Nanocomposite Materials for electrical insulation based on polyolefins and their blends

In the last years, different research studies investigated the possibility to find out a new class of materials in the field of medium/high voltage insulation. Nowadays, considering cables, the conventional materials commonly used, both non-recyclable, are cross-linked polyethylene (XLPE) and EPR. In the last decades, the importance of the material recycling has grown, and a switch to thermoplastic polymer based materials has started. Nanocomposites showed markedly improved mechanical, thermal, optical and physical- chemical properties ( e.g. flammability heat resistance, barrier properties) with respect to their unfilled counterparts. Nanocomposites blends based on polyolefins could represent a new class of insulating materials with low environmental impact and good electrical performances. This research activity investigated the electrical behaviour of some nanocomposites based on thermoplastic polymers. The materials have been prepared by adding different kinds of fillers to polyolefin matrices, namely nanosilica, sepiolite fibers and laminar silicates.

The considered matrices are syndiotactic polypropylene sPP, low density polyethylene LDPE/high density polyethylene HDPE blend in 80:20 proportion, low density polyethylene LDPE/ethylene vinyl acetate EVA blend in 80:20 proportion. The materials have been prepared, as is usual in thermoplastics, by extrusion, in fact, this process could be easily scaled up to industrial production. Dielectric strength measurements have been performed to evidence the short term behaviour of the materials and also to evaluate the quality of the blends and of the prepared specimens; a first screen on the prepared materials permitted to choose the best processing procedures. To study the materials medium/long term behaviour, electrical aging tests have been carried out. The aging tests have been performed applying industrial frequency, 50 Hz, sinusoidal voltage waveforms to the specimens. Various voltage amplitudes have been considered and the end-point criterion has been the breakdown condition. The relevant times to breakdown have been collected and used to plot the materials life curves. By the analysis and the comparison of the obtained results, different behaviours among the nanocomposites and the pure polymers have been highlighted. The surface partial discharges electrical aging tests have been carried out inside an environmental chamber in order to control both the temperature and the humidity. The electrical treeing tests have been performed applying constant sinusoidal (50 Hz) voltage amplitude; the tree inception voltage has been determined to evaluate both the effect of the filler and the good adhesion of the needle to the polymeric matrix. During the tests the partial discharge activity has been measured by means of a Phase Resolved Partial Discharge Analyser PRPDA. In this way it has been possible to trace the evolution during the test of the parameters relevant to the partial discharges, to identify the kind of tree growth (bush, branch, bush-branch) and to evaluate the effects of the different nanofillers employed.