Physico-chemical diagnosis of EPDM electrical aging by tree phenomenon

This paper presents a physico-chemical experimental study of treeing structures expanding in EPDM (Ethylene Propylene Diene Monomer) in a point - plane electrode geometry. Accelerated electrical degradation tests, under alternating 50 Hz electrical field, were realised on this polymer to initiate trees and to follow their propagation (i.e. the tree length) by measuring the partial discharges (PDs) occurring within this insulating material. A chemical analysis by Scanning Electron Microscopy (SEM) and by Infrared spectroscopy (IR) are undertaken to determine the damaged polymer microstructures and changes in its morphology. The chemical groups existing in this material and the changes due to its degradation under electrical field have been identified by Infrared spectroscopy. The microstructure modification of this material, by breaking chemical bonds and splitting molecular chains, explains the formation of tree channels, and accompanying off-gases that promote the appearance of partial discharges and develop the defect. This process is related to the amplitude and the duration of the electrical field which governs the tree forms and which depends on the diameter of the channels and their internal texture revealed by SEM analysis.