Influence of moisture absorption on the DC conduction and space charge property of MgO/LDPE nanocomposite

Experimental studies and theoretical explanations were carried out in order to understand the influence of moisture absorption on the dielectric properties of magnesium oxide (MgO)/low density polyethylene (LDPE) nano-composite. After exposing LDPE and our MgO/LDPE to 40% relative humidity (RH) for 9 days, the absorbed water content in LDPE and our MgO/LDPE nanocomposite was about 2.3 0 / 00 and about 3.6 0 / 00 respectively. Experimental results show that moisture absorption has a negative influence on the dielectric properties of our MgO/LDPE nanocomposite at high electric fields of about 40kV/mm. Damp nanocomposites can still maintain the ability to suppress space charge and keep the conducting current from increasing (which normally occurs when moisture is absorbed) at low electric fields below 20 kV/mm. Absorbed water molecules can be considered to be divided into three forms, free water molecules which are assumed into the polymer matrix, weakly bound water molecules which may gather around nano-particles and tightly bonded water molecules, bound by hydrogen bonds. Thermogravimetry (TG) test results indicate that tightly bonded water molecules cannot be removed from the nanocomposite after a drying process at 80 °C in vacuum conditions. For damp MgO/LDPE nanocomposite after the drying process, the j-E characteristics were completely recovered up to an electric field of 40 kV/mm, whereas the space charge suppression ability partially recovered. The reason for this partial recovery may be attributed to the influence of tightly bonded water molecules on the interfacial charge in nano-particles and matrix polymers.