Evolution of some dielectric properties of polypropylene-nanoclay composites with DC poling

Space charge and DC conductivity evolution with DC poling of two types of polypropylene-based nanocomposites containing different concentrations of nanofiller were investigated. The two nanofillers used were natural and synthetic organoclays. It was observed that the optimal concentration of a nanofiller for mitigating space charge in polypropylene (PP) is 2-wt% and between 2 and 4-wt% for the natural and synthetic clay, respectively. Excessive quantity of nanofiller could lead to overlapping of the interaction zone of double layers formed at the nanoparticle/host material interfaces, which promotes charge transport. Under DC field this overlapping increases conductivity of a nanocomposite and thus could minimize the benefit of incorporating nanofillers into PP. The total charge stored in unfilled PP increased continuously with time but increased only slightly for filled specimens. After only 504 h of DC poling, at -25 kV/mm, the conductivity of specimens containing 6-wt% of natural clay and 8-wt% of synthetic clay reached ≈ 6 times the level for unfilled PP. This observation could be related to crossing the percolation threshold of these composites. The effect of platelet size on space charge mitigation reported by other authors has been confirmed in this work, i.e. nanofillers with smaller platelets mitigate space charge more efficiently then nanofillers with larger platelets.