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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
3
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.
4
Background
• Introduce nano-sized filler into polymer.
– Polymer nanocomposites / nanodielectrics.•Polymers with nanometre-sized fillers
homogeneously dispersed at just a few wt%.
5
Research Trend
The number of publications in nanodielectrics (Nelson, 2010)
• Improved breakdown strength, mitigated space charge formation, enhanced partial discharge resistance, etc. Opportu
nity!
6
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…
7
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.
Materials and Preparation
9
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.
10
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.
Results and Discussion
12
Thermal Analysis
Non-linear Avrami fitting on unfilled PE Non-linear Avrami fitting on 5 wt% nanofilled PE
All nanofilled PE exhibited reduced induction time and faster crystallisation.
Nanoparticles act as nucleation sites.
Time / s
0 100 200 300 400 500 600 700 800
Cry
stal
lise
d F
ract
ion
0.0
0.1
0.2
0.3
0.4
0.5
0.6111 °C
113 °C115 °C 117 °C 119 °C
121 °C(partly shown)
Time / s
0 100 200 300 400 500 600 700 800C
ryst
alli
sed
Fra
ctio
n
0.0
0.1
0.2
0.3
0.4
0.5
0.6
115 °C
111 °C113 °C
117 °C 119 °C121 °C
13
Crystallisation Rate Constant, K3• At any given temperature, nanofilled
PE shows higher K3.
• 2 wt% - increased K3.
• 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?
Plot showing the content of nano-SiO2 on K3 parameter of PE
Tc / °C
110 112 114 116 118 120 122 124
K3
/ s-3
1e-10
1e-9
1e-8
1e-7
1e-6
1e-5
1e-4
1e-3
0 wt%2 wt%5 wt%10 wt%
14
Subsequent Melting Behaviour
• The melting behaviour was similar, except at Tc = 111 ºC.
– Pronounced double peak (unfilled PE) vs. more singular peak (nanofilled PE).
Temperature / °C
60 80 100 120 140
En
do
the
rmic
Tc = 111 °C
Tc = 113 °C
Tc = 115 °C
Tc = 117 °C
Tc = 119 °C
Temperature / °C
60 80 100 120 140
En
do
the
rmic
Tc = 111 °C
Tc = 113 °C
Tc = 115 °C
Tc = 117 °C
Tc = 119 °C
Unfilled PE Nanofilled PE
15
Crystallinity
• The addition of nano-SiO2 does not affect the final crystallinity.
• A hint to similar melting trace?
Sample
Tc = 111 ºC Tc = 115 ºC Tc = 119 ºC
X / % X / % X / %
0 wt% 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.
Polarised Optical Microscopy• For crystallised unfilled
PE, spherulites can be clearly observed.
• For nanofilled PE:
– The size of the spherulites was smaller.
– Nano-inclusion appears dramatically to suppress spherulitic development.
0 wt% 2 wt%
5 wt%
16
10 wt%
Crystallised at 117 ºC
Scanning Electron Microscopy• Unfilled PE, crystallised 115 ºC:
– Open banded spherulitic structures, space filling.
• 2 wt% nanofilled PE:
– Banded spherulites can still be observed.
– Smaller spherulites size.
– Nucleation effect.
– Nanofiller well distributed, but agglomeration could not be avoided.
0 wt%
2 wt%17
Scanning Electron Microscopy
5 wt%
10 wt%18
• 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.
• At 10 wt%, the growth of spherulite is largely suppressed, resulted in highly disordered system.
19
AC Breakdown Test
• No difference between 0 wt%, 2 wt% and 5 wt%?
• Severe aggregations in 10 wt% nanofilled PE reduced the breakdown strength.
Breakdown Field / kV mm-1
60 80 100 120 140 160 180 200
We
ibu
ll C
um
ula
tive
Fa
ilure
Pro
ba
bili
ty /
%
1
2
5
10
20
30
50
70
90
99
0 wt%2 wt%5 wt%10 wt%
Sample 𝛼 / kV mm-1 𝛽0 wt% 152 ± 3 19 ± 6
2 wt% 152 ± 2 33 ± 10
5 wt% 150 ± 2 26 ± 7
10 wt% 121 ± 2 21 ± 7
Crystallised at 115 ºC
20
AC Breakdown Test
• Same breakdown trend in quenched systems.
• Nanosilica does not alter AC breakdown strength.
• At severe aggregations, AC breakdown strength would be reduced.Breakdown Field / kV mm-1
60 80 100 120 140 160 180 200
We
ibu
ll C
um
ula
tive
Fa
ilure
Pro
ba
bili
ty /
%
1
2
5
10
20
30
50
70
90
99
0 wt%2 wt%5 wt%10 wt%
Quenched
Sample 𝛼 / kV mm-1 𝛽0 wt% 148 ± 4 16 ± 5
2 wt% 147 ± 4 16 ± 4
5 wt% 144 ± 3 23 ± 7
10 wt% 115 ± 3 16 ± 5
Conclusions and Future Work
22
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.
23
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.
24
Future Work
• Dielectric spectroscopy:
– Dielectric response of the nanocomposites.
– Water absorption behaviour.
• Pulse electro-acoustic:
– Space charge behaviour.
• Surface treatment of nano-SiO2.
Thank you!
Appendices
27
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
28
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.
29
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 ball-bearing 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.
30
Avrami Analysis• The crystallinity fraction at time t:
• 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
𝜒= 1−exp[−𝐾𝑒𝑥𝑝(𝑡− 𝑡𝑖)𝑛 ]
31
Crystallisation Rate Constant, K3
• N = the number of nucleation sites per unit volume
• G = the growth rate of the crystallising objects
𝐾3 = 43𝜋𝑁𝐺3 ≅ (𝐾𝑒𝑥𝑝)3𝑛
32
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 = 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.
𝑋= ∆𝐻𝜔∆𝐻𝑜 × 100
33
Weibull Analysis• Two-parameter Weibull distribution:
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 = progressive order of failed tests
n = total number of tests
𝑃ሺ𝐸ሻ= 1−𝑒ቈ−ቀ𝐸𝜶ቁ𝛽
𝑃ሺ𝐸ሻ= 𝑖−0.3𝑛+0.4
34
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