Characteristics and annealing behavior of polyethylene designed for use as underground cable insulation

A series of polyethylenes developed for use as high voltage electric cable insulation have been characterized. All crosslinked polyethylene materials show similar melting behavior, whereas linear-low-density polyethylene shows complicated melting behavior due to the variation of copolymer content. The IR investigation shows that tree retardant polyethylenes contain additional chemical additives. The basic functions of the additives are thought to be antioxidation and voltage stabilization, on the basis of higher oxidative induction time (OIT) and higher breakdown strength. The density, heat of fusion and crystallinity of TR-XLPE was found to be higher than the other crosslinked polyethylenes, presumably due to the presence of the additive.

SAXS results of various polyethylenes were interpreted based on model simulations. Among the various models, the variable stack model was found to fit marginally better than others for XLPE, LDPE and LLDPE. The absence of a definite Bragg peak in the SAXS pattern of LLDPE was considered to be due to the wide distribution of local crystallinity, resulting from fractionation during crystallization according to branch levels.

DSC experiments showed that isothermal annealing of polyethylene results in the generation of a new peak or a shoulder next to the annealing temperature. The origin of the double melting peak in XLPE annealed at 107°C was investigated using in-situ methods. The crystallinity determined from DSC, WAXD and density measurement was found to increase with annealing time. Morphological changes with annealing time based on the SAXS studies indicated a large increase of crystallinity distribution among the stacks, a small decrease of thickness distribution within each stack and an increase of mean long periodicity.

In the DSC study, clear evidence of room temperature annealing was found for all XLPEs and LLDPE. It was found that room temperature annealing causes a shoulder around 45°C as well as a shift of melting peak, resulting in an increase of crystallinity.

Large spherulites clustered in dense patterns were observed in the electrically ruptured regions of all polyethylenes. IR spectra of the ruptured region shows clear evidence of severe oxidation and chain scission during breakdown. Molecular chain scission appears to occur at the crosslink and branch points within the amorphous region, due to oxidation and temperature rise. The resulting short chain sections are believed to have crystallized to form large spherulites.