Low-density Polyethylene Insulation Research Articles

Modification mechanism of low-density polyethylene insulation by hydrophilic and hydrophobic porous SiO2 nanoparticles

To study the modification effect and mechanism of porous nanoparticles on electrical insulation performance, SiO2/low-density polyethylene (LDPE) nanocomposites with different filler concentrations are prepared by filling two kinds of porous SiO2 nanoparticles. The microstructure and dispersion of SiO2 in LDPE are characterized by scanning electron microscope [ the effect of nanofilling on the crystallinity of LDPE is analyzed by Differential Scanning Calorimetry (DSC)]. The mechanism of insulation modification of nanocomposites is studied by space charge and direct current conductance analysis of charge transport and Weibull distribution of breakdown field strength. By filling nanoscale SiO2 particles, the crystallinity of the LDPE matrix can be changed effectively. It is indicated that the hydrophobic SiO2 nanofillers with higher dispersity in LDPE matrix are more effective than hydrophilic SiO2 nanofillers to improve the crystallinity of LDPE matrix. The LDPE crystallinity of hydrophobic 1 wt% SiO2/LDPE nanocomposite approaches the highest value through heterogeneous nucleation. Compared with pure polyethylene, the conductivity of nanoporous SiO2 filled polyethylene composites is reduced obviously and the lowest conductivity is obtained for the nanocomposites filled with hydrophobic SiO2 nanoparticles in 1 wt% filling rate. The space-charge distribution and electric breakdown field strength further demonstrate that the filling of 1 wt% hydrophobic SiO2 nanoparticles can efficiently inhibit carrier injection and increase the breakdown field strength. The heterogeneous nucleation of porous SiO2 nanofillers increases the crystallinity of LDPE, and thus decreases the carrier mobility and increases the probability of charge charge trapping. The dispersion of nanofillers in composite is improved significantly by the symmetrical group –CH3 on the surface of hydrophobic SiO2 nanoparticles, which effectively suppresses the space-charge accumulation and evidently increases the electric breakdown field strength. It is proved that the special nanoscale interface in nanodielectrics can substantially improve the dielectric properties of polyethylene.

Thermal degradation properties of LDPE insulation for new and aged fine wires

The thermal behavior of new and aged low-density polyethylene (LDPE) insulations was investigated using an SDT Q600 thermal analyzer. The activation energy and pyrolysis reaction model were estimated using the non-isothermal and masterplots methods. The thermal degradation processes present different behaviors of the LDPE insulations before and after thermal aging. The thermogravimetric curves shift to the direction of higher temperature, for the aged LDPE insulation. The values of activation energy evaluated using the Kissinger–Akahira–Sunose (KAS) and Flynn–Wall–Ozawa (FWO) methods are almost same. However, the activation energy values estimated by the Friedman method were slightly higher than those obtained using the KAS and FWO methods. The suitable pyrolysis reaction models of the new and aged LDPE insulations were attributed to the “Contracting area” (R2) model, which was determined using the generalized masterplots method. In addition, the pre-exponential factor and compensation effect are discussed. Finally, it should be stressed that the aged LDPE insulation generally pyrolyzes more weakly and with more difficulty than the new insulation, i.e., the ignition and flame spread of aged wire in old buildings are not relatively easy.

The influence of defects on the short-term breakdown characteristics and long-term dc performance of LDPE insulation

Extruded polyethylene is used as the bulk insulation for ac HV cables because of its high electric resistivity and breakdown strength. Although the material at present has limited use in dc power cables, it is used extensively in submarine optical communication cable systems. This paper reports on the study of the short-term characteristics and long-term performance of low-density polyethylene (LDPE) insulation under dc electric stress. The results are presented in which controlled defects as found in practical systems (voids, metallic and non-metallic particles) were introduced into well-characterized polymer material so that their influence on electrical strength and breakdown mechanisms could be determined. Samples were compression-molded under laboratory conditions and subjected to ramp-to-failure and electrical aging tests at various stresses. Weibull statistics are used to analyze the results. Examination of electrically aged samples indicates an oxidation degradation around the defects due to stress enhancement. From the failed samples a value of n in the inverse power model was estimated. The residual life of aged samples was also examined.

Investigation of space charge in low-density polyethylene using a field probe technique

A test method that uses a capacitive field probe to investigate the space charge distribution in low-density polyethylene (LDPE) is described. Specimens of 7-mm thickness were stressed under 100 kV DC at room temperature and for different time periods. The results indicate that the LDPE insulation layer between electrodes is occupied by positive and negative homocharges. The dependence of space charge distribution on the stressing time is also evident. >