polymers
Article
Electrical Characteristics of Polypropylene Mixed
with Natural Nanoclay
Huseyin R. Hiziroglu * and Iosif E. Shkolnik
Department of Electrical & Computer Engineering, Kettering University, Flint, MI 48504, USA;
shkolniki@comcast.net
* Correspondence: hhizirog@kettering.edu; Tel.: +1-810-762-7962
Received: 20 July 2018; Accepted: 20 August 2018; Published: 24 August 2018
Abstract: Polypropylene has been used in radio-frequency capacitors and has also started to be
employed in cables as insulation. The objective of this study was to evaluate the electrical properties
of polypropylene filled with natural clay as a nano-material. Polypropylene samples having 0%,
2% and 6% natural clay by weight were exposed to 60-Hz sinusoidal voltages at two different
rates of rise. The breakdown voltage of each sample was recorded at these different ramp rates.
Also, the Root-mean-squared (rms) current was measured as the voltage was increased across
the test samples. The important findings of this study were (a) the breakdown strength of the
natural nanoclay-filled polypropylene was higher than the unfilled polypropylene, and the optimum
concentration of nanoclay appeared to be 2% by weight; (b) the current density as a function of
the electric-field intensity indicated a non-linear behavior with saturation, and the saturation onset
took place at a higher electric-field intensity in nanoclay-filled polypropylene, wherein 2% nanoclay
seemed to be the optimum concentration as well for the onset electric field of saturation.
Keywords: breakdown; isotactic polypropylene; nanocomposites; natural nanoclay
1. Introduction
Nano-particles have found a wide variety of applications in order to modify the properties
of polymer dielectrics. Since the beginning of this millennium, studies into nanocomposites with
polymer-based materials for electrical applications as dielectrics and insulators have been intensified
considerably in order to better understand various phenomena in these materials so that their effective
usage will be possible in numerous applications [1].
It is known that the addition of nano-particles to a polymer seems to enhance the overall properties
of the composite [1]. In such a nanocomposite, interfaces between two phases, one being a nanoparticle
and the other being the host polymer, play a significant role [2] in its properties. This is primarily
due to the fact that the volume of the interfaces dominates the whole volume of the composite as the
size of the nanoparticle decreases [2]. This effect could also substantially modify the overall dielectric
characteristics of the composite material [2]. Thus, studies into polymer-based nanocomposites have
been underway in order to use them in high-voltage cables as an insulating material [3]. At present,
polymeric materials used in cables in high-voltage applications are primarily various versions of
polyethylene (PE), with cross-linked polyethylene (XLPE) being one of the most widely used [4].
With the present technologies the most powerful extruded high-voltage cables with PE as an insulating
material used in high-voltage, direct-current (HVDC) systems can handle up to 640 kV [3].
A polymeric nanocomposite based on PE filled with nanometric size metal-oxide has been
introduced recently due to the lower charge mobility than that of pure PE as a potential candidate
for HVDC cable insulation [3]. Also, a combination of PE and metal-oxide nano-particles seemed
to be promising in ultra-high-voltage applications due to the interface role of the nanomaterial in
Polymers 2018, 10, 942; doi:10.3390/polym10090942
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Polymers 2018, 10, 942
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the host material [3]. Therefore, it is a fact that polymer-based nano-dielectrics have shown not only
enhanced electrical properties but also indicated better mechanical and thermal properties than the
plain polymeric material. Also, various studies have revealed that nanocomposites with polymer
dielectrics have exhibited significant improvements in optical and physico-chemical properties [5].
Along with PE and other polymers, polypropylene (PP) has found a place in electrical systems as
a dielectric [6]. The isotactic form of PP has a melting temperature of 481 K with high crystallinity that
leads to having high stiffness and tensile strength [7]. Although isotactic PP has high crystallinity it is
considered to be a semicrystalline material since crystallization of PP is a random process influenced
by the probability of a molecule’s ability to develop a crystalline structure with adjacent molecules [7].
Therefore, crystalline phase, amorphous phase and crystalline/amorphous interface have importance
in the properties of charge transfer in PP similar to the phenomena taking place in PE as described
in [3,8]. Due to these highly desirable properties, PP films are used as the dielectric medium in
high-performance pulse and low-loss RF capacitors [9,10]. Also, PP has been employed as an insulating
medium in power cables [11] primarily due to its relatively high melting temperature that might
possibly facilitate higher current-carrying capacity [12] in addition to other such microstructure
properties as crystallinity, amorphous phase as well as crystalline/amorphous interface [3]. Moreover,
it should be noted that an appropriate process such as extrusion of cables should be selected in order
to optimize the mechanical and electrical strengths since they could be influenced by the crystallinity
and crystal thickness in the material [3,13,14].
Different electrical properties of PP have been investigated in various studies [9–11,15–17]. One of
the studies done with PP was to evaluate its dielectric spectrum at low voltages [18,19]. Another work
was the experimental investigation of the space charge distribution in PP and its nanocomposites [20].
In fact, improvements have been observed in the space-charge distribution when PP was loaded
with natural clay. Consequently, it is hypothesized that the breakdown behavior of PP could also
improve when filled with nano-material. However, there is very little information on the electrical
breakdown [21] and the current density in PP mixtures with nano-particles and it needs to be further
studied. Therefore, the objectives of this study are to evaluate and compare; (a) the breakdown strength
and (b) the variation of current-density as a function of applied electric-field intensity of PP-based
nanocomposite films with various contents of nano-size natural clay in PP. It is expected that the initial
data from this work could be beneficial to better understand PP loaded with 2 and 6 wt % natural clay
so that they can be used with enhanced properties in different applications in the electrical industry.
2. Materials and Methods
2.1. Materials
The polyolefin used in this study is isotactic PP, and the natural nanoclay is montmorillonite,
[All.67 Mg0.33 (Na0.33 )]Si4 Ol0 (OH)2 . Since PP is a hydrophobic material and purified natural
clay is hydrophilic, they are immiscible thermodynamically [22]. Consequently, natural nanoclay
needs to be intercalated and later in the process compatibilized [22]. After the intercalation
process of montmorillonite with dimethyl di(hydrogenated tallow) ammonium chloride, also known
by its trade name as Cloisite® 20A [22,23], the natural nanoclay turned into hydrophobic from
hydrophilic. In the next step, compatibilization was done using Polybond® 3150 with a composition
of PP-maleic anhydride modifier (PP-MA) [22]. Also included in the batch was Irganox® B 225,
an antioxidant [22], with a blend of 50% tris(2,4-ditert-butylphenyl)phosphite and 50% pentaerythritol
tetrakis[3-[3,5-di-tert-butyl-4-hydroxyphenyl]propionate], in order to avoid the degradation of the
nanocomposite due to oxidation, which is very important especially when used as cable insulation [3].
After being pelletized, the nanocomposite was brought to 135 µm by a film blowing technique at 180 ◦ C.
Then, the films were rolled at 115 ◦ C. Further details of this material preparation were presented in
a paper from National Research Council of Canada [22], the institution that prepared and supplied
the samples for this study. In this experimental study, three sets of samples were used. Sample 1 was
Polymers 2018, 10, 942
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trade
name
of Pro-fax
HL-451H
from Basell
the control
sample.
Its composition
isotactic PP
PPwith
withthethe
trade
name
of Pro‐fax
HL‐451H
fromasBasell
as the
control
sample. Its
was
87
wt
%
PP,
0.2
wt
%
antioxidants
and
12.8
wt
%
compatibilizer.
Sample
2
was
a
nanocomposite
composition was 87 wt % PP, 0.2 wt % antioxidants and 12.8 wt % compatibilizer. Sample 2 was a
with a composition
ofa85.3
wt % PP, 0.2
% wt
antioxidants,
12 wt
% compatibilizer
and%2 compatibilizer
wt % nanoclay.
nanocomposite
with
composition
of wt
85.3
% PP, 0.2 wt
% antioxidants,
12 wt
Finally,
3 was Finally,
also a nanocomposite
having
81.8 wt % PP, 0.2
wt %81.8
antioxidants,
12 wt
wt %
%
and
2 wtSample
% nanoclay.
Sample 3 was also
a nanocomposite
having
wt % PP, 0.2
compatibilizer
and
6
wt
%
nanoclay.
antioxidants, 12 wt % compatibilizer and 6 wt % nanoclay.
Each nanocomposite
nanocomposite sample
sample had a thickness of 135 µm
10%. Before
Before being
Each
μm with
with aa variation
variation of
of ±
±10%.
situated between
between two
two stainless‐steel
stainless-steel electrodes,
electrodes, the
the thickness
thickness of
of each
each sample
sample was
was recorded
recorded using
using a
situated
electrode had
had a circular flat surface with a diameter of 33 mm and rounded edges
micrometer. Each electrode
non-uniformity. The electrode‐film
electrode-film assembly was immersed into
in order to minimize electric-field
electric‐field non‐uniformity.
highly refined transformer oil in a stainless-steel
test
cell.
stainless‐steel
2.2. Methods
Methods
2.2.
Figure 11 shows
shows the
the experimental
experimentaltest
testapparatus
apparatusthat
thatconsisted
consistedofofa astainless‐steel
stainless-steeltest
test
cell
and
Figure
cell
and
a
a
high-voltage
transformer
with
a
model
number
of
TEO
100/10
from
Messwandler
Bau,
Germany,
high‐voltage transformer with a model number of TEO 100/10 from Messwandler Bau, Germany,
along with
with auxiliary
auxiliary elements
elements in
in order
order to
along
to make
make the
the measurements
measurements possible.
possible.
Because
the
thickness
of
the
test
sample
is
much
smaller
than
diameter
of electrodes,
the electrodes,
Because the thickness of the test sample is much smaller
than
thethe
diameter
of the
the
the electric-field
distribution
in the
sample
can
consideredasasalmost
almostuniform.
uniform.Also,
Also,in
inorder
order to
to
electric‐field
distribution
in the
testtest
sample
can
bebe
considered
avoid
the
possibility
of
breakdown
in
air
around
the
electrode
edges,
the
assembly
of
the
electrodes
avoid the possibility of breakdown in air around the electrode edges, the assembly of the electrodes
and the
the test
test sample
sample were
were immersed
immersed in
in highly
highly refined
and
refined transformer
transformer oil
oil filling
filling the
the test
test cell.
cell.
Figure 1. Principle circuit schematic of the experimental setup.
Figure 1. Principle circuit schematic of the experimental setup.
The rating of the high‐voltage test transformer was 220/100,000 V (rms) with output power of 5
rating of the high-voltage test transformer was 220/100,000 V (rms) with output power of
kVA The
at 60‐Hz.
5 kVA
60-Hz. voltage divider CM from Messwandler Bau, Germany, was incorporated in order
A at
capacitive
A
capacitive
voltage
divider
CM
Messwandler
Bau, Germany,
was1.incorporated
inof
order
to measure the applied
rms
voltage
tofrom
the test
sample as indicated
in Figure
The accuracy
the
to
measure
the
applied
rms
voltage
to
the
test
sample
as
indicated
in
Figure
1.
The
accuracy
of
the
voltmeter (34405A, Agilent Technologies, Santa Clara, CA, USA) and connected to the low‐voltage
voltmeter
(34405A,
Agilent
Technologies,
Santa
Clara,
CA,
USA)
connected
to
the
low-voltage
arm
arm of the voltage divider was better than 1%. A 1‐kΩ precision resistor with a tolerance of ±
of the voltage
dividerbetween
was better
1%. A 1-kΩ
precision
with as
a tolerance
± 0.01%
was
0.01%was
connected
thethan
low‐voltage
electrode
andresistor
the ground
shown inofFigure
1 since
connected
between
electrode
and the ground
as shown
in Figure
1 since
the rms
current
the
rms current
in the
thelow-voltage
circuit is directly
proportional
to the
rms voltage
across
1‐kΩ
resistor.
A
in
the
circuit
is
directly
proportional
to
the
rms
voltage
across
1-kΩ
resistor.
A
voltmeter
(34401A,
voltmeter (34401A, Agilent Technologies, Santa Clara, CA, USA) having an acV accuracy of less than
Agilent
Technologies,
Clara,
CA,
USA)
having
an acV
accuracy
of less
than
0.1% was
employed
0.1%
was
employed toSanta
measure
the
rms
voltage
across
the 1‐kΩ
resistor
The
applied
voltage
across
to
measure
the
rms
voltage
across
the
1-kΩ
resistor.
The
applied
voltage
across
the
test
sample
was
the test sample was a 60‐Hz sinusoidal waveform and was raised from zero volt up to the voltage
at
a
60-Hz
sinusoidal
waveform
and
was
raised
from
zero
volt
up
to
the
voltage
at
which
the
failure
which the failure occurred in the sample. While increasing the voltage at a rate of rise of 90 V/s,
occurred in
the voltage
at a voltage
rate of rise
90 V/s, typically
15up
to 20
typically
15the
to sample.
20 data While
pointsincreasing
were recorded
for the
andof current
from zero
to data
the
points
were
recorded
for
the
voltage
and
current
from
zero
up
to
the
breakdown
voltage.
From
the
breakdown voltage. From the experimental results, electric‐field intensity and the current density
experimental
results,
electric-field
intensity
and
the
current
density
were
calculated.
In
order
to
verify
were calculated. In order to verify the reproducibility of the experimental results experiments were
the reproducibility
of the experimental
results
experiments
performedMoreover,
on 5 different
with
performed
on 5 different
samples with
the same
amountwere
of nanofiller.
thesamples
breakdown
voltages were also recorded with a voltage rate of rise of 1050 V/s for assessing if there were
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2018,
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significant changes
changes in
in the
the breakdown
breakdown voltages.
voltages. These
These breakdown
breakdown tests
tests were
were also
also performed
performed on
on 55
significant
Polymers
2018, 10,
x FOR
PEER
REVIEW
11
different
samples
with
the
same
nanofillercontent.
content.
different
samples
with
the
same
nanofiller
the same
amount
of
nanofiller.
Moreover,
the
breakdown voltages were also recorded with4aofvoltage
rate of rise of 1050 V/s for assessing if there were significant changes in the breakdown voltages.
significant changes in the breakdown voltages. These breakdown tests were also performed on 5
Results
3.3.These
Results
breakdown tests were also performed on 5 different samples with the same nanofiller content.
different samples with the same nanofiller content.
3. Results
3.1.
BreakdownStrength
Strength
3.1.
Breakdown
3. Results
Sincethe
theelectric
electric
fieldhas
hasalmost
almostaauniform
uniformdistribution
distributionin
inthe
thesamples,
samples,the
thebreakdown
breakdownstrength
strength
field
3.1.Since
Breakdown
Strength
Breakdown
Strength
canbe
be3.1.
calculated
as:
can
calculated
as:
SinceSince
the electric
field
has
almost
distributionininthe
the
samples,
breakdown
strength
the electric
field
has
almostaauniform
uniform distribution
samples,
thethe
breakdown
strength
E
V
/d
(1)
E
bb==V
bb
/d
(1)
can be
calculated
as: as:
can
be calculated
Ethe
=V
(1)
bthe
b /d
whereEEbbisisthe
thebreakdown
breakdownstrength,
strength,VVbbisis
breakdown
voltageand
andddisisthe
thethickness
thicknessof
ofthe
the
where
breakdown
voltage
Eb = Vb/d
(1)
sample.
Figure
exhibitsthe
thestrength,
trendof
ofthe
meanbreakdown
breakdownvoltage
strengths
ofdthe
the
PPand
andits
itsmixtures
mixtures
with
sample.
exhibits
trend
mean
breakdown
strengths
PP
with
where EFigure
is the22breakdown
Vthe
andof
is the
of the
sample.
b where
b is the
Eb is the breakdown
strength,
Vb is the breakdown
voltage
and
d is thickness
the thickness
of the
different
concentrations
of
natural
nanoclay.
different
concentrations
of
natural
nanoclay.
Figure 2 exhibits the trend of the mean breakdown strengths of the PP and its mixtures with different
sample. Figure 2 exhibits the trend of the mean breakdown strengths of the PP and its mixtures with
concentrations
of natural nanoclay.
different concentrations
of natural nanoclay.
135
135
135
130
Breakdown strength (kV/mm)
Breakdown
Breakdown strength
strength (kV/mm)
(kV/mm)
130
130
125
125
125
120
120
115
115
110
110
120
115
110
105 105
105
100 100
100
00 0
22 2
444
66
8 88
Concentration
of
nanoclayby
by
weight
)) )
Concentration
of
nanoclay
byweight
weight(%
(%
Concentration
of
nanoclay
(%
Figure
2. Breakdown
strength
vs.the
concentration of
natural
clay
inin
and
its nanocomposites
with with
Figure
Breakdown
strength
vs.the
concentration
naturalclay
clay
PP
and
its
nanocomposites
Figure
Breakdown
strength
vs.
thethe
concentration
ofofnatural
natural
clay
inPP
PP
and
its
nanocomposites
with
Figure
2.2.2.Breakdown
strength
vs.
concentration
of
in
PP
and
its
nanocomposites
with
voltage rate of rise being parameter [21].
90 V/s;
1050 V/s.
voltagerate
rateof
ofrise
risebeing
beingparameter
parameter[21].
[21].
V/s;
1050
voltage
rate
of
rise
being
parameter
[21].
90V/s;
V/s;
1050V/s.
V/s.
voltage
90
1050
V/s.
As can be seen from Figure 2, the mean breakdown strength of five PP samples was evaluated
As
can
be
seen
from
Figure
2,the
themean
meanof
breakdown
strength
offive
fivePP
PPsamples
samples
was
evaluated
As
seen
2,
breakdown
strength
of
five
PP
samples
was
evaluated
As
canbe
be
seenfrom
fromaFigure
Figure
2,
the
mean
breakdown
of
evaluated
as
ascan
105.5
kV/mm
with
standard
deviation
5.7 kV/mm strength
at
90 V/s voltage
rate
of
rise. was
as
105.5
kV/mm
with
a
standard
deviation
of
5.7
kV/mm
at
90
V/s
voltage
rate
of
rise.
as
105.5
kV/mm
with
a
standard
deviation
of
5.7
kV/mm
at
90
V/s
voltage
rate
of
rise.
In Figure
the bars on
the dataofpoints
indicateatthe
errorrate
of the
mean that was
105.5 kV/mm
with2,a standard
deviation
5.7 kV/mm
90 standard
V/s voltage
of rise.
In
Figure
the
bars
on
the
data
points
indicate
the
standard
error
of the
mean
that
was
In
Figure
the
bars
data
points
indicate
the
of
mean
that
determined
2.55
kV/mm.
The
mean
breakdown
strength
of standard
nanocomposite
with
athe
concentration
ofwas
In
Figure 2,
2,2,as
the
bars
onon
thethe
data
points
indicate
the standard
error oferror
the mean
that
was determined
2 wt
%as
clay,
however,
wasbreakdown
109.2
kV/mm
a of
standard
deviation
of
5.8aakV/mm
and
determined
asnatural
2.55The
kV/mm.
The
mean
breakdown
strength
ofnanocomposite
nanocomposite
with
concentration
of%
determined
2.55
kV/mm.
The
mean
strength
concentration
as 2.55
kV/mm.
mean
breakdown
strength
ofwith
nanocomposite
with a with
concentration
of 2awtof
standard
error
ofhowever,
2.6 kV/mmwas
as shown
in
Figure with
2.with
There
is
an improvement
ofof
about
3.5%
on the
wt %
%
natural
clay,
however,
was
109.2
kV/mm
a
standard
deviation
of
5.8
kV/mm
and
22natural
wt
natural
clay,
109.2
kV/mm
a
standard
deviation
5.8
kV/mm
and
aa
clay, however, was 109.2 kV/mm with a standard deviation of 5.8 kV/mm and a standard
breakdown
strength
of
nanocomposite
with
2
wt
%
of
natural
clay
concentration
with
respect
to
the
standard
error
of2.6
2.6kV/mm
kV/mm
as
shown
in
Figureis
There
animprovement
improvement
ofabout
about
3.5%
onthe
the
standard
error
of
2.2.an
There
isisan
of
3.5%
on
error of 2.6
kV/mm
as
shownas
inshown
Figurein
2. Figure
There
improvement
of about 3.5%
on the
breakdown
controlstrength
PP. The mean
breakdown strength
of
PP%
loaded
with 6clay
wt %
natural clay was
110.5
kV/mm
breakdown
strength
of
nanocomposite
with
2
wt
%
of
natural
clay
concentration
with
respect
to
breakdown
of
nanocomposite
with
2
wt
of
natural
concentration
with
respect
to
the
strength
of nanocomposite with 2 wt % of natural clay concentration with respect to the controlthe
PP.
with a standard deviation of 2.4 kV/mm and a standard error of 1.06 kV/mm. The improvement in
control
PP.
The
meanbreakdown
breakdown
strength
ofPP
PP
loaded
with
wt%
%natural
natural
clay
was
110.5kV/mm
kV/mm
control
PP.
The
mean
strength
of
loaded
with
66wt
clay
was
110.5
The mean
breakdown
strength
of
PP
loaded
with
6
wt
%
natural
clay
was
110.5
kV/mm
with
a
the breakdown strength was 4.7% with respect to the control PP, while the improvement was only
with
a
standard
deviation
of
2.4
kV/mm
and
a
standard
error
of
1.06
kV/mm.
The
improvement
in
with
a
standard
deviation
of
2.4
kV/mm
and
a
standard
error
of
1.06
kV/mm.
The
improvement
in
standard
of 2.4tokV/mm
and a standard
of 1.06 kV/mm.
aboutdeviation
1% with respect
the nanocomposite
with 2%error
concentration
of natural The
clay. improvement in the
the
breakdown
strength
was
4.7%
with
respect
tothe
thea control
control
PP,while
while
the
improvement
was
only
the
breakdown
strength
was
4.7%
with
respect
to
PP,
the
was
breakdown
strength
was
4.7%
with
respect
to the
control
PP,speed
while
the
only
about
Experimental
procedures
were
the
same
when
ramp
of
1050improvement
V/simprovement
was
used was
as with
theonly
about
1%
with
respect
to
the
nanocomposite
with
2%
concentration
of
natural
clay.
about
1%
with
tonanocomposite
the nanocomposite
withconcentration
2%strength
concentration
of samples.
natural
90 V/s
raterespect
oftorise
order
to assess the
breakdown
ofof
thenatural
test
At this ramp speed,
1% with
respect
thein
with
2%
clay. clay.
Experimental
procedures
were
the
same
when
a
ramp
speed
of
1050
V/s
was
used
as
with
the
Experimental
procedures
were
the
same
when
a
ramp
speed
of
1050
V/s
as
the
breakdown
strength
of
5
samples
is
also
presented
in
Figure
2
with
different
nanoclay
Experimental procedures were the same when a ramp speed of 1050 V/swas
wasused
usedcontents
aswith
withthe
the
in
PP.
90
V/s
rate
of
rise
in
order
to
assess
the
breakdown
strength
of
the
test
samples.
At
this
ramp
speed,
90
V/s
rate
of
rise
in
order
to
assess
the
breakdown
strength
of
the
test
samples.
At
this
ramp
speed,
90 V/s rate of rise in order to assess the breakdown strength of the test samples. At this ramp speed,
Figurestrength
2 revealsof
that
the
controlisisPP
haspresented
the smallestin
breakdown
strength
as compared
to the
PP‐
the
breakdown
strength
of555samples
samples
also
presented
Figure
with
different
nanoclay
contents
the
samples
also
Figure
22with
different
nanoclay
contents
thebreakdown
breakdown
strength
of
is also
presented
in in
Figure
2 with
different
nanoclay
contents
in PP.
based
composites.
The
mean
value
of
the
breakdown
strength
of
the
control
PP
is
127
kV/mm.
The
inPP.
PP.Figure 2 reveals that the control PP has the smallest breakdown strength as compared to the
in
standard
deviation
and
the
standard
error
are,smallest
as indicated
in Figure strength
2 with error
bars, foundto
tothe
be PP‐
Figure
reveals
that
the
control
PPhas
has
the
smallest
breakdown
ascompared
compared
to
the
PP‐
Figure
22reveals
that
the
control
PP
the
breakdown
as
PP-based
composites.
The
mean
value
of the
breakdown
strengthstrength
of the control
PP is 127
kV/mm.
3.5 and 1.5 kV/mm, respectively. The mean breakdown strength of PP‐based nanocomposite loaded
based
composites.The
Themean
meanthe
valueof
ofthe
theerror
breakdown
strengthof
ofFigure
thecontrol
control
PP
127kV/mm.
kV/mm.
The
based
composites.
value
breakdown
strength
the
PP
isis127
The standard
are,
indicated
with
error
found The
to
be
with 2 wtdeviation
% natural and
nanoclaystandard
is determined
as
132askV/mm.
Forin
this mean2value
1.7
andbars,
0.8 kV/mm
standard
deviation
and
the
standard
error
are,
as
indicated
in
Figure
2
with
error
bars,
found
to
be
standard
deviation
and
the
standard
error
are,
as
indicated
in
Figure
2
with
error
bars,
found
to
be
3.5 and 1.5 kV/mm, respectively. The mean breakdown strength of PP-based nanocomposite loaded
3.5
and
1.5kV/mm,
kV/mm,
respectively.
Themean
meanbreakdown
breakdown
strength
of
PP‐based
nanocomposite
loaded
3.5
and
strength
PP‐based
nanocomposite
with
2 1.5
wt
%
naturalrespectively.
nanoclay is The
determined
as 132 kV/mm.
Forof
this
mean value
1.7 and 0.8 loaded
kV/mm
with22wt
wt%
%natural
naturalnanoclay
nanoclayisisdetermined
determinedas
as132
132kV/mm.
kV/mm.For
Forthis
thismean
meanvalue
value1.7
1.7and
and0.8
0.8kV/mm
kV/mm
with
Polymers 2018, 10, 942
5 of 11
are, respectively, the standard deviation and the standard error. The PP nanocomposite with 6 wt %
nanoclay content, however, has a mean breakdown strength of 133 kV/mm. The standard deviation
and standard error, for this case, are determined as 4.7 and 2.1 kV/mm, respectively.
At Polymers
a higher
ramp-speed of 1050 V/s, nanocomposite with 2 wt % nanoclay content indicated
about
2018, 10, x FOR PEER REVIEW
5 of 11
3.9% improvement on the breakdown strength as compared to control PP that has no natural clay content.
are, respectively,
the standard
deviation
and the
standard
error.
The PP
with 6the
wt %
The increase
in breakdown
strength,
however,
was
not, once
again,
asnanocomposite
substantial when
nanoclay
nanoclay
content,
however,
has
a
mean
breakdown
strength
of
133
kV/mm.
The
standard
deviation
content was increased from 2 to 6 wt %. When compared, the variations of the breakdown strengths
and standard error, for this case, are determined as 4.7 and 2.1 kV/mm, respectively.
as a function of the natural clay concentration in PP at two different rates of rise of the applied voltage,
At a higher ramp‐speed of 1050 V/s, nanocomposite with 2 wt % nanoclay content indicated
in Figure
2, it3.9%
is apparent
that the
breakdown
strength
when
the ramp
speed
about
improvement
on the
breakdown
strengthincreases
as compared
to control
PP that
has is
noincreased
natural from
90 to 1050
V/s.
The
data
presented
in
Figure
2
have
a
confidence
level
of
95%
on
the
error
bars.
clay content. The increase in breakdown strength, however, was not, once again, as substantial when
the nanoclay content was increased from 2 to 6 wt %. When compared, the variations of the
3.2. Characteristics
on the Variation
of Current
Densityclay
as aconcentration
Function of Electric-Field
Intensity
breakdown strengths
as a function
of the natural
in PP at two different
rates of
rise of the applied voltage, in Figure 2, it is apparent that the breakdown strength increases when the
The
current-density vs. electric-field intensity characteristics of PP and its nanocomposites with
ramp speed is increased from 90 to 1050 V/s. The data presented in Figure 2 have a confidence level
natural ofclay
within the scope of this study. In this context, current density
95%were
on thealso
error evaluated
bars.
essentially corresponds to the applied current injected to the sample. Due to the nature of dielectrics,
Characteristics
on the Variation
of Current
Density as a Function
of Electric‐Field
Intensity
applied3.2.
current
can be modelled
with
two components
under sinusoidal
steady-state.
These currents
are the conduction
current due
to the transport
ofcharacteristics
charge, andofthe
currentwith
due to the
The current‐density
vs. electric‐field
intensity
PPdisplacement
and its nanocomposites
natural clay were
also evaluated
within with
the scope
of this
In the
this material.
context, current
density
rate-of-the-change
of electric-flux
density
respect
to study.
time in
Using
a Tektronix
essentially
to the applied
current injected
to the sample.
Due toleads
the nature
of dielectrics,
oscilloscope
(TBScorresponds
1052B, Tektronix,
Beaverton,
OR, USA)
the current
the applied
voltage by
applied current can be modelled with two components under sinusoidal steady‐state. These currents
1.5359 rad at 60-Hz. Because this angle is close to π/2 rad, which is for perfect dielectric, it is
are the conduction current due to the transport of charge, and the displacement current due to the
apparent
that the conduction
current
is insignificant
compared
to theUsing
displacement
rate‐of‐the‐change
of electric‐flux
density
with respect toastime
in the material.
a Tektronixcurrent.
Thus, the
conduction
current
at
60-Hz
is
neglected
and
the
measured
current
is
oscilloscope (TBS 1052B, Tektronix, Beaverton, OR, USA) the current leads the applied considered
voltage by to be
1.5359 radthe
at 60‐Hz.
Because this
angle isIn
close
to π/2
which of
is for
perfect
dielectric,
it isintroduced
apparent
approximately
displacement
current.
fact,
therad,
amount
error
in the
current
with
that the conduction
current is insignificant
as compared
the displacement
Thus,
the
this approximation
was calculated
±0.061%. Therefore,
thetocurrent
density iscurrent.
primarily
governed
by
conduction current at 60‐Hz is neglected and the measured current is considered to be approximately
the permittivity of the material under consideration.
the displacement current. In fact, the amount of error in the current introduced with this
approximation was calculated ±0.061%. Therefore, the current density is primarily governed by the
J = I/A
permittivity of the material under consideration.
(2)
J =the
I/A applied current to the material and
(2) A is the
Here, J is the current density (A/m2 ), I is
2
2), I is the applied
J ismaterial
the current
density (A/m
current
the material
A is 3
thepresents
surface areaHere,
of the
effectively
sandwiched
between
thetoelectrodes
(m(A)). and
Figure
2). Figure 3 presents the
surface
area
of
the
material
effectively
sandwiched
between
the
electrodes
(m
the current-density vs. applied electric-field intensity for the plain PP as a control sample. The graph
current‐density vs. applied electric‐field intensity for the plain PP as a control sample. The graph in
in Figure
3 points out that there is a steep linear increase in current density at lower electric fields.
Figure 3 points out that there is a steep linear increase in current density at lower electric fields.
Although the current density continues to increase above the applied electric-field intensity of about
Although the current density continues to increase above the applied electric‐field intensity of about
30 kV/mm,
this increase
is no
longer
asititwas
was
below
30 kV/mm.
Therefore,
30 kV/mm,
this increase
is no
longer as
as steep
steep as
below
30 kV/mm.
Therefore,
the J vs. Ethe J vs.
E characteristic
is non-linear,
with
forthe
thecontrol
control
characteristic
is non‐linear,
withsaturation
saturation for
PP.PP.
0.55
0.5
Current density (μA/mm2)
0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
0
10
20
30
40
50
60
70
Electric field intensity (kV/mm)
80
90
100
110
Figure 3. Variation of current density with electric‐field intensity in unfilled PP.
Figure 3. Variation of current density with electric-field intensity in unfilled PP.
Polymers 2018, 10, 942
6 of 11
Polymers
2018,Maxwell’s
10, x FOR PEER
REVIEW
From
equations,
6 ofthe
11
the displacement current density is directly proportional to
electric-flux density in frequency domain as:
From Maxwell’s equations, the displacement current density is directly proportional to the
electric‐flux density in frequency domain as: J = ωD
(3)
J = D
(3)
where ω is the angular frequency (rad/s) and D is the electric flux density vector (C/m2 ).
2).
where
 is due
the angular
frequency (rad/s)
and D
the electric
flux density
vector
Also,
to the constitutive
properties
ofisdielectric
materials,
electric
flux(C/m
density
is directly
Also,
duepermittivity
to the constitutive
properties
ofwith
dielectric
materials,
electric flux
density
related
to the
of the medium
along
the applied
electric-field
intensity
as: is directly
related to the permittivity of the medium along with the applied electric‐field intensity as:
D = εE
(4)
D = εE
(4)
Here, E
E is the applied electric-field
electric‐field intensity
intensity(V/m)
(V/m) and ε is the permittivity of the medium
medium (F/m),
(F/m),
which can also be expressed as:
as:
ε = ε o (1 + χe )
(5)
(5)
1
−12 F/m being the free-space permittivity and χ being the electric susceptibility
with εo = 8.85 × 10−12
e
with εo = 8.85 × 10 F/m being the free‐space permittivity and
being the electric susceptibility
that is proportional to the polarization vector,
that is proportional to the polarization vector,
P = ε o χe E
(6)
(6)
of
of the
the material
material [24].
[24]. If
If the
the material
material is
is polarizable
polarizable with
with electric
electric field,
field, the
the permittivity
permittivity will
will be
be larger
larger than
than
the
free‐space
permittivity.
the free-space permittivity.
Relative
permittivity, which
which is
is often
often referred
referred to
to as
as the
the dielectric
dielectricconstant,
constant,isisalso
alsodefined
definedas:
as:
Relative permittivity,
εr = ε/εo
(7)
εr = ε/εo
(7)
For the unloaded PP, raising the electric‐field intensity to 30 kV/mm from zero yields a
permittivity
of 2.27, which
is nearly
constant, as intensity
shown intoFigure
4. From
the
same
figure,
at higher
For the unloaded
PP, raising
the electric-field
30 kV/mm
from
zero
yields
a permittivity
electric
fields than
30 kV/mm,
relative
diminishes
about
1.46 at
110
of 2.27, which
is nearly
constant,however,
as shownthe
in Figure
4. permittivity
From the same
figure, at to
higher
electric
fields
kV/mm.
than 30 kV/mm, however, the relative permittivity diminishes to about 1.46 at 110 kV/mm.
2.4
Relative Permittivity
2.2
2
1.8
1.6
1.4
0
10
20
30
40
50
60
70
80
90
100
110
Electric field intensity (kV/mm)
Figure
Figure 4.
4. Relative
Relative permittivity
permittivity as
as aa function
function of
of electric‐field
electric-field in
in unfilled
unfilled PP.
PP.
It should also be noted that these values may differ from one kind of PP to another since the
It should also be noted that these values may differ from one kind of PP to another since the
composition may have different concentrations of PP along with different component materials.
composition may have different concentrations of PP along with different component materials.
Nevertheless, the relative permittivity of PP under consideration is within the limits of the dielectric
Nevertheless, the relative permittivity of PP under consideration is within the limits of the dielectric
constant (2.2–2.5) in the linear region of the J vs. E characteristic as stated in standards [25].
constant (2.2–2.5) in the linear region of the J vs. E characteristic as stated in standards [25].
The variation of current density with the electric‐field intensity is given in Figure 5 for PP doped
The variation of current density with the electric-field intensity is given in Figure 5 for PP doped
with 2 wt % nanoclay.
with 2 wt % nanoclay.
Polymers 2018, 10, 942
Polymers 2018, 10, x FOR PEER REVIEW
Polymers 2018, 10, x FOR PEER REVIEW
7 of 11
7 of 11
7 of 11
0.55
0.55
0.5
0.5
2) 2
Current
density
(μA/mm
Current
density
(μA/mm
)
0.45
0.45
0.4
0.4
0.35
0.35
0.3
0.3
0.25
0.25
0.2
0.2
0.15
0.15
0.1
0.1
0.05
0.05
0
0 0
0
10
10
20
20
30
30
40
50
40
50
Electric field
Electric field
60
70
80
60
70
80
intensity (kV/mm)
intensity (kV/mm)
90
90
100
100
110
110
120
120
Figure 5.
5. Variation of current‐density with electric-field
electric‐field intensity in PP composite
filled
with
wt %
Figure
composite filled
filled with
with 22 wt
Figure
5. Variation of current-density
current‐density with electric‐field intensity in PP composite
%
nanoclay.
natural nanoclay.
nanoclay.
natural
It can be observed from Figure 5 that the current density varies with applied electric‐field
It can be observed from Figure 5 that the current density varies with applied electric-field
electric‐field
intensity in the nanocomposite similar to that of the unfilled, control PP samples, and is non‐linear
intensity in the nanocomposite
non‐linear
nanocomposite similar
similar to
to that of the unfilled, control PP samples, and is non-linear
for the electric‐field intensities of 35 kV/mm and above. The relative permittivity, on the other hand,
for the electric‐field
kV/mm and
electric-field intensities of 35 kV/mm
andabove.
above.The
Therelative
relativepermittivity,
permittivity, on
on the
the other
other hand,
hand,
remains constant around 2.25 up to about 35 kV/mm in PP filled with 2 wt % natural clay as shown
remains constant
constant around
around2.25
2.25up
uptotoabout
about3535kV/mm
kV/mmininPP
PPfilled
filledwith
with2 2wtwt%%
natural
clay
shown
natural
clay
as as
shown
in
in Figure 6 for the linear region of the J–E characteristic of the material. For electric fields higher than
in
Figure
6 for
linear
region
theJ–E
J–Echaracteristic
characteristicofofthe
thematerial.
material.For
For electric
electric fields
fields higher than
Figure
6 for
thethe
linear
region
ofof
the
35 kV/mm, however, there is a substantial reduction on the relative permittivity from 2.25 to 1.38 at
35 kV/mm,
kV/mm,however,
however,there
there is
is aa substantial
substantial reduction
reduction on
on the
the relative
relative permittivity
permittivity from 2.25 to 1.38 at
110 kV/mm due to the non‐linearity of the material.
kV/mm due
110 kV/mm
dueto
tothe
thenon‐linearity
non-linearity of
of the
the material.
material.
Relative
Relativepermittivity
permittivity
2.35
2.35
2.15
2.15
1.95
1.95
1.75
1.75
1.55
1.55
1.35
1.35 0
0
10
10
20
20
30
40
30
40
Electric field
Electric field
50
60
70
50
60
70
intensity (kV/mm)
intensity (kV/mm)
80
80
90
90
100
100
110
110
120
120
Figure 6. Relative permittivity as a function of electric‐field in PP composite filled with 2 wt % natural
Figure
permittivity
as aasfunction
of electric‐field
in PPin
composite
filled with
wt % natural
Figure 6.
6. Relative
Relative
permittivity
a function
of electric-field
PP composite
filled2 with
2 wt %
nanoclay.
nanoclay.
natural nanoclay.
PP filled with 6 wt % natural clay also revealed a non‐linear, saturation‐type J vs. E characteristic
PP filled with 6 wt % natural clay also revealed a non‐linear, saturation‐type J vs. E characteristic
PP
filled
6 wt %7.natural
clay also revealed a density
non-linear,
saturation-type
J vs.
E characteristic
as pointed
outwith
in Figure
The displacement‐current
started
to saturate at
approximately
33
as pointed out in Figure 7. The displacement‐current density started to saturate at approximately 33
as
pointed
out
in
Figure
7.
The
displacement-current
density
started
to
saturate
at
approximately
kV/mm.
kV/mm.
33 kV/mm.
The relative permittivity of this nanocomposite appears to be nearly constant at 2.24 up to the
The relative permittivity of this nanocomposite appears to be nearly constant at 2.24 up to the
The
relative
permittivity
of thisasnanocomposite
appears
electric‐field
intensity
of 33 kV/mm
presented in Figure
8. to be nearly constant at 2.24 up to the
electric‐field intensity of 33 kV/mm as presented in Figure 8.
electric-field
intensity
of PP
33 kV/mm
as presented in filled
Figure
8. 2 wt % nanoclay, the material exhibits
Similar to
the plain
and the nanocomposite
with
Similar to the plain PP and the nanocomposite filled with 2 wt % nanoclay, the material exhibits
Similar
to
the
plain
PP
and
the
nanocomposite
filled
with
2 wt
nanoclay,
material
a reduction in its relative permittivity to 1.42 at 110 kV/mm. As
can%be
observedthe
from
theseexhibits
results,
a reduction in its relative permittivity to 1.42 at 110 kV/mm. As can be observed from these results,
a
reduction
in
its
relative
permittivity
to
1.42
at
110
kV/mm.
As
can
be
observed
from
results,
natural nanoclay in PP did not contribute significantly to the current density vs. these
electric‐field
natural nanoclay in PP did not contribute significantly to the current density vs. electric‐field
characteristic or to the relative permittivity.
characteristic or to the relative permittivity.
Polymers 2018, 10, 942
8 of 11
natural nanoclay in PP did not contribute significantly to the current density vs. electric-field
characteristic
to the
relative
permittivity.
Polymers
x
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REVIEW
8
Polymers 2018,
2018, 10,
10,or
x FOR
FOR
PEER
REVIEW
8 of
of 11
11
0.55
0.55
0.5
0.5
Current
Currentdensity
density(μA/mm
(μA/mm2)2)
0.45
0.45
0.4
0.4
0.35
0.35
0.3
0.3
0.25
0.25
0.2
0.2
0.15
0.15
0.1
0.1
0.05
0.05
0
0
0
0
10
10
20
20
30
30
40
50
60
70
40
50
60
70
Electric
Electric field
field intensity
intensity (kV/mm)
(kV/mm)
80
80
90
90
100
100
110
110
Figure 7.
7. Variation
Variation of
of current-density
current‐density with
with electric-field
electric‐field intensity
intensity in
in PP
PP composite
composite
filled
with
6 wt
wt %
%
Figure
composite filled
filled with
with 6
current‐density
electric‐field
natural nanoclay.
nanoclay.
natural
nanoclay.
2.35
2.35
Relative
Relativepermittivity
permittivity
2.15
2.15
1.95
1.95
1.75
1.75
1.55
1.55
1.35
1.35
0
0
10
10
20
20
30
40
50
60
70
30
40
50
60
70
Electric
Electric field
field intensity
intensity (kV/mm)
(kV/mm)
80
80
90
90
100
100
110
110
Figure 8.
8. Relative permittivity
permittivity as aa function
function of the
the electric‐field in
in PP composite
composite filled with
with 6 wt %
%
Figure
Figure
8. Relative
Relative permittivity as
as a function of
of the electric‐field
electric-field in PP
PP composite filled
filled with 66 wt
wt %
natural
nanoclay.
natural nanoclay.
nanoclay.
natural
4.
4. Discussion
Discussion
4.
Discussion
The
dielectric
of
PP
is
affected
by
the
polymer
morphology
when
PP
is
subjected
to
The dielectric
dielectric breakdown
breakdown
of PP
PP is
is affected
affected by
by the
the polymer
polymer morphology
morphology when
when PP
PP is
is subjected
subjected to
to
The
breakdown of
sinusoidal voltage.
voltage. PP
PP usually
usually breaks
breaks down
down electrically
electrically in
in locations
locations where
where the
the material
material is
is less
less dense.
dense.
sinusoidal
sinusoidal voltage. PP usually breaks down electrically in locations where the material is less dense.
Andritsch
al.
indicated
that
the smallest
breakdown
voltages
took place
in the
Andritsch etet
etal.
al.indicated
indicatedthat
that
smallest
breakdown
voltages
the interspherulitic
interspherulitic
Andritsch
thethe
smallest
breakdown
voltages
tooktook
placeplace
in theininterspherulitic
zones
zones
of
PP,
whereas
the
highest
breakdown
voltages
took
place
in
the
spherulites
regions
[12].
zones
of
PP,
whereas
the
highest
breakdown
voltages
took
place
in
the
spherulites
regions
[12].
of PP, whereas the highest breakdown voltages took place in the spherulites regions [12].
Therefore,
Therefore, the
the improvement
improvement observed
observed in
in the
the breakdown
breakdown strength
strength of
of nanocomposites
nanocomposites of
of PP
PP could
could be
be
Therefore,
the improvement observed in the breakdown strength of nanocomposites of PP could be due to the
due
to
the
natural
nanoclay
that
fills
the
regions
less
dense
in
PP.
due to the
naturalthat
nanoclay
fills the
less
natural
nanoclay
fills thethat
regions
lessregions
dense in
PP.dense in PP.
Also,
an
optimum
content
of
nanoclay
is
proposed
according to
the
results
on
the
breakdown
Also,
an
optimum
content
of
nanoclay
is
proposed
the results
results on
on the
the breakdown
breakdown
Also, an optimum content of nanoclay is proposed according
according to
to the
strengths in
in this
this investigation.
investigation. It
It was
was observed
observed that
that the
the breakdown
breakdown strength
strength did
did not
not improve
improve
strengths
strengths in this investigation. It was observed that the breakdown strength did not improve
substantially
for
the
nanocomposite
filled
with
6
wt
%
nanoclay
when
compared
to
2
wt
%.
Therefore,
substantially for
for the
the nanocomposite
filled with
with 66 wt
wt %
substantially
nanocomposite filled
% nanoclay
nanoclay when
when compared
compared to
to 22 wt
wt %.
%. Therefore,
Therefore,
nanocomposite loaded
loaded with
with 22 wt
wt %
% natural
natural clay
clay was
was considered
considered to
to be
be the
the optimum
optimum amount
amount of
of natural
natural
nanocomposite
clay.
Abou‐Dakka
et
al.
concluded
similarly
with
the
similar
nanocomposites
of
PP
in
which
clay. Abou‐Dakka et al. concluded similarly with the similar nanocomposites of PP in which all
all the
the
homocharges
could
possibly
be
reduced
for
the
optimum
nanoclay
content
[11].
homocharges could possibly be reduced for the optimum nanoclay content [11].
When the
the rate
rate of
of rise
rise of
of the
the applied
applied voltage
voltage was
was increased
increased from
from 90
90 to
to 1050
1050 V/s
V/s across
across unfilled
unfilled PP
PP
When
and
its
nanocomposite
samples,
a
significant
increase
was
observed
on
the
breakdown
strength
and its nanocomposite samples, a significant increase was observed on the breakdown strength of
of
Polymers 2018, 10, 942
9 of 11
nanocomposite loaded with 2 wt % natural clay was considered to be the optimum amount of natural
clay. Abou-Dakka et al. concluded similarly with the similar nanocomposites of PP in which all the
homocharges could possibly be reduced for the optimum nanoclay content [11].
When the rate of rise of the applied voltage was increased from 90 to 1050 V/s across unfilled PP
and its nanocomposite samples, a significant increase was observed on the breakdown strength of the
materials. The increase in breakdown strength is due to the insufficient time to transfer energy to the
particles in the unfilled PP as well as in the natural nanoclay-filled samples to initiate the ionization
activity when the rate of rise of the applied voltage is higher. Thus, a higher electric-field intensity is
required to maintain higher energy for the inception of ionization activity that might possibly cause a
sustained discharge leading to the failure of the material.
The saturation effect in the current density vs. electric-field intensity characteristics in unfilled PP
and its nanocomposites loaded with natural nanoclay can be explained based on the net polarization
of the materials. From Equation (3) it should be noted that the electric flux density is an angular
frequency reduced displacement current under sinusoidal steady state. Also, the electric flux density
is proportional to the net polarization of the material as indicated in Equations (4)–(6). When the
applied electric field is raised starting from zero up to a certain level, the displacement current
density increases linearly since it is directly proportional to the net polarization in the material.
Above that certain electric-field intensity, on the other hand, the rate-of-change of net polarization
with respect to the electric-field intensity becomes lower. Consequently, the saturation effect starts
occurring in the material. In the linear region of the J–E characteristics of the materials, the relative
permittivity remains almost constant since the net polarization of the material takes place at a
constant rate of change with respect to the electric field. But, with respect to a higher electric field,
net enhancement rate of polarization diminishes and leads to a significant reduction in the permittivity
of the material. In a another work it was also stated that although polymeric materials have almost
constant permittivity at relatively low electric fields, at high electric fields, substantial drops could be
seen in the permittivity [26].
5. Conclusions and Future Work
As discussed in Section 4, one of the important conclusions of this study was, PP loaded with
2 and 6 wt % natural nanoclay indicated an improved breakdown strength when compared to the
unfilled isotactic PP. However, it was observed that breakdown strength improved significantly as
soon as the PP was doped with 2 wt % natural nanoclay. When the nanoclay content increased to
6 wt % there was still an increase in breakdown strength, but not as sharp as it was from plain PP
to 2 wt % nanoclay loaded PP. Also, as the ramp speed of the applied voltage increased, breakdown
strength of all the materials under consideration increased.
Current density vs. electric-field intensity characteristics of all the materials in this study have
shown very similar trends with some saturation effect. When the electric field was raised in the
material up to a certain level, it seemed to have a linear behavior. However, above a critical applied
electric field, materials responded non-linearly as discussed in Section 4. This was essentially due to
the reduction of the enhancement-rate of the net electric polarization in the PP as well as its composites
with nanoclay.
Surface modification seems to be important for the contribution of the filler material to the
properties of nanocomposites as indicated in [3,13,14,27]. As a future study, the influence of surface
modification could be investigated on the breakdown and permittivity properties of the PP loaded
with natural nanoclay. Also, further study should be performed on the aging of nanocomposites of PP
with natural clay since their life prediction is important especially when used in capacitors and cables.
Author Contributions: H.R.H. performed the experiments; H.R.H. and I.E.S. analyzed the data; H.R.H. wrote
the manuscript.
Funding: This research received no external funding.
Polymers 2018, 10, 942
10 of 11
Acknowledgments: The authors are grateful to M. Abou-Dakka of National Research Council Canada for
supplying the isotactic polypropylene and polypropylene nanocomposite filled with 2 and 6 wt % Cloisite® 20A.
The equipment used in this work was partially funded by National Science Foundation grant # USE-8851806.
Conflicts of Interest: The authors declare no conflict of interest.
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