A Raman Microprobe Study of Corona Ageing in a Controlled Atmosphere

N.A. Freebody, A.S. Vaughan19 January 2011

Problem• A Raman microscope has a potential lateral resolution of ~1 μm and

is able to characterise the chemical composition of a material [1]

• Exact characterisation of the processes and chemicals involved in electrical treeing via Raman microscopy has proven to be difficult

• In 2003 Liu, Vaughan and Chen discovered that the deposits left on the HV electrode after the ageing LDPE via corona discharge, similar to those in electrical trees [2]

• This study aims to repeat and extend the work of Liu, Vaughan and Chen to include a similar analysis involving various polymers in a closed chamber where the atmosphere can be controlled

[1] R. Tabaksblat, R.J. Meier, and B.J. Kip, “confocal Raman Mircrospectroscopy – Theory and application to Thin Polymer samples” Appl. Spectrosc. Vol.46, pp. 60-68, Jan 1992

[2] X.S. Liu, A.S. Vaughan, and G. Chen, “A Raman spectroscopic study of bulk and surface ageing phenomena in polyethylene” Annual Report Conference on Electrical Insulation and Dielectric Phenomena: p. 145-148, 2003

Samples• Sheets of additive free LDPE were cut into samples (30 x 30 x 1

mm). 

• PS samples (30 x 30 x 1 mm) were made by hot press moulding in moulds against a glass like surface and quenched from the melt.

• PEEK samples (30 x 30 x 5 mm) were cut from a larger sheet supplied by directplasticsonine 

• Silicone rubber samples (30 x 30 x 5 mm) were made using the Dow Corning Sylgard 184 Silicone Elastomer kit

Experimental setup

• Samples characterised using Renishaw RM1000 Raman microprobe system including a Renishaw NIR 780TF diode laser (wavelength 785 nm) with a maximum output power of 25 mW and a x50 objective lens.

• Spectra analysed using a combination of Wire 3 and Sigma plot 10 software.

Diagram of test cell (a) stainless steel hypodermic needle electrode (tip radius approximately 3 µm) (b) Aluminium sample base plate, (c)

Perspex cylinder with rubber seals,

Material Atmosphere Needle separation / mm Ageing voltage / kV Ageing time / h

LDPE Air 1 15 6

LDPE Nitrogen 2 12.5 3

PS Air 2 20 6

PEEK Air 3 18 6

Silicone rubber Air 3 17.5 6

Silicone rubber Nitrogen 2 15 3

a)

b)

c)

Surface Characterisation

Optical micrographs showing (a) un aged LDPE, (b) corona aged LDPE, (c) un aged silicone rubber, (d) corona aged silicone

rubber

10 μm

a) b)

c) d)

Wavenumber (cm-1)

100020003000In

tens

ity

Surface Characterisation

Optical micrographs showing a) un aged LDPE,

b) corona aged LDPE, c) un aged silicone rubber,

d) corona aged silicone rubber

10 μm

Raman spectra for various positions on LDPE surface aged by corona discharge.

a) b)

c) d)

Wavenumber (cm-1)

100020003000In

tens

ity

Surface Characterisation

Optical micrographs showing a) un aged LDPE,

b) corona aged LDPE, c) un aged silicone rubber,

d) corona aged silicone rubber

Wavenumber (cm-1)

100020003000In

ten

sity

10 μm

Raman spectra for various positions on PS surface aged

by corona discharge.

a) b)

c) d)

Wavenumber (cm-1)

100020003000In

tens

ity

Surface Characterisation

Optical micrographs showing a) un aged LDPE,

b) corona aged LDPE, c) un aged silicone rubber,

d) corona aged silicone rubber

10 μm

Raman spectra for various positions on PEEK surface aged by corona discharge.

a) b)

c) d)Wavenumber (cm-1)

100020003000In

ten

sity

Wavenumber (cm-1)

100020003000In

tens

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Wavenumber (cm-1)

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Surface Characterisation

Optical micrographs showing a) un aged LDPE,

b) corona aged LDPE, c) un aged silicone rubber,

d) corona aged silicone rubber

Always try and provide a caption next to your picture in this style

10 μm

Raman spectra for various positions on Silicone rubber

surface aged by corona discharge.

• Little or no variation of spectra from original matrix indicates no chemical changes

a) b)

c) d)

Wavenumber (cm-1)

100020003000

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nsi

ty

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Needle Deposits

Optical image of deposit found on electrode of aged Si rubber

10 μm

Needle Characterisation - Air• Peaks at higher energy levels indicate possible oxygen related

bi-products• Traces of fluorescence in all spectra thought to be precursor

to ageing

Wavenumber (cm-1)

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nsity

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nsity

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Raman spectra of (a) LDPE, (b) PS, (c) PEEK and (d) Silicone rubber electrodes, aged in air

a) b) c) d)

Needle characterisation - Nitrogen

Wavenumber (cm-1)

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Wavenumber (cm-1)

100020003000In

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• No peaks indicating oxygen related bi-products• Large amounts of fluorescence• Evidence of carbon related peaks similar

Raman spectra of (a) LDPE and (b) Silicone rubber aged in Nitrogen

a) b)

Electrical Treeing

• Previous Raman studies on treeing in LDPE indicate:

– Presence of carbon

– Presence of fluorescence

– Presence of original polymer matrix

Raman spectra published by Vaughan et al showing the Raman spectra acquired from a

conducting electrical tree grown in PE a) relates to a position away from a tree channel,

b) at the edge of a tree channel and c) the centre of a tree channel [3]

[3] A.S. Vaughan, I.L. Hosier, S.J. Dodd, S.J. Sutton, “On the structure and chemistry of electrical trees in polyethylene” J. Phys. D-App. Phys. vol 39(5): pp. 962-978, 2006.

Conclusions• LDPE, PEEK and PS in air: evidence of a link with electrical

treeing

• Silicone rubber in air: deposits have a similar chemical fingerprint to original matrix

• Evidence of oxygenated bi-products

• LDPE in nitrogen: close spectral resemblance to electrical trees (fluorescence and the D and G bands of sp2 hybridized carbon)

• Common processes involved between corona surface ageing and electrical treeing especially when an inert atmosphere is used

• Possible ex situ to reproduce, in bulk, the processes involved in treeing by using corona discharge

Future work• Temperature control

• Moisture control

• Other polymers (i.e. Epoxy)

• Other gasses (i.e. Methane, SF6)

• Function of voltage, needle separation and ageing time

• Apply other characterisation methods (i.e. SEM, FTIR)

Questions?