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Maleated Natural Rubber as a Coupling Agent for Recycled High Density Polyethylene/Natural Rubber/ Kenaf Powder Biocomposites
Xuan Viet Cao a , Hanafi Ismail a , Azura A Rashid a , Tsutomu Takeichi b & 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, Polymer-Plastics Technology and Engineering, 51:9, 904-910, DOI: 10.1080/03602559.2012.671425
To link to this article: http://dx.doi.org/10.1080/03602559.2012.671425
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Trang 2Maleated 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
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 165C 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
absorp-tion 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
biocompo-sites 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
sup-ply in the near future and environmental concerns This
class of composites indicated as biocomposite, which shows
various benefits and good properties inherited from its
con-stituents Fillers (bio-fibers or powders) used in polymer
composites mainly include banana, sisal, hemp, jute,
pine-apple, 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
consump-tion, and environmental friendliness[1–3]
Kenaf is gaining a lot of attention in the composite industry, since they can be applied as filler in polymer com-posites 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 com-position 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 plas-tics is expected to be different from those of the corre-sponding 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 rub-ber (NR) blend is still very limited In this study, the poten-tial 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 vari-ous 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
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
ISSN: 0360-2559 print=1525-6111 online
DOI: 10.1080/03602559.2012.671425
904
Trang 3compatibilizer 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
mech-anical 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
tem-perature of 135C 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 80C for 24 h Mixing
pro-cess was carried out at 165C 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
165C 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
extracted the unreated MAH by Soxhlet extraction in acetone for 24 h, and further dried in a vacuum oven at
40C for 24 h prior to FTIR measurement FTIR spectrum was recorded in the transmittance range from 4000 to
600 cm1 with a resolution of 4 cm1 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 3C according to ASTM D 412 Tensile strength, ten-sile 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 (25C) 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 ðwt woÞ=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 micro-scope (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
TABLE 1 Formation of rHDPE=NR=KP biocomposites Materials Composition (phr) Recycled high density polyethylene
(rHDPE)
70
Natural rubber (SMR L) 30 Kenaf powder (KP) 0, 10, 20, 30, 40
Note (phr)-part per hundred resin
a
5% of KP
Similar biocomposites but without MANR were also prepared
Trang 4RESULTS AND DISCUSSION
FTIR Analysis
The FTIR spectra of NR and MANR are shown in
Figure 1 The peaksat 1662 cm1and 833 cm1were
corre-sponded to C¼ C of NR and observed for both cases For
the MANR spectrum, the absence of the absorption peak
at 698 cm1 suggested the unreacted MAH has been
com-pletely removed from the MANR A broad and intense
characteristic peak at 1779 cm1 and a weak absorption
peak at ca 1862 cm1 were observed These peaks were
assigned to grafted anhydride, which are due to symmetric
(strong) and asymmetric (weak) C¼ O stretching
vibra-tions of succinic anhydride rings, respectively[18,19]
There-fore, 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
shear-ing stability of the composites The addition of
compatibi-lizers 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
com-posites (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
corre-sponding 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
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 1 FTIR spectra of NR (a) and MANR (b).
FIG 2 Torque development for rHDPE=NR=KP biocomposites with MANR at different KP content.
FIG 3 Stabilization torque at 12 min of rHDPE=NR=KP biocomposites.
Trang 510 and 40 KP content are depicted in Figure 4 The
differ-ence 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
hydro-philic 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
Figure 5, tensile strength of rHDPE=NR=KP biocompo-sites 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 com-posites 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 4 Stress-strain behavior of rHDPE=NR=KP biocomposites at
various filler content (Color figure available online.)
FIG 5 Tensile strength of rHDPE=NR=KP biocomposites at various
filler content.
FIG 6 Elongation at break of rHDPE=NR=KP biocomposites at various filler content.
FIG 7 Possible hydrogen bonding formed between KP and MANR.
Trang 6similar 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
simi-lar 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
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 num-ber of –OH groups could be reduced via the hydrogen bond between MANR and –OH of KP fiber The improve-ment 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 8 Tensile modulus of rHDPE=NR=KP biocomposites at various
filler content.
FIG 9 Water absorption of rHDPE=NR=KP biocomposites without
MANR.
FIG 10 Equilibrium water uptake at 63 days of rHDPE=NR=KP biocomposites with and without MANR.
Trang 7or 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
inter-face 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
CONCLUSIONS Maleated natural rubber was prepared and used in this study to improve the interfacial adhesion between hydro-philic 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 biocom-posites with MANR provided an enhancement in tensile strength, tensile modulus, elongation at break, and water absorption compared to the biocomposites without MANR
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