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