Crosslinked polyethylene (XLPE) is a key material used for power cable insulation due to its electrical properties and structure. Both need to be controlled for long term use. It is made by taking low-density polyethylene (LDPE) and connecting chains together using dicumyl peroxide (DCP). The crosslinks are a source of stability and reduced conductivity, but the crosslinking process also leaves behind DCP byproducts that are sources of excess conductivity and shallow trap states. As such, they are typically removed via degassing, a procedure where XLPE is annealed in evacuated environments at temperatures within its melting range (50-110°C) and which enhances crystallization. This combination of byproduct removal and crystallization would be implicated in the improvement of insulating properties, so a combined analysis of changes in morphology and electrical properties of XLPE due to annealing has clear utility.<br/> <br/>In this work, XLPE with and without degassing was analyzed using broadband dielectric spectroscopy (BDS) to determine these impacts. The effect of variation in fabrication parameters such as such as the amount of DCP and crosslinking temperature were also examined. BDS was selected to allow for combined analysis of structure via measuring the temperature coefficient of capacitance (TCC) and of electrical properties with the dielectric loss. It was determined that degassing reduces the dielectric loss in XLPE, and also in LDPE. The two also had similar loss values with and without the thermal treatment. The use of the treatment was also correlated with altered TCC and increased crystallinity. This implied that structural changes played a bigger role in improving the dielectric properties than removal of byproducts in this case. Intentionally incorporating in byproducts to measure their impact led to drastic increases in the dielectric loss and heavily altered TCC due to the presence of these small molecules. They also altered the temperature-dependence of the dielectric loss, leading to a maximum in the loss around room temperature, rather than exhibiting enhanced loss only at elevated temperatures. Using higher amounts of DCP (e.g. 8 wt%) can also lead to increases in the dielectric loss and altered TCC, even with degassing. Meanwhile, the choice of crosslinking temperature, which impacts XLPE morphology, was able to influence the dielectric loss, TCC, and crystallinity, but had only subtle impacts on each one.<br/>

Combined dielectric and structural characterization of polyethylene via broadband dielectric spectroscopy

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.