38 / 2018-08-14 14:09:31

Low Density Polyethylene - Clay Nano dielectric prepared by melt extrusion process: Dielectric and Thermal properties

Nano dielectric, LDPE/clay, dielectric response, melt extrusion

In recent times polymer Nano dielectrics has drawn greater attention as electrical insulation in HVDC cables due to its superior electrical, thermal & mechanical properties and find numerous application in power equipment. Nano dielectrics are prepared by adding a small quantity of inorganic nanometer scale fillers into conventional polymers. By adding nano fillers like SiO2, MgO, silica, layered silicate, alumina, etc. the possibility of improving the properties of insulating materials such as polyethylene (LLDPE, LDPE, MDPE, HDPE), polypropylene, epoxy and silicone rubbers are being explored. These polymer Nano dielectrics are characterized by the fact that they have interfaces with enormous specific surface between polymers and Nano fillers and that such interfaces tend to determine whole properties of the polymer nanocomposites. Dielectric evaluation of polymer nanodielectrics is usually made determining the properties like permittivity, dielectric loss, electrical conductivity, space charge, dielectric breakdown, partial discharge resistance, treeing characteristics etc. Much of the research activity on the use of polyethyelene-metal oxide nano dielectrics for use in HVDC cables is reported by several investigators. Studies on the PE/clay nanocomposites are reported but scanty. Dielectric response studies of PE/clay nanocomposites is conducted at 145 Vrms but its behavior at higher voltages are not explored. The evaluation of nano dielectrics at higher voltage stress is very essential in order to ascertain its suitability for high voltage equipment.

A study was undertaken in the laboratory of CPRI to understand the dielectric and thermal properties of low density polyethylene (LDPE)/clay Nano dielectric prepared by melt extrusion process for use as electrical insulation. Nano clay has been investigated as fillers in low density polyethylene in concentrations of 1 phr to 4 phr ( 1gm in 100 gm). LDPE- clay melt compound was pressed under a plate vulcanizer at 140o C for 10 minutes to produce film samples of 100 micron and 200 micron. Finally the samples of LDPE/clay Nano dielectric was put in vacuum over at 100o C for 24 hours so as to make uniform initial thermal and mechanical conditions. The distribution and dispersion of the nano particles in the LDPE is determined by SEM (Scanning Electron Microscopy). Evaluation studies were carried out by measuring surface resistivity, volume resistivity, Glass transition temperature (Tg), deposition temperature using Differential scanning calorimeter. Dielectric response studies were carried out using spectroscopic techniques like Dielectric spectroscopy (dielectric loss angle, permittivity, capacitance measurements as a function of frequency from few mHz to tens of kHz) at 2 kV and at different temperature form 30 oC to 70 oC, Fourier Infrared spectrometer for identifying the functional groups. The surface and volume resistivity have increased by an order of two and permittivity has increased with increase in filler content and frequency. The dielectric losses has shown an increasing trend with increase in temperature. The stress strain curves indicate the mechanical stability decrease with increase in filler content. The results are presented and discussed.