Effect of aspect ratio and water contamination on the electric properties of nanostructured insulating materials

Organically-modified nanofiller clays can have significantly different aspect ratios as well as accumulate a relatively large amount of water in the composite bulk due to the contribution of the filler itself and the interaction between filler and polymer matrix. This paper investigates the effect of water absorption in a nanostructured thermoplastic polymer, namely ethylene-vinyl-acetate (EVA), on electrical property modifications considering the contribution of aspect ratio. The change of electrical properties (particularly space charge accumulation, electric strength, bulk conductivity and permittivity/losses) is studied as a function of water content absorbed by nanofillers having different aspect ratio, i.e. fluorohectorite and bohemite. An increase of the space charge build up and of the conductivity (and decrease of the electric strength) is observed as a function of the water content for specimens containing layered silicates as nanofillers (fluorohectorite), while the electric properties of bohemite specimens do not show significant variations with the absorbed water content. This behavior can be associated with the different aspect ratio of the nanofillers. A filler having higher aspect ratio, in fact, seems to be more effective in worsening the electrical properties of the final nanocomposite. This could be explained considering that the higher the aspect ratio of the nanoparticles, the larger the percolation probability. To quantify how water content may affect electric properties of the final nanocomposite a simplified model providing the percolation probability as a function of water content and geometry of the particles is developed in this paper.