DSpace at VNU: Effect of Filler Surface Treatment on the Properties of Recycled High-Density Polyethylene (Natural Rubber) (Kenaf Powder) Biocomposites

Effect of Filler Surface Treatment on the Properties of
Recycled High-Density Polyethylene/(Natural Rubber)/(Kenaf
Powder) Biocomposites

Xuan Viet Cao,1 Hanafi Ismail,1 Azura A. Rashid,1 Tsutomu Takeichi,2 Thao Vo-Huu3
1
School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia,
14300, Nibong Tebal, Malaysia
2

School of Materials Science, Toyohashi University of Technology, Tempaku-cho, Toyohashi, 4418580, Japan

3

Department of Polymer Materials, Faculty of Materials Technology, Ho Chi Minh University of Technology,
Ho Chi Minh City, Vietnam

Biocomposites were prepared from a kenaf core powder and recycled high-density polyethylene/(natural
rubber) blend by using an internal mixer at 165oC and
50 rpm. The effect of the filler content and the filler
surface treatment was studied. Chemical modification
of kenaf filler was performed with alkali pretreatment
followed by treatment with silane. Scanning electron
microscopy and infrared spectroscopy studies confirmed changes in the chemical compositions and
structural characteristics induced through the modification. It was found that treated biocomposites offered
higher tensile strength and tensile modulus, but lower
elongation at break compared with untreated biocomposites. Lower water absorption and higher thermal
stability of the resultant biocomposites were also
obtained when treated fillers were used. J. VINYL ADDIT.
C
2014 Society of Plastics

TECHNOL., 00:000–000, 2014. V
Engineers

INTRODUCTION
The development of biocomposites by use of recycled
or recyclable polymers and natural organic fillers is on
the rise because of the exhaustion of petroleum resources
and growing public concern of the effect on the environment [1–3]. The advantages of natural fillers over inorganic counterparts include availability in large quantities,
low cost, low density, reasonable strength, reduced energy
consumption, and biodegradability [4].
Kenaf (Hibiscus cannabinus L.) is recognized as green
lignocelluloses plants with both economic and ecological
advantages. Unlike kenaf bast fibers, kenaf core is usually
used as a source material for paper products, fiberboard,
absorbents, and animal feeds. It was reported that kenaf
Correspondence to: Hanafi Ismail; e-mail:
DOI 10.1002/vnl.21374
Published online in Wiley Online Library (wileyonlinelibrary.com).
C 2014 Society of Plastics Engineers
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JOURNAL OF VINYL & ADDITIVE TECHNOLOGY—2014

core fibers are more homogeneous than hardwood fibers
[5], and paper from kenaf core has a high tensile and
burst strength compared with hardwood pulps [6]. In
addition, the availability of kenaf core, together with the
ease of its processability, could make it a good substitute
for inorganic fillers for biocomposites based on polymers
(i.e., rubber, plastics, and thermoplastic elastomers).

Thermoplastic elastomer (TPE), a blend of natural rubber (NR) and polyethylene, has received considerable
attention recently. Because of its unique microstructure, it
demonstrates elastic properties at room temperature and
flowability at high temperature. In the TPE system, NR, a
biodegradable polymer, plays a functional role as a toughener to overcome the brittleness in thermoplastic composites. A further benefit of TPEs is that TPEs provide high
value-added products if the components are derived from
waste sources (“upcycling”) [7]. In this study, recycled
HDPE (rHDPE)/NR blend was used as a matrix to produce biocomposites. Therefore, (kenaf core)-reinforced
rHDPE/NR biocomposites could provide environmental
advantages and cost reduction.
In previous studies, we have successfully prepared biocomposites on the basis of a kenaf core powder (KP) and
rHDPE/NR blend. However, it was found that the kenaf
has inherently low compatibility with nonpolar polymer
matrices, such as polyethylene and cis-polyisoprene (NR).
This drawback caused difficulties in achieving good dispersion and strong interfacial adhesion between the components, which led to composites with rather poor
mechanical properties. The use of (maleic anhydride)grafted polyethylene and (maleic anhydride)-grafted NR
enhanced the properties of rHDPE/NR/KP biocomposites
to some extent [8, 9].
Chemical modification on natural fiber presents a
promising approach for the establishment of covalent
bonding between the filler and matrix [10, 11]. It is


generally carried out with the use of reagents that contain
functional groups that are able to react with the hydroxyl
groups
from
fibers.
In
this

study,
gaminopropyltriethoxysilane (APTES) was used as a
silane-coupling agent for KP surface treatment. In addition, KP was pretreated with sodium hydroxide (NaOH)
to remove impurities and promote the possible reaction
between silane and filler. The effect of filler content and
filler treatment on the performance of the biocomposites
was evaluated.
MATERIALS AND METHODS
rHDPE was obtained from Zarm Scientific and Supplies Sdn. Bhd. (Penang) with a melt flow index of 0.237
g/10 min. NR used was SMR L grade from the Rubber
Research Institute of Malaysia (RRIM). APTES was supplied by Sigma Aldrich. Other chemicals, such as ethanol,
acid acetic, and NaOH, were used as received and were
provided by Bayer Chemicals (M) Sdn. Bhd. Kenaf core
fibers were obtained from Forest Research Institute
Malaysia (FRIM). Kenaf powder was produced by grinding kenaf core in a table-type pulverizing machine and
sieving to obtain the powder size in range of 32 to 150
mm.
Filler Surface Treatment with APTES
First, KPs were pretreated with NaOH. KP was
immersed in NaOH solution (5% w/v) for 2 h at room
temperature. Then, the fillers were washed with distilled
water containing a few drops of acetic acid. Subsequently,
fillers were washed thoroughly with distilled water. After
washing, the fillers were kept in air and dried in an oven
at 80oC for 6 h.
Second, silane treatment was carried out in a mixture
of water/ethanol (30/70 v/v) for the pretreated NaOH KP
(KP-NaOH). One gram of APTES was dissolved for
hydrolysis in 1,000 mL of the water/ethanol mixture. The
pH of the solution was adjusted to 4 with acetic acid and

stirred for 1 h [12]. Then, 10 g of KP was soaked in the
solution and stirred continuously for 3 h at room temperature. The filler was filtered and dried in air. Finally,
APTES-treated KP (KP-APTES) was dried in a vacuum
oven at 80oC for 24 h prior to compounding.
Preparation of rHDPE/NR/KP Biocomposites
The 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 80oC for 24 h. The
rHDPE was first added to the mixer and melted for 3
min. After 3 min, NR was added. After 6 min, KP (or
KP-APTES) was added. The blend was mixed further for
another 6 min, at which time the stabilization torque was
received, indicating the formation of a homogeneous sample. The total mixing time was 12 min for all samples.
The blend composites were then compression-molded in a
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JOURNAL OF VINYL & ADDITIVE TECHNOLOGY—2014

TABLE 1. Formulation of rHDPE/NR/KP biocomposites.a
Materials

Composition (php)

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

70
30
0, 10, 20, 30, 40

php, parts per hundred polymer.
a
Similar biocomposites with KP-APTES were also prepared.

hydraulic hot press into 1-mm sheets for preparation of
test samples. The hot-press procedure involved preheating
at 165oC 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 testing was done by using a Perkin-Elmer 2000
testing instrument. The FTIR spectrum was recorded in
the transmittance range from 4,000 to 500 cm21 with a
resolution of 4 cm21. There were eight scans for each
spectrum. All FTIR spectra were obtained by using attenuated total reflectance. About 5 mg of KP was mixed
with 95 mg of potassium bromide and pressed to form
pellets. FTIR was performed on the pellets to obtain the
information on the chemical modification of KP.

Tensile Properties
The tensile properties were measured by using an Instron 3366 machine at a cross-head speed of 50 mm/min
at 25 6 3oC according to ASTM D 412. Tensile strength,
tensile modulus, and elongation at break (Eb) of the each
sample were obtained from the average of five
specimens.

Water Absorption
A water absorption test was carried out by immersing
samples in distilled water at room temperature (25oC).

After immersion in water, samples were removed, patted
dry with a soft cloth, and weighed at regular intervals on
an electronic balance. The percentage of water absorption,
Mt, was calculated by
¸ ðwt –wo Þ=wo
Mt 5100C

(1)

where wo and wt are the original dry weight and weight
after exposure, respectively.

Scanning Electron Microscopy (SEM)
The topology of filler and tensile fractured specimens
was analyzed with a Supra-35VP field emission scanning
electron microscope. All samples were coated with gold/
palladium by a sputter-coating instrument (Bio-Rad
DOI 10.1002/vnl


loss temperature (T50%), and maximum degradation temperature (Td).
RESULTS AND DISCUSSION
Filler Characterization

FIG. 1. FTIR spectra of untreated KP (a), KP-NaOH (b), and KPAPTES (c). [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]

Polaron Division) for 45 min to prevent electrostatic
charging during evaluation.
Thermogravimetric Analysis (TGA)

Analysis by TGA was carried out by using a PerkinElmer TG-6 Analyzer to determine the thermal stability
of the composites. About 10–20 mg of sample was heated
at 10 C/min from 30 C to 600 C with a nitrogen flow
rate of 20 mL/min. The weight loss curve (TGA) and
derivative weight loss curve (DTG) were analyzed to
obtain the 5% weight loss temperature (T5%), 50% weight

Infrared spectra of untreated KP, KP-NaOH, and KPAPTES are presented in Fig. 1. Chemical modification of
KP led to a change of molecular interactions that showed
wave number shifts in the FTIR spectra. A peak at 1,732
cm21 was assigned to unconjugated C5O groups in hemicellulose of the untreated KP. This peak fully disappeared after pretreatment with NaOH. Treatment of filler
with amino silane also showed some peaks shifts at 710
and 460 cm21, corresponding to the Si--O--Si asymmetric
stretching and Si--O--C asymmetric bending, respectively
[13, 14]. Slight changes in the peaks found in the 1,0301,060 cm21 region and peak at 1,267 cm21 should also
be noted. These changes could be attributed to the presence of asymmetric stretching of Si--O--Si and/or Si--O-C bonds [13, 15]. The appearance of siloxane bonds was
indicative of a polysiloxane depositing on the filler,
whereas the alkoxysilane bonds seemed to confirm the
occurrence of a condensation reaction between APTES
and KP. In addition, bands in the 3,200-3,600 cm21 range
became broader, which might be because of the NH2
stretch vibration from APTES [16].
The change in surface morphology of the treated KP
was examined by analysis by SEM. Figure 2 shows the

FIG. 2. SEM micrographs of untreated KP (a), KP chemically treated with NaOH (b) and APTES (c).

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time and reached stabilization torque at the end of mixing
time as the composites became homogenous. However,
higher torque values in the case of KP-APTES were
observed after the sixth minute. Figure 4 illustrates the
effect of filler treatment on stabilization torque of
rHDPE/NR/KP biocomposites with respect to filler content. It can be observed that the stabilization torque of
treated composites was higher than that of the untreated
composites. Silane modification of KP might result in
increased melt viscosity of the composites owing to
enhanced interaction between filler and polymer.

Tensile Properties

Mixing torque of rHDPE/NR/KP biocomposites at 0
and 20 KP (parts per hundred polymer, php) is shown in
Fig. 3. Unlike the blend, the composite torque curve has
three peaks corresponding to loading peaks of rHDPE,
NR, and KP. Generally, torque decreased with mixing

Generally, the incorporation of KP into the rHDPE/NR
blend reduced the tensile strength and Eb while increasing
the tensile modulus of the composites. More detailed discussion of filler content has been presented elsewhere [8,
9]. This study mainly focused on the influence of filler
treatment on the properties of rHDPE/NR/KP biocomposites. Figure 5 depicts the effect of silane treatments on
the tensile strength of the composites. APTES pretreatment of filler showed a positive effect on the tensile
strength but the increment was only between 3.7% and

6.6%. The reason for improvement in tensile strength
might be because of better filler dispersion in the matrix
and a fair degree of adhesion at the interface [17, 18].
Indeed, previous NaOH treatment could remove impurities and waxy substances from the fiber surface and create a rougher topography. Thus, the mechanical
interlocking would be promoted, and the interface quality
was enhanced further by silane treatment [13]. The interaction between phases was expected to improve because
the KP surface became less hydrophilic because of chemical bonding between APTES and the OH groups at the
filler surface. Nevertheless, the low silane concentration
and difficulty of grafting reaction might render the effectiveness of APTES [[19]].
Figure 6 illustrates the effect of filler treatment on the
Eb of the rHDPE/NR/KP biocomposites. It was clearly

FIG. 4. Effect of filler treatment on stabilization torque of rHDPE/NR/
KP biocomposites.

FIG. 5. Effect of filler treatment on tensile strength of rHDPE/NR/KP
biocomposites.

FIG. 3. Effect of filler treatment on the torque-time curves of rHDPE/
NR/KP biocomposites.

SEM micrographs of filler surface before and after chemical treatment. It can be seen that the KP-NaOH surface
(Fig. 2a) appeared rougher and cleaner than the untreated
KP (Fig. 2b) because external impurities were mostly
removed from the surface of KP. The pretreatment of KP
with NaOH was expected to remove hemicelluloses, lignin, and waxes present on the surface of the fiber. This
observation was in good agreement with FTIR results,
which confirmed the disappearance of the peak at 1,732
cm21. Figure 2c shows the topography of KP-APTES. It
can be observed that there was no significant change in

surface morphology of KP-APTES compared with KPNaOH. However, after silane treatment, the surface of KP
seemed to be covered with an additional layer, which corresponded to the deposition of siloxane.
Mixing Torque

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FIG. 6. Effect of filler treatment on elongation at break of rHDPE/NR/
KP biocomposites.

observed that KP-APTES treatment had an adverse effect
on Eb. The lower Eb of composites with KP-APTES was
associated with enhanced adhesion between the filler and
matrix. Better adhesion gives way to more restriction of
the deformation capacity of the composites; thus, catastrophic failure occurs after small strain deformations.
Figure 7 shows that with increasing KP content, there
was an increase in tensile modulus for untreated and treated
biocomposites. Significant improvement in the modulus of
the treated composites could be related to better adhesion
between the fiber and the matrix through a grafting reaction,

FIG. 7. Effect of filler treatment on tensile modulus of rHDPE/NR/KP
biocomposites.

because the silane coupling agent reduced incompatibility
between the fibers and the rHDPE/NR matrix. Therefore, it

increased their interfacial adhesion. Better adhesion led to
more restriction of the deformation capacity of the matrix in
the elastic zone and increased modulus [20, 21].
Morphological Study
Analysis by SEM was used to evaluate the effect of
the filler treatment on the morphology of the tensile fracture surface of the composites. The SEM micrographs of
untreated and (KP-APTES)-treated composite samples at

FIG. 8. SEM micrographs of rHDPE/NR/KP biocomposites at 40 phr of KP (a) untreated (32003), (b) KP-APTES (3200), and (c) KP-APTES
(3400).

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KP was pretreated with NaOH to remove lignin, hemicelluloses, and other impurities. This pretreatment is an
effective means of advocating better property retention of
composites when exposed to moisture [22]. In addition,
the improved interaction between the matrix and filler
after APTES treatment through hydrogen bonding led to
the reduction of water absorption of the composites.
Thermogravimetric Analysis

FIG. 9. Effect of filler treatment on water absorption of rHDPE/NR/
KP biocomposites.

40 phr of KP are shown in Fig. 8. In the case of the

untreated composites, some fiber detachment and voids
can be seen (Fig. 8a). In contrast, in Fig. 8b, the presence
of a number of fibers sticking out of the matrix was visible and less fiber pullout was observed on the fractured
surface of the treated composites. This detachment and
voids occurred because the rough surface of treated filler,
in addition to the chemical bond, facilitated the mechanical locking between the KP filler and the matrix. A closer
examination of the fracture surface revealed that the level
of adhesion between filler and matrix was greatly
improved because the filler was embedded in the matrix
and broken under the tensile load (Fig. 8c).

Figure 11a and b shows the TGA and DTG curves of
untreated and treated rHDPE/NR/KP biocomposites at different filler contents. All composites were less thermally
stable than the rHDPE/NR blend because of the lower
thermal stability of the kenaf fiber. As expected, two
peaks of DTG curve were observed for all samples, which
corresponded to two main degradation stages that
occurred from the matrix materials. In the rHDPE/NR/KP
composites, two other peaks were also obtained. A peak
was observed at 140oC corresponding to the dehydration
of KP fiber and a major peak at 330 C was caused by the
thermal degradation of cellulose [23]. TGA parameters
for rHDPE/NR/KP biocomposites can be seen in Table 2.
Generally, silane treatment improved the thermal stability of rHDPE/NR/KP biocomposites to some extent,

Water Absorption
Water absorption is one of the key parameters in the
evaluation of quality of lignocellulosic fiber composites.
Water absorption of rHDPE/NR/KP biocomposites at 0
and 20 phr of KP as a function of the immersion time is

shown in Fig. 9, whereas Fig. 10 shows the water uptake
of the rHDPE/NR/KP biocomposites at 63 days. It was
obvious that silane treatment resulted in lowering of the
water uptake when compared with the untreated samples.

FIG. 10. Water uptake at 63 days of rHDPE/NR/KP biocomposites.

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FIG. 11. (a) Typical TGA curves of rHDPE/NR/KP biocomposites. (b)
Typical DTG curves of rHDPE/NR/KP biocomposites. [Color figure can be
viewed in the online issue, which is available at wileyonlinelibrary.com.]

DOI 10.1002/vnl


REFERENCES

TABLE 2. TGA data for rHDPE/NR/KP biocomposites.

Samples
rHDPE/NR
10KP
10KP-APTES
40KP
40KP-APTES

Temperature

at 5% weight
loss T5%
( C)

Temperature
at 50%
weight
loss T50%
( C)

Maximum
degradation
temperature
Td ( C)

Weight
residue
(%)

301.3
304.7
331.0
277.4
289.1

480.5
464.3
470.0
463.0
461.6

487.0
487.3
488.6
488.0
488.2

0.0
0.17
0.98
7.3
5.92

particularly at low filler content. All the degradation temperatures in the first degradation region were shifted to a
higher temperature, which suggested that at low temperatures fiber with high lignin and hemicelluloses content
exhibited low thermal stability, whereas fiber with higher
cellulose content showed better thermal stability [24]. The
alkaline-silane filler treatment did reduce the hemicelluloses and lignin to a considerable extent [25] and thus led to a
better thermal stability over this temperature range (disappearance of hemicelluloses peak in DTG curve). However,
lignin seems to be more stable than celluloses and hemicelluloses at high temperatures; hence, lower lignin content
resulted in a lower thermal resistance at a high temperature
range. Alkaline pretreatment also caused a decrease in the
char yield because it removed a portion of the cell structure
(hemicelluloses or lignin) and eliminated some inorganic
matter [26].
CONCLUSIONS
Spectroscopic analysis (FTIR) and analysis by SEM
revealed that alkali pretreatment increased the surface roughness by removing hemicelluloses and lignin, which could
promote a better mechanical interlocking with the matrix.
Silane treatment further enhanced the compatibility of kenaf

with the polymer matrix by introducing a compatible molecular structure on the filler surface. Surface modification of
kenaf filler showed a positive effect on tensile strength of
the composites. Tensile modulus of the treated biocomposites
was increased but Eb was reduced compared with the
untreated biocomposites. This improvement was a result of
the enhanced interfacial adhesion between the rHDPE/NR
matrix and KP via physical and chemical bonding between
the components. Water absorption was found to reduce with
fiber treatment. In addition, the filler treatment also improved
the thermal stability of rHDPE/NR/KP biocomposites to
some extent, particularly at low filler content.
ACKNOWLEDGMENT
The authors thank AUN/SEED–Net/JICA for the financial support through a Collaborative Research (CR) grant.

DOI 10.1002/vnl

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