The chemical degradation of epoxy resin by partial discharges.
Epoxy res~ a major component of solid electrical insulation systems, degrades when
subjected to electrical discharges. Identification of the epoxy resin degradation
mechanism might indicate improvements which can be made in the chemical
formulation of the resin to enhance the insulation systems.
Samples of a commonly used epoxy res~ bisphenol-A epoxy crosslinked with
phthalic anhydride, were manufactured and then aged by applying lOkV AC to an
electrode 2mm above the resin surface. The following experimental conditions were
manufacturing system: (i) moulded slab and (ii) slice cut from moulded cylinder;
atmosphere: (i) nitrogen, (ii) dry, (iii) normal and (iv) moist air;
high voltage electrode:(i) brass and (ii) copper.
In addition, ageing due to chemical, thermal and radiative stressing was also
examined. The changes in the stressed resin samples were determined using the
following techniques: diffuse reflectance Fourier Transform infrared spectroscopy
(DRIFT), attenuated total reflectance FTIR (ATR-FTIR), thermogravimetric
analysis (TGA), thermogravimetric FTIR (TG-FTIR) and atomic force microscopy
The changes in the electrode materials were determined using X-ray diffratometry
(XRD) and Fourier Transform infrared (FTIR) techniques.
The method of production of the specimens was shown to affect the degradation.
Silicone release agent, used in the moulding of the resin slabs, was found on the
surface of degraded moulded resin slabs: the contamination of the moulded samples
was not detectable prior to partial discharge stressing. Crazing and flaking of the
stressed resin surface were found on the moulded slabs but not on the slices of resin.
Anhydride, acid and amine species were identified on the surface of the electrically
stresses resin slices. Chemical reactions accounting for the changes found on the
surface of the stressed resin slices are given. The radical species formed by methyl
group dissociation, reacting with hydroxyl and activated oxygen species, lead to the
formation of linear anhydrides, acids and peracids on the resin surface. Reactive
nitrous oxide species in the discharge atmosphere react with the resin to form amines.
Zinc formate dihydrate was identified on brass electrodes after the resin ageing
process, whilst basic copper nitrate was identified on copper electrodes. The
difference in deposit found on the electrode indicates that zinc reacts with carbon
species from the discharge environment; copper reacts, not with carbon species, but
with nitrogen species. The variation in chemical interaction at the high voltage
electrode, dependant upon electrode material, has been found to correlate with
changes on the resin surface.
Correlations are made between the effects of partial discharge and other stresses
applied. None of the applied stresses generated the anhydride structure found in
partial discharge stressed resin samples. However, in common with p.d. stress, UV
radiation increases the level of crosslinking in the epoxy resin and produces carbonyl
structures, nitric acid fumes produce acid, peracid and nitroso structures.