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Article
Effect of thermal
conductive fillers on
the properties of
polypropylene
composites
MS Nurul and M Mariatti
Abstract
We investigated the effects of various fillers such as carbon nanotube (CNT), syn-
thetic diamond (SND), boron nitride (BN), and copper (Cu) on the properties of
polypropylene (PP) composites. The thermal conductivity and stability of PP were
enhanced upon the addition of thermally conductive fillers. Young’s modulus increased
with filler loading, while tensile strength increased at up to 2 vol.% then decreased
with elongation in all filler types. The morphology of the composite samples showed
agglomeration and void content in PP/Cu composites, leading to the deterioration of
thermal and mechanical properties at high-volume loading. Findings indicate that PP/
CNT has better thermal and mechanical properties compared with the other types
of fillers.
Keywords
Thermal properties, tensile properties, conductive fillers, polypropylene, composites
Introduction
Conductive polymer composites (CPCs) are among the versatile materials that can be
used in several applications such as self-regulated heating, electromagnetic shielding,
vapor sensing, and bipolar plates in the fuel cell.
1
Reinforcement of polymers with con-
ductive fillers such as carbon nanotube (CNT), silica, synthetic diamond (SND), silicon
School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, Engineering Campus, Nibong
Tebal, Penang, Malaysia
Corresponding author:
M Mariatti, School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, Engineering
Campus, 14300 Nibong Tebal, Penang, Malaysia.
Email: mariatti@eng.usm.my
Journal of Thermoplastic Composite
Materials
26(5) 627–639
ªThe Author(s) 2011
Reprints and permissions:
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DOI: 10.1177/0892705711427345
jtc.sagepub.com
nitride, boron nitride (BN), copper (Cu), ferrite, bronze, and aluminum nitride can be
adapted to satisfy the required characteristics of CPCs.
2–5
Thus, CPCs are emerging
as one of the most economical and effective ways to cope with thermal management
issues.
6
In general, the effectiveness of reinforcing fillers in composites is inversely propor-
tional to the size of the filler. Previous studies reported that the absorption energy of a
smaller particle is higher than that of a larger particle due to its high surface energy.
7
Jung et al.
8
and Boudenne et al.
9
proved that the nano-sized conductive fillers in com-
posites give better thermal conductive and stability characteristics, since smaller parti-
cles have better interaction and can more easily form the conductive path than the
micron-sized particles.
Moreover, the geometry of the particle is an important factor in achieving the optimal
properties of composites. A greater surface-to-volume ratio of filler results in greater
effectiveness. Volume fraction is another factor that affects the effectiveness of the rein-
forcing filler; it should be as high as 20 by vol.%to afford satisfactory conductivity prop-
erties. However, filler loading at higher content is generally required to yield these
positive effects of fillers. This would detrimentally affect some important properties
of the polymers matrix, including processability, appearance, density, and ageing
performance.
10
In this study, we investigated the effects of four types of conductive fillers, specifi-
cally CNT, SND, BN, and Cu, in polypropylene (PP) composites. The correlations
between filler loading ranging from 0 to 4 vol.%and thermal and mechanical properties
of these composites were investigated.
Experimental
Materials
Homopolymer PP (Titanpro 6431) is a commercial product from Titan Polymer (M) Sdn.
Bhd, with a melt index of 7 g/10 min and a density of 0.9 g/cm
3
. CNT, SND, BN, and Cu
were supplied by Shenzhen Nanotech Port Co., Ltd, Heyuan Zhong Lian Nano-
technology, TaijiRing Nano-products, and Sigma Aldrich, respectively. The properties
of these fillers are presented in Table 1.
Table 1. Typical properties of thermal conductive fillers used in the study.
Properties (units) SND CNT BN CU
Thermal conductivity (W/mK) 2000 2000 300 385
Particle size distribution (nm) 5–6 2–645 30–43 379–492
Mean particle size (d
50
) 5.5 69 36 434
Density (g/cm
3
) 3.3 1.3 2.2 8.9
Shape Sphere Tube Sphere Sphere
BN: boron nitride, CNT: carbon nanotube, CU: copper, SND: synthetic diamond.
628 Journal of Thermoplastic Composite Materials 26(5)
Sample preparation
Conductive nanofillers were dried in oven at 100C for 3 h to remove moisture before
mixing with PP ranging at 1, 2, 3 and 4 vol.%of filler loading. Compounding between PP
and fillers was performed in a two-roll mill heater at a constant temperature of 185C and
at 50 rpm for 20 min. Then, the composite sheet was compression molded in an electri-
cally heated hydraulic press at 185C and subsequently cooled at 1000 psi for 3 min.
Filler characterizations
Particle size of the fillers was measured by Nanophox particle size analysis, model
NX0064. Data on particle size distribution were presented as cumulative distribution as a
function of particle size. Thermal stability of the filler was determined by thermogravi-
metric analysis (TGA)/differential thermal analysis (DTA) using Linseis model L75/04.
Fillers were heated from room temperature to 800C at a heating rate of 10C/min.
Composites characterizations
Flow behaviors of samples were determined using Dynisco Polymer Test model 4004
following the method described in American Society for Testing and Materials (ASTM)
D 1238-90b with a load of 2.16 kg at 230C and a melt time of 360 s. Cutting samples
within an interval of 10 s were weighed and melt index values were calculated in g/10 s.
Physical ashing test was performed according to ASTM D2584 to determine filler weight
fraction (W
f
) in the composites after compounding. Void content was determined from a
relationship between the theoretical density and the experimental density of the compo-
sites. Thermal conductivity was tested using a hot disc thermal constant analyzer model
TPS 2500 according to ASTM D792-98. The heat source was placed between two
448 mm samples and connected to thermal conductivity detector. TGA was per-
formed using model Perkin Elmer Pyris TGA-6. The sample was heated from room tem-
perature to 600Cat10
C/min in a nitrogen environment. Melting and crystallization
behavior of the composites was studied, employing differential scanning calorimeter
(DSC) using a Perkin-Elmer DSC-6 at a heating rate of 10C/min. Melting temperature
T
m
and crystallization temperature T
c
were derived from endothermic and exothermic
peak temperatures. The degree of crystallinity X
c
was calculated from heat of fusion
by taking 207 J/g as the enthalpy to crystallize 100%PP.
11
Tensile test was conducted
by Instron 3366 with gauge length of 50 mm and speed of 50 mm/min according to
ASTM D 638-98. The morphology of tensile fracture specimens was captured by ZEISS
SUPRA 35 VP field emission scanning electron microscope (FESEM).
Results and discussion
Melt flow index
Figure 1 illustrates the decreasing trends of melt flow index (MFI) as the conductive
filler loading was increased. These trends were expected because the incorporation of
Nurul and Mariatti 629
fillers hinders polymer flow and increases the viscosity of composites. PP/CNT exhib-
ited the lowest MFI due to the high aspect ratio of CNT, leading to strong intermolecular
interaction between the nanotubes. In contrast, the greater size of Cu (micron-sized)
resulted in a higher MFI value, which slightly increased at high Cu loading (i.e. 3 and
4 vol.%). This trend can be attributed to the metallic properties of Cu, such that it is able
to induce and catalyze the degradation of polymer composites. In addition, the heat
energy absorbed by Cu will spread to the surrounding PP matrix. Thus, the polymer
chains will be cut down, allowing MFI to increase.
12
Tensile properties
The correlation between average tensile strength and void content of PP and PP com-
posites is presented in Figure 2. SND and BN systems exhibited higher tensile strength
compared with CNT and Cu systems. The maximum tensile strength was observed at
2 vol.%, after which a decreasing trend was observed. Tensile strength was reduced at
higher nanofiller loading due to strong interactions between particle–particle rather than
particle–matrix. This trend is supported by the increasing void content as filler content
was increased. In the CNT- and Cu-filled PP systems, a decreasing trend in tensile
strength compared with that of PP was observed. This may be related to the large particle
size of Cu, which functions as a defect, and the high void content in the two-composite
systems. Increasing void content could cause detrimental effects on mechanical proper-
ties that create stress concentration and inhibit stress transfer from the matrix to the fil-
ler.
13–16
The distribution of fillers in the PP matrix at 4 vol.%was revealed by sectional
fractography of tensile test by SEM (Figure 3). PP surface (Figure 3a) was dramatically
Filler loading (vol.%)
01234
Melt flow index (g/10 s)
0
5
10
15
20
25
PP/CNT
PP/SND
PP/BN
PP/CU
Figure 1. Melt flow index (MFI) curves of polypropylene (PP) and PP composites as a function of
filler loading.
630 Journal of Thermoplastic Composite Materials 26(5)
changed by the presence of thermal conductive particles. SND and BN in Figure 3(b) and
(c) were well dispersed; the filler appears to be embedded in the PP matrix, suggesting
that the tensile strength of these systems is high. The worst dispersion and distribution
were observed in PP/Cu composites, as indicated by the presence of agglomerations and
voids in Figure 3(d). CNT was poorly distributed but was well dispersed in PP matrix
(Figure 3e). These properties of PP/Cu and PP/CNT are responsible for the increased
stress behavior and the ineffective transfer of load applied in PP composites, leading
to decreased tensile strength of CNT and Cu systems.
X
c
also influences the mechanical properties of composites.
17,18
Theoretically, the
mechanical strength of a crystalline polymer is determined by its crystalline structure.
Table 2 presents the X
c
,T
m
, and T
c
values for PP and PP composites in this study. At
4 vol.%filler loading, X
c
of SND and BN was higher than that of PP because the
crystalline region acts as a physical crosslink that enhances the tensile strength of PP
composites. In contrast, CNT and Cu systems exhibited low X
c
values since the
superficial area interferes with crystal growth, thus leading to reduced tensile strength
of the composites.
19,20
T
m
values were not significantly changed by the addition of
conductive fillers and an increase in filler loading. This may be attributed to the
maintenance of the flexibility of the polymer chain even when fillers are dispersed in
the polymer matrix.
8
T
c
values of composites were higher than that of PP and were
within the range of 110–135C, indicating that the conductive fillers can act as
nucleating agents.
Figure 4 illustrates the correlation of Young’s modulus and filler content (W
f
)ofthe
PP and PP composites. In general, the trends markedly increased with respect to the W
f
.
The highest Young’s modulus were found in CNT followed by BN, SND, and Cu fillers,
Tensile strength (MPa)
26
28
30
32
34
36
38
40
PP
PP/CNT
PP/SND
PP/BN
PP/CU
Void content (%)
0
5
10
15
20
25
30
35
Filler loading (vol.%)
01234
Figure 2. Tensile strength and void content of polypropylene (PP) and PP composites as a function
of filler loading. Bar graph refers to the tensile strength and line plot refers to the void content,
respectively.
Nurul and Mariatti 631
with increases of up to 31%,27%,25%, and 9%from that of PP, respectively. Incor-
poration of rigid and stiff reinforcement into the polymer enhanced the stiffness of the
polymer composites. Higher rigid filler content increased the Young’s modulus signif-
icantly. The PP/CNT system exhibited the highest maximum Young’s modulus due to
the high aspect ratio of CN, which leads to greater stiffening compared with particulate
composites. The lower interfacial area of the sphere shape of SND, BN, and Cu results in
lower Young’s modulus compared with CNT system. The Cu system had the lowest
Young’s modulus because of the large particle size of Cu, which results in less inter-
action between filler–filler and filler–matrix. Figure 5 illustrates the trends of descending
Figure 3. Scanning electron microscope (SEM) micrograph of the 4 vol.% filler loading at 5 K
magnifications. (a) Polypropylene (PP), (b) PP/synthetic diamond (SND), (c) PP/boron nitride (BN),
(d) PP/copper (CU), and (e) PP/carbon nanotube (CNT).
632 Journal of Thermoplastic Composite Materials 26(5)
elongation at break with addition of stiff reinforcement, which decreases the ductility of
the matrix.
Thermal conductivity
Figure 6 presents the thermal conductivity of PP composites at room temperature. We
found that the thermal conductivity of composites increased monotonically from that of
PP and increased directly with increased filler amount. This ascending trend may be
attributed to the ease of heat transfer obtained by increasing contact in the composites.
CNT was the most effective filler for enhancing thermal conductivity, followed by SND,
Cu, and BN; this trend seems to follow the hierarchy of thermal conductivities of the
Young's modulus (GPa)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Weight fraction (%)
0
5
10
15
20
25
30
35
PP
PP/CNT
PP/SND
PP/BN
PP/CU
Filler loading (vol.%)
01234
Figure 4. Young’s modulus and weight fraction of polypropylene (PP) and PP composites as a
function of filler loading. Bar graph refers to the Young’s modulus and line plot refers to the weight
fraction, respectively.
Table 2. DSC and thermal interface resistance (R
i
) data for PP and PP composites filled at 4 vol.%
of CNT, SND, BN, and CU.
Composites X
c
(%) T
m
(C) T
c
(C) R
i
(nm
2
/w K)
PP 34.5 164.4 118.4 -
PP/CNT 30.0 163.9 123.7 0.98
PP/SND 38.7 163.9 124.3 0.14
PP/BN 42.4 164.0 125.8 1.3
PP/CU 33.9 163.9 120.7 1.1
BN: boron nitride, CNT: carbon nanotube, CU: copper, DSC: differential scanning calorimeter, PP:
polypropylene, SND: synthetic diamond.
Nurul and Mariatti 633
filler (refer to Table 1). The correlation with thermal interface resistance would influence
the effectiveness of the phonon to pass through in the composites systems. The thermal
resistance at the interface between the matrix and the filler, known as Kapitza resistance
(R
i
), was analyzed according to Eq. (1).
21
Elongation at break (%)
2
3
4
5
6
7
8
9
Filler loading (vol.%)
01234
PP
PP/CNT
PP/SND
PP/BN
PP/CU
Figure 5. Elongation atbreak of polypropylene (PP) and PP composites as a function of filler loading.
Thermal conductivity
(W/m.K)
0.22
0.24
0.26
0.28
0.30
0.32
0.34
0.36
PP/CNT
PP/SND
PP/BN
PP/CU
Filler loading (vol.%)
01234
Figure 6. Thermal conductivity of the polypropylene (PP) composites as a function of filler loading.
634 Journal of Thermoplastic Composite Materials 26(5)
Kc¼KmþKmL
2RiKfþLvf
3ð1Þ
where K
c
,K
m
,andK
f
are the thermal conductivity of the composite, matrix, and filler,
respectively; R
i
is the interfacial thermal resistance; Lis the length of filler assumed
at diameter d
50
;andv
f
is the volume fraction taken at 4 vol.%filler loading. The
predicted values of R
i
are summarized in Table 2. Lower thermal resistance was
exhibited by the SND and CNT systems due to their high thermal conductivity.
However, the CNT filler can produce higher thermal conductivities at identically lower
filler content due to its high aspect ratio, so that it is able to form a conductive network
for easier phonon-dominated ballistic heat transport compared with the spherical SND.
High R
i
was observed in the Cu and BN systems due to their low thermal conductivity.
However, the PP/Cu system exhibited minimum thermal conductivity at 2 vol.%load-
ing only, with decreasing values obtained with further addition of filler loadings. This
isrelatedtothepooradhesionandpoordispersionanddistributionofCuseeninSEM
morphology (Figure 3d). Variations in agglomeration size, high void content, and the
lack of contact between particles suggest that Cu particles were relatively nonhomo-
genously dispersed in the matrix. This subsequently resulted in low heat transfer in the
Cu system, which led to low thermal conductivity of the composite. In contrast, the
nearly uniform size of particles indicatinggooddispersioninthePPmatrix(Figure
3b, c, and e) resulted in better thermal interaction in CNT-, SND-, and BN-filled
PP composites.
Thermogravimetry analysis
The TGA curves for PP and PP composites at 4 vol.%filler loading are presented
in Figure 7. The curve shows single-step degradation where it shifted to the right
(i.e. higher temperature) with the addition of filler. This indicates that PP composites
achieve a stabilization effect through the barrier effect of filler loading, which hinders
volatilization of bulk samples into gas phase.
22
TGA curves reveal that composites
arestableatupto350
C, with weight reduction of around 0.5%. The TGA profile can
be clearly depicted by the derivative weight %(DTG) curve in Figure 8. The points
where degradation starts shifted to a higher temperature in composites compared
with PP. TGA trends of the composite materials are supported by the TGA analysis
of fillers (Figure 9), which revealed weight reduction in fillers as indicated by the
minus ()signintheyaxis as a function of temperature. CNT exhibited the highest
curve, suggesting that CNT has better thermal stability compared with the other
fillers. Weight reductions at different temperature (Table 3) follow the sequence of
CNT, SND, Cu, and BN. As shown in Figure 7, increasing filler loading leads to
increased thermal behavior of the composites due to the higher thermal stability of
fillers compared with the matrix. In general, all composite systems exhibited slightly
similar weight residue, which explains why fillers are retained without decomposition.
Most of the fillers will decompose at very high temperatures, while PP will be com-
pletely degraded.
Nurul and Mariatti 635
Conclusions
In this study, we performed characterization of fillers and investigation of the effect of
thermally conductive fillers on the mechanical, flow, and thermal properties of PP
composites. Findings suggest that CNT has better thermal properties compared with
Temperature (°C)
300 350 400 450 500 550
Weight loss (%)
0
20
40
60
80
100
PP
PP/CNT1
PP/CNT4
PP/SND4
PP/BN4
PP/CU4
Figure 7. Thermogravimetric analysis (TGA) curve of polypropylene (PP) and PP composites as a
function of temperature. The numbers 1 and 4 refer to 1 and 4 vol.% of filler loading, respectively.
Derivative weight %
(%/m)
–30
–20
–10
0
PP
PP/CNT1
PP/CNT4
PP/SND4
PP/BN4
PP/CU4
Temperature (°C)
350 400 450 500 550
Figure 8. Derivative weight percentage (DTG) of polypropylene (PP) and PP composites as a
function of temperature. The numbers 1 and 4 refer to 1 and 4 vol.% of filler loading.
636 Journal of Thermoplastic Composite Materials 26(5)
other conductive fillers. Results demonstrate that CNT, SND, BN, and Cu particles
variably affect the properties of PP composites. In general, the MFI of composites
decreased with increased filler loading due to the ability of fillers to hinder plastic flow.
PP/CNT exhibited the greatest thermal conductivity and thermal stability due to the high
aspect ratio of CNT, which facilitates the formation of bridges for phonon transformation
compared with the spherical fillers. However, entanglements of CNT lead to stress
concentration, resulting in reduced tensile properties. In general, the overall thermal
properties of composites improved with filler addition. For particulate fillers, lower d
50
results in higher tensile strength, Young’s modulus, higher R
i
, and lower thermal con-
ductivity values. The thermal conductivity and R
i
of the composite materials generally
seems to follow the hierarchy of thermal conductivities of the filler. Cu with the highest
d
50
showed poor thermal and tensile properties due to the agglomeration and voids which
0 200 400 600 800
Delta-M (mg)
–25
–20
–15
–10
–5
0
CNT
SND
BN
CU
Temperature (°C)
Figure 9. Thermogravimetric analysis (TGA) curve for conductive fillers used as a function of
temperature.
Table 3. Weight reduction (mg) in conductive fillers at 100 and 500C.
Conductive fillers
Weight reduction (mg)
At 100C At 500C
CNT 0.9 7.0
SND 1.7 8.6
CU 3.1 12.7
BN 4.0 13.2
BN: boron nitride, CNT: carbon nanotube, CU: copper, SND: synthetic diamond.
Nurul and Mariatti 637
existed in the system. PP/SND exhibited the greatest tensile strength possibly due to
better distribution of filler in the PP matrix.
Funding
This study was supported by Universiti Sains Malaysia under Postgraduate Research
Fund USM-RU-PGRS (Project no. 8033053) and Short Term Grant (Project no.
6035279).
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