Effect of reverse polarity on space charge evolution in polypropylene with different concentration of natural and synthetic nano clay

Abstract

Space charge evolution in Polypropylene (PP) loaded with two types of nanoparticles, synthetic and natural clay were investigated using weekly reversing polarity. The space charges were measured with the PEA technique after certain periods of poling with both polarities. The charge distribution in the host materials was substantially smaller with reverse polarity compared to single negative polarity. This majority of charge injected with single polarity is likely due to compatibilizers, which are polar materials. The apparent charges due to dipoles are neutralized when the field is inversed and the remaining charges that fall in deep traps require more than ± 25 kV/mm to be released. To obtain and maintain space charge suppression for both polarities the nanoclay concentration in PP should be below the percolation threshold and ensure uniform distribution of nano-particles throughout polymer matrix. The PNC with 2-wt% of synthetic clay stores lots of charges with reverse polarity, especially after the end of the positive periods. So the synthetic PNC with 2-wt% will not be an optimal concentration. It is seen that 4-wt% (PP1-S4%) for the synthetic PNC and the 2-wt% (PP2-N2%) for the natural PNC are the optimal PNC concentrations. They store minimal charge, particularly in central zones. The absolute value of charge with reverse polarity of all the filled and unfilled materials could store charges relatively of same order. The quantity of charge in the unfilled material, PP1-0% shows continuous increase of charges and it becomes higher than the filled PNC, (except the PP1-S2%), after the 1st W of single polarity poling and the 12th W of reverse polarity poling. But the quantity of charge in the natural PNC was small and unaffected by the loading with nanoclay. Therefore, more poling time is necessary to examine the effect of long term poling on the charge evolution as we have reported earlier for single poling.