The Morphology of Cross-Linked Polyethylene Insulation

Cross-linked polyethylene, now commonly used for insulation of underground electric distribution cables, is susceptible to electrochemical treeing, in common with other polymeric dielectrics. This treeing phenomenon sometimes originates at holes or cavities present in the cable. A polishing and etching technique has been developed which, when combined with scanning electron microscopy, permits the relation of the cavity structure to the crystalline morphology to be determined. The larger cavities, 5 to 10 μm in size, are to be found at the points of impingement of spherulites. Smaller cavities, revealed by the etching techniques, occuir in the boundaries of spherulites. These cavities are believed to be due to condensation of aromatic by-products of the initiator. A cavity network exists throughout the material giving rise to a weak pathway which is likely to facilitate tree formation.