A Raman Microprobe Study of Corona Ageing in a Controlled Atmosphere

1. A Raman Microprobe Study of Corona Ageing in a Controlled Atmosphere N.A. Freebody, A.S. Vaughan19 January 2011

2. 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

3. 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

4. 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. c) a) b) 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,

5. Surface Characterisation a) b) 10 μm d) c) Optical micrographs showing (a) un aged LDPE, (b) corona aged LDPE, (c) un aged silicone rubber, (d) corona aged silicone rubber

6. Surface Characterisation Raman spectra for various positions on LDPE surface aged by corona discharge. a) b) 10 μm c) d) Optical micrographs showing a) un aged LDPE, b) corona aged LDPE, c) un aged silicone rubber, d) corona aged silicone rubber

7. Surface Characterisation Raman spectra for various positions on PS surface aged by corona discharge. a) b) 10 μm c) d) Optical micrographs showing a) un aged LDPE, b) corona aged LDPE, c) un aged silicone rubber, d) corona aged silicone rubber

8. Surface Characterisation Raman spectra for various positions on PEEK surface aged by corona discharge. a) b) 10 μm c) d) Optical micrographs showing a) un aged LDPE, b) corona aged LDPE, c) un aged silicone rubber, d) corona aged silicone rubber

9. Surface Characterisation Raman spectra for various positions on Silicone rubber surface aged by corona discharge. a) b) Always try and provide a caption next to your picture in this style 10 μm c) d) Optical micrographs showing a) un aged LDPE, b) corona aged LDPE, c) un aged silicone rubber, d) corona aged silicone rubber • Little or no variation of spectra from original matrix indicates no chemical changes

10. Needle Deposits 10 μm Optical image of deposit found on electrode of aged Si rubber

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

12. Needle characterisation - Nitrogen • No peaks indicating oxygen related bi-products • Large amounts of fluorescence • Evidence of carbon related peaks similar a) b) Raman spectra of (a) LDPE and (b) Silicone rubber aged in Nitrogen

13. 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.

14. 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

15. 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)

16. Questions?