ELECTRICAL DEGRADATION OF BINARY POLYETHYLENE BLENDS

Films of binary mixtures of a low-molecular-mass linear polyethylene (LPE) fraction (MBAR(m) = 2500) and higher-molecular-mass LPE (MBAR(m) = 76 000) and ethyl-branched PE (MBAR(m) = 150 000; 1.8 mol% ethyl branches) were exposed to external partial discharges (PD). For both series of binary blends, the average time to breakdown decreased markedly with increasing content of low-molecular-mass LPE component. The PD experiments also showed that rapid cooling was a more favourable thermal treatment than isothermal crystallization at low degree of supercooling. The low-molecular-mass component is the major cause of the electrical breakdown, and electron microscopy showed that rapid cooling promotes co-crystallization between different molecular-mass species. Blends of the low-molecular-mass LPE and the branched PE with intimately mixed components exhibit a resistance towards PD similar to that of the rapidly cooled binary LPE blends. Transmission electron microscopy on a PD exposed, isothermally crystallized binary LPE blend revealed the formation of 100-500 nm wide cracks occurring in the segregated low-molecular-mass domains in the near-surface layer. It is suggested that the inferiority of the low-molecular-mass component is due to its extreme mechanical brittleness, causing a great many cracks to be formed in the surface layer due to thermally induced mechanical stresses caused by the cold plasma.