its utilization in thermoplastic elastomer
composites has been less studied and remained
promising. In previous study[16], the authors reported that
maleic anhydride grafted polyethylene which is more
favorable for rHDPE phase was successfully used as a

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rHDPE=NR=KP BIOCOMPOSITES

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compatibilizer to enhance the properties of rHDPE=NR=
KP biocomposites.
To the best of our knowledge, no attempt has been made
towards the employment of maleated natural rubber
(MANR) as a coupling agent in polyolefin natural rubber
composites. In the present work, (MANR) was prepared

and used as a coupling agent for this system. MANR
was expected to facilitate the interactions between rubber
phase (NR) and filler (KP) as well as plastic (rHDPE)
and rubber (NR) phase. The effect of MANR on the mechanical properties, water absorption, and morphology of the
biocomposites was investigated.
EXPERIMENTAL
Materials and Chemicals
Recycled high density polyethylene (rHDPE) was
obtained from Zarm Scientific and Supplies Sdn Bhd,
Penang with melt flow index of 0.237 g=10 min. Natural
rubber used was SMR L grade from the Rubber Research
Institute of Malaysia (RRIM). Maleic anhydride (MA) was
supplied by Sigma Aldrich. Kenaf powder was produced
by grinding kenaf core in a table-type pulverizing machine
and sieved to obtain the powder size in range of 32 to
150 mm.
Preparation of Maleic Anhydride Grafted
Natural Rubber
Maleic anhydride grafted natural rubber (MANR) was
prepared in an internal mixer (Haake Rheomix) at a temperature of 135 C for 10 min and a rotor speed of 60 rpm
according to a procedure reported by Nakason et al.[17]
The maleic anhydride content used in this study was 6 phr.
Preparation of rHDPE/NR/KP Biocomposites
Formulation of rHDPE=NR=KP biocomposites is given
in Table 1. Prior to compounding, rHDPE and KP were
dried by using a vacuum oven at 80 C for 24 h. Mixing process was carried out at 165 C and a rotor speed of 50 rpm.
The rHDPE was first charged into the mixer and melted for
3 min. NR was added at third minute. The MANR and KP
were added at the 6th min, respectively. The blend was
allowed to further mixing for another 6 min to obtain the

stabilization torque. The total mixing time was 12 min for
all samples. The blends were then compression molded in
a hydraulic hot press into 1 mm sheets for preparing test
samples. The hot press procedure involved preheating at
165 C for 6 min followed by compressing for 3 min at the
same temperature, and subsequent cooling under pressure
for 2 min.
Fourier Transform Infrared Spectroscopy (FTIR)
FTIR (Perkin Elmer System 2000) was used to confirm
the grafting reaction between NR and MAH. Sample was

905

TABLE 1
Formation of rHDPE=NR=KP biocompositesÃ
Materials

Composition (phr)

Recycled high density polyethylene
(rHDPE)
Natural rubber (SMR L)
Kenaf powder (KP)
MANRa

70
30
0, 10, 20, 30, 40
5

Note. (phr)-part per hundred resin.
a
5% of KP.
Ã
Similar biocomposites but without MANR were also
prepared.

extracted the unreated MAH by Soxhlet extraction in
acetone for 24 h, and further dried in a vacuum oven at
40 C for 24 h prior to FTIR measurement. FTIR spectrum
was recorded in the transmittance range from 4000 to
600 cmÀ1 with a resolution of 4 cmÀ1. There were 8 scans
for each spectrum. All FTIR spectra were obtained using
attenuated total reflectance (ATR).
Tensile Properties
The tensile properties were measured using an Instron
3366 machine at a cross-head speed of 50 mm=min at
25 Æ 3 C according to ASTM D 412. Tensile strength, tensile modulus, and elongation at break of the each sample
were obtained from the average of five specimens.
Water Absorption
A water absorption test was carried out by immersing
the samples in distilled water at room temperature
(25 C). The water absorption was determined by weighing
the samples at regular intervals on an electronic balance.
The percentage of water absorption, Mt, was calculated by
Mt ¼ 100 Â ðwt À wo Þ=wo

ð1Þ

where wo and wt are the original dry weight and weight

after exposure, respectively.
Scanning Electron Microscopy (SEM)
The morphology of the composites was also analyzed
with a Supra-35VP field emission scanning electron microscope (SEM). The objective was to get some information
regarding filler dispersion and bonding quality between
matrix and filler. The fracture surfaces obtained from
tensile test were coated with gold=palladium by a sputter
coating instrument (Bio-Rad Polaron Division) for
45 min to prevent electrostatic charging during evaluation.


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X. V. CAO ET AL.

RESULTS AND DISCUSSION
FTIR Analysis
The FTIR spectra of NR and MANR are shown in
Figure 1. The peaksat 1662 cmÀ1 and 833 cmÀ1 were corresponded to C ¼ C of NR and observed for both cases. For
the MANR spectrum, the absence of the absorption peak
at 698 cmÀ1 suggested the unreacted MAH has been completely removed from the MANR. A broad and intense
characteristic peak at 1779 cmÀ1 and a weak absorption
peak at ca 1862 cmÀ1 were observed. These peaks were
assigned to grafted anhydride, which are due to symmetric
(strong) and asymmetric (weak) C ¼ O stretching vibrations of succinic anhydride rings, respectively[18,19]. Therefore, it can be confirmed that the MAH was successfully
grafted onto NR backbone.A possible reaction mechanism
can be found elsewhere[17].
Processing Characteristics

The torque development provides information regarding
the effectiveness of mixing, thermal and mechanical shearing stability of the composites. The addition of compatibilizers or coupling agents can also be studied through the
torque versus time curves.
Figure 2 shows the torque behavior of rHDPE=NR=KP
biocomposites with MANR as a coupling agent. Generally,
a similar pattern of torque curves was observed for all composites (except for the rHDPE=NR blend). The first rise in
torque was attributed to the resistance exerted by solid
rHDPE against the rotors. The torque decreased as
rHDPE melted with mechanical shearing and the rise of
internal temperature. The second peak was detected corresponding to NR charging.
These two peaks were obtained for all composites and
expressed the different amount of rHDPE and NR charged
into the mixing chamber. However, the change of torque
observed as MANR added was unobvious. The third peak
appeared at around the 7th min due to the introduction of
KP, which presented proportionally to the filler content.
Upon completion of filler dispersion, the torque started

FIG. 1.

FTIR spectra of NR (a) and MANR (b).

FIG. 2. Torque development for rHDPE=NR=KP biocomposites with
MANR at different KP content.

to decrease gradually due to a reduction in viscosity as
the stock temperature increased.
The stabilization torque of the composites is presented
in Figure 3. It can be seen that stabilization torque
increased gradually with increasing filler loading. This

was due to the higher the filler content the lower the
mobility of polymer chains and thus increased the viscosity
and stabilization torque. However, stabilization torque of
composites with MANR was found to be higher than that
of composites without MANR. Therefore, the addition of
MANR improved the filler-matrix interfacial bonding,
which resulted in the higher stabilization torque in the
composites with MANR[20].
Tensile Properties
The typical stress-strain curves of rHDPE=NR blend,
rHDPE=NR=KP composites with and without MANR at

FIG. 3. Stabilization torque at 12 min of rHDPE=NR=KP biocomposites.


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rHDPE=NR=KP BIOCOMPOSITES

907

FIG. 4. Stress-strain behavior of rHDPE=NR=KP biocomposites at
various filler content. (Color figure available online.)
FIG. 6. Elongation at break of rHDPE=NR=KP biocomposites at
various filler content.

10 and 40 KP content are depicted in Figure 4. The difference originated from the incorporation of filler and the
addition of coupling agent was evident from these curves.
The curve of rHDPE=NR blend displayed typical yield
behavior and ductile nature. However, rHDPE=NR=KP

composites exhibited more brittle behavior under tensile
load, which expressed shorter elongation and higher initial
slope (higher tensile modulus). This is common effect of
incorporation of short fiber into a thermoplastic or rubber
matrix.
Due to the weak interfacial bonding between the hydrophilic lignocellulosic filler and the hydrophobic polymer
matrixes, the stress propagation was obstructed. Therefore,
the composites could not elongate and broke when internal
stress increased at interface of filler and matrix, resulted in
lower tensile at break compared to yield strength and yield
strength then was reported as tensile strength. As shown in

FIG. 5. Tensile strength of rHDPE=NR=KP biocomposites at various
filler content.

Figure 5, tensile strength of rHDPE=NR=KP biocomposites decreased gradually with increasing filler content.
Increasing filler content from 10 phr to 40 phr, yield
strength was slightly reduced; hence tensile strength was
only decreased ca 1 MPa. The other reason caused poor
stress transfer in composites was the irregular morphology
of KP. This hindered the KP orientation during tensile test
and resulted in the deterioration of elongation of the composites. This explained the rather lower elongation at break
of composites after 20 phr KP compared to 10 phr KP as
shown in Figure 6.
As expected, the addition of MANR as coupling agent
enhanced the composites properties. MANR improved
interfacial adhesion between KP and matrix by forming
hydrogen bonding between KP and MANR. Coupling
mechanism of MANR in rHDPE=NR=KP composites is
proposed in Figure 7. The better interfacial bonding also

prevented fiber-fiber contact, hence gave the better filler
dispersion. As a result, tensile strength and elongation at
break increased with the addition of MANR.
This was also responsible for the higher tensile modulus
for composites with MANR. As shown in Figure 8, at a

FIG. 7.

Possible hydrogen bonding formed between KP and MANR.


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X. V. CAO ET AL.

FIG. 8. Tensile modulus of rHDPE=NR=KP biocomposites at various
filler content.

similar filler loading, composites with MANR exhibited
higher tensile modulus than those without MANR. The
incorporation of KP was expected to increase the modulus
resulting from the inclusion of rigid filler particles in the
soft matrix. These results indicated that tensile modulus
of the KP filled rHDPE=NR biocomposites followed the
same trend with the filled plastic and rubber composites.
Water Absorption
Water absorption of rHDPE=NR=KP biocomposites
without MANR is presented in Figure 9. All rHDPE=

NR=KP biocomposites with MANR also displayed a similar pattern of sorption, where the samples absorbed water
very rapidly during the first stages, followed by gradual
increase until reaching a certain value (saturated point).
Obviously, the water uptake of the composites increased
as filler content increased. The hydrophilic character of
natural fiber was responsible for the water absorption in

FIG. 9. Water absorption of rHDPE=NR=KP biocomposites without
MANR.

the biocomposites by forming hydrogen bonding between
water and the hydroxyl group of cellulose, hemicellulose
in the cell wall. As KP content increased, the number of
hydrogen bonding also increased. In rHDPE=NR=KP
biocomposites without MANR, because fiber-matrix
adhesion is weak, water can easily enter into the interfacial
gaps.
Figure 10 presents equilibrium water uptake at 63 days
of rHDPE=NR=KP biocomposites with and without
MANR. It is clear that the addition of MANR resulted
in lowering of water uptake compared to the composites
without MANR. As mentioned here, the water absorption
is dependent on the availability of free –OH groups on the
surface of the fiber. In composites with MANR, the number of –OH groups could be reduced via the hydrogen
bond between MANR and –OH of KP fiber. The improvement of interfacial adhesion between fiber and matrix also
reduced the water accumulation in interfacial gaps, hence,
limiting the penetration of water into the composites[21].
Morphology of Biocomposites
SEM was used to evaluate the effect of filler content and
the addition of coupling agent on the morphology of the

composites. These morphology observations are correlated
to the mechanical properties as well as the water absorption
of the biocomposites as discussed earlier. Tensile fracture
surfaces of the biocomposites with and without MANR at
10 and 40 phr of KP are shown in Figure 11. In the case
of composites without MANR at 10 phr (Fig. 11(a)),
the composites had matrix fibrillation and deformed in
ductile mode.
However, the fibers were not well oriented and the poor
adhesion at the interface can be deduced from the clean
surface of the fibers. This poor adhesion is clearly visible
at 40 phr (Fig. 11(b)), where some fibers were pulled out

FIG. 10. Equilibrium water uptake at 63 days of rHDPE=NR=KP
biocomposites with and without MANR.


rHDPE=NR=KP BIOCOMPOSITES

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CONCLUSIONS
Maleated natural rubber was prepared and used in this
study to improve the interfacial adhesion between hydrophilic kenaf powder and the rHDPE=NR hydrophobic
matrices. The SEM micrographs showed the better
adhesion at the fiber-matrix interfaces as MANR was
added to the composites. It was attributed to the hydrogen
bonding formed between the hydroxyl groups of fiber and

the maleic anhydride of MANR. rHDPE=NR=KP biocomposites with MANR provided an enhancement in tensile
strength, tensile modulus, elongation at break, and water
absorption compared to the biocomposites without
MANR.
REFERENCES
FIG. 11. Tensile fracture surfaces of rHDPE=NR=KP biocomposites at
200X: (a) 10 phr of KP, (b) 40 phr of KP, (c) 10 phr of KP with MANR,
and (d) 40 phr of KP with MANR.

or remained loosely with the matrix. Voids were also
presented in the samples. These features were evidences
for poor mechanical properties and high water uptake of
the uncoupled composites.
The addition of MANR significantly improved adhesion
to the fiber. Figures 11(c) and (d) shows the microstructure
of KP filled rHDPE=NR composites with MANR at 10 phr
and 40 phr, respectively. The morphology was clearly
different compared to the composites without MANR.
All the micrographs of the composites with MANR also
showed better dispersed fillers compared to the composites
without MANR. The better fiber-matrix adhesion can be
measured by the fact that more matrix fibrillation, rougher
fracture surfaces, and less fiber pull out were observed.
Interestingly, the improvement of the adhesion at the interface was still obvious at 30 phr by looking at a good
bonding between KP fiber and matrix (Fig. 12). The KP
fiber was coated and there were NR matrix tearing bridges
between fiber and matrices.

FIG. 12. Tensile fracture surface of rHDPE=NR=KP biocomposites at
30 phr of KP with MANR.

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