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Polyethylene Nanocomposites –
A Solution Blending Approach
by
Kwan Yiew Lau1,2
with Prof. Alun S. Vaughan1
Dr. George Chen1
Dr. Ian L. Hosier1
1 University
of Southampton
2 Universiti
Teknologi Malaysia
Introduction
Background
• Polymeric insulators – widely used as standard materials
in power delivery systems.
• The current commercial trend is to add micro-sized filler
into polymer.
– Benefits: Enhanced mechanical & thermal properties.
– Trade-off: Worsened electrical properties.
3
Background
• Introduce nano-sized filler into polymer.
– Polymer nanocomposites / nanodielectrics.
• Polymers with nanometre-sized fillers
homogeneously dispersed at just a few wt%.
4
Research Trend
The number of publications in nanodielectrics (Nelson, 2010)
• Improved breakdown strength, mitigated space charge
formation, enhanced partial discharge resistance, etc.
5
Problems
• Lots of uncertainties concerning nanocomposite
applications in dielectric systems remain unanswered.
• The mechanisms that lead to the unique dielectric
properties of nanocomposites remain unclear.
Lack of understanding!
especially of the underlying physics and chemistry…
6
Challenges
• Dispersion of nanoparticles in polymers.
• Small size = agglomeration ≠ single particles.
• Various preparation techniques are proposed to obviate, or
at least minimise, unwanted clustering effects.
7
Materials and
Preparation
Materials
• Polymers:
– 80 wt% LDPE grade LD100BW (ExxonMobil Chemicals)
– 20 wt% HDPE grade Rigidex HD5813EA (BP
Chemicals)
• Nanofiller:
– SiO2 nanopowder (Sigma Aldrich), 10 - 20 nm.
– Unfunctionalized.
9
Preparation of Materials
• Solution blending method:
– Nano-SiO2 was added into xylene, sonicated for 1 hour.
– PE blend was then added.
– The mixture was heated to the boiling point of xylene &
stirred simultaneously.
– The hot mixture was precipitated in methanol.
– Filtering, drying and melt pressing.
• Unfilled PE prepared in the same way.
10
Results and
Discussion
Thermal Analysis
0.6
0.6
All nanofilled
PE exhibited
reduced
induction time
and faster
crystallisation.
119 °C
Crystallised Fraction
0.5
0.4
0.3
0.2
121 °C
(partly shown)
Nanoparticles
act as
nucleation
sites.
0.1
0.0
111 °C
113 °C
115 °C
117 °C 119 °C
121 °C
0.5
Crystallised Fraction
111 °C
113 °C
115 °C 117 °C
0.4
0.3
0.2
0.1
0.0
0
100
200
300
400
500
600
700
800
Time / s
Non-linear Avrami fitting on unfilled PE
0
100
200
300
400
500
600
700
800
Time / s
Non-linear Avrami fitting on 5 wt%
nanofilled PE
12
Crystallisation Rate Constant, K3
• At any given temperature, nanofilled
PE shows higher K3.
1e-3
• 2 wt% - increased K3.
1e-4
1e-5
1e-6
K3 / s-3
• 5 wt% - higher K3 data
– Increased interactions?
– Increased nucleation sites?
• 10 wt% - K3 values saturated.
– Suppression effect caused by the
reduced growth rate?
– Indicative of the onset of nanosilica
aggregation?
0 wt%
2 wt%
5 wt%
10 wt%
1e-7
1e-8
1e-9
1e-10
110
112
114
116
118
120
122
124
Tc / °C
Plot showing the content of nano13
SiO2 on K3 parameter of PE
Subsequent Melting Behaviour
Nanofilled PE
Tc = 119 °C
Tc = 119 °C
Tc = 117 °C
Tc = 117 °C
Endothermic
Endothermic
Unfilled PE
Tc = 115 °C
Tc = 115 °C
Tc = 113 °C
Tc = 113 °C
Tc = 111 °C
Tc = 111 °C
60
80
100
120
140
60
Temperature / °C
•
80
100
120
140
Temperature / °C
The melting behaviour was similar, except at Tc = 111 ºC.
– Pronounced double peak (unfilled PE) vs. more singular peak (nanofilled PE).
14
Crystallinity
• The addition of nanoSiO2 does not affect the
final crystallinity.
• A hint to similar melting
trace?
Tc = 111 ºC
Tc = 115 ºC
Tc = 119 ºC
Sample
0 wt%
X/%
X/%
X/%
66.3
58.5
53.4
2 wt%
65.7
58.9
48.4
5 wt%
66.9
57.8
49.8
10 wt%
65.5
58.4
49.2
– The thickness of the lamellae is similar.
• Nano-SiO2 acts as nucleating agent but does not
increase the final crystallinity.
– Nucleation effect + topological confinement.
15
Polarised Optical Microscopy
• For crystallised unfilled
PE, spherulites can be
clearly observed.
• For nanofilled PE:
– The size of the
spherulites was
smaller.
0 wt%
2 wt%
5 wt%
10 wt%
– Nano-inclusion
appears dramatically
to suppress
spherulitic
development.
Crystallised at 117 ºC
16
Scanning Electron Microscopy
• Unfilled PE, crystallised 115 ºC:
– Open banded spherulitic
structures, space filling.
• 2 wt% nanofilled PE:
– Banded spherulites can still be
observed.
0 wt%
– Smaller spherulites size.
– Nucleation effect.
– Nanofiller well distributed, but
agglomeration could not be
avoided.
2 wt%
17
Scanning Electron Microscopy
• Aggregation becomes more
apparent with increasing amount of
nanofiller.
• At 5 wt%, the effect of spherulite
banding becomes less
pronounced, and the texture was
significantly perturbed.
5 wt%
• At 10 wt%, the growth of spherulite
is largely suppressed, resulted in
highly disordered system.
18
10 wt%
AC Breakdown Test
Crystallised at 115 ºC
99
0 wt%
2 wt%
5 wt%
10 wt%
Weibull Cumulative Failure Probability / %
90
70
Sample
𝛼 / kV mm-1
𝛽
0 wt%
152 ± 3
19 ± 6
2 wt%
152 ± 2
33 ± 10
5 wt%
150 ± 2
26 ± 7
10 wt%
121 ± 2
21 ± 7
50
30
20
• No difference between 0 wt%,
2 wt% and 5 wt%?
10
• Severe aggregations in 10 wt%
nanofilled PE reduced the
breakdown strength.
5
2
1
60
80
100
120
140
Breakdown Field / kV mm-1
160 180 200
19
AC Breakdown Test
Quenched
99
0 wt%
2 wt%
5 wt%
10 wt%
Weibull Cumulative Failure Probability / %
90
70
Sample
0 wt%
𝛼 / kV mm-1
148 ± 4
𝛽
16 ± 5
2 wt%
147 ± 4
16 ± 4
5 wt%
144 ± 3
23 ± 7
10 wt%
115 ± 3
16 ± 5
50
30
20
• Same breakdown trend in
quenched systems.
10
• Nanosilica does not alter AC
breakdown strength.
5
2
1
60
80
100
120
140
Breakdown Field / kV mm-1
160 180 200
• At severe aggregations, AC
breakdown strength would
be reduced.
20
Conclusions and
Future Work
Conclusions
• Nano-SiO2 enhances the nucleation density.
– Evidenced from the shorter crystallisation process and
higher value of crystallisation rate constant.
• The DSC melting traces of the nanocomposites were
similar to unfilled PE.
– Nano-SiO2 did not exert on appreciable effect on the
melting behaviour.
• Nano-SiO2 did not possess significant effect towards the
final crystallinity.
22
Conclusions
• From POM & SEM:
– Nano-SiO2 suppresses spherulitic development and
thus perturbed the morphological structure of the
isothermally crystallised material.
• From SEM, nanosilica is well-distributed in PE through
solution blending approach.
– Agglomeration is unavoidable.
• Nano-SiO2 does not alter AC breakdown strength of PE.
– But the breakdown strength will reduce if the dispersion
is poor.
23
Future Work
• Dielectric spectroscopy:
– Dielectric response of the nanocomposites.
– Water absorption behaviour.
• Pulse electro-acoustic:
– Space charge behaviour.
• Surface treatment of nano-SiO2.
24
Thank you!
Appendices
Experimental Techniques
• DSC
– Perkin Elmer DSC 7 with Pyris software.
– Sample ~5 mg in a sealed aluminium pan.
– Nitrogen atmosphere.
– Avrami analysis was performed by DSC.
• Heating rate: 10 ºC min-1
• Cooling rate: 100 ºC min-1
• POM
– Linkam THM600 hot stage.
– Melt press sample between two microscope slides
27
Experimental Techniques
• SEM
– JEOL Model JSM-5910.
• Gun voltage = 15 kV; Working distance = 11 mm.
– Standard permanganic etching technique.
• Permanganic reagent composed of 1 % w/v
solution of potassium permanganate in an acid
mixture composed of concentrated sulphuric acid,
phosphoric acid & water at ratio 5: 2: 1.
• After etching, the reagent was quenched using
hydrogen peroxide & dilute sulphuric acid at ratio
4: 1.
28
Experimental Techniques
• Dielectric Breakdown Test
– Samples of ~85 µm in thickness were prepared by using
a Specac press (150 ºC, 3 tonne).
– Dielectric breakdown test based upon ASTM Standard
D149-87.
– The test sample was placed between two 6.3 mm ballbearing electrodes immersed in silicone fluid.
– An AC voltage of 50 Hz and a ramp rate of 50 V(RMS) s-1
was applied until failure.
29
Avrami Analysis
• The crystallinity fraction at time t:
𝜒 = 1 − exp⁡
[−𝐾𝑒𝑥𝑝 (𝑡 − 𝑡𝑖 )𝑛 ]
• The obtained experimental values of X and t were fitted to
the equation using a non-linear approach to estimate the
Kexp, ti and n.
• Kexp = experimental rate constant or overall crystallisation
rate constant containing contributions from both nucleation
and growth
• n = Avrami exponent or dimensionality of the growth
30
Crystallisation Rate Constant, K3
3
4
3
𝐾3 = 𝜋𝑁𝐺 ≅ (𝐾𝑒𝑥𝑝 )𝑛
3
• N = the number of nucleation sites per unit volume
• G = the growth rate of the crystallising objects
31
Crystallinity Calculation
• The enthalpies of melting was determined as a function of
crystallisation temperature for each material and then
converted into the percentage of HDPE present in each
blend that was involved in each phase transition
(Mandelkern, 1992).
∆𝐻
𝑋=
× 100
𝜔∆𝐻𝑜
∆H = melting enthalpy
∆Ho = the value of enthalpy corresponding to the melting of a
100 % crystalline material (293 J g-1 PE)
ω = the weight fraction of the crystallisable material.
32
Weibull Analysis
• Two-parameter Weibull distribution:
𝑃 𝐸 =1−𝑒
−
𝐸 𝛽
𝜶
P(E) = cumulative probability of failure at E
E = experimental breakdown strength
α = scale parameter, represents the breakdown strength at the cumulative failure probability
of 63.2 %
β = shape parameter
• The cumulative probability of failure, P(E) was approximated using the
median rank method:
𝑖 − 0.3
𝑃 𝐸 =
𝑛 + 0.4
i = progressive order of failed tests
n = total number of tests
33
SEM Micrographs
“Dielectric properties
of XLPE/SiO2
nanocomposites
based on CIGRE
WG D1.24
cooperative test
results”
(Tanaka et al., 2011)
IEEE TDEI, 18(5),
1484-1517
XLPE containing 5wt% of
unfunctionalized nanosilica
XLPE containing 5wt% of
functionalized nanosilica
34