Electrical aging of low density polyethylene cable insulation

Nano-sized BaSrTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> particles and a dispersant were incorporated in samples of low density polyethylene (LDPE). The nanoparticle loading was 2% or 10% by weight, and the dispersant loading was 4 parts per hundred relative to BaSrTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> . dc conductivity measurements were made in the temperature range 30-70°C in vacuum, and in air at 30 °C. The vacuum dc conductivity in LDPE with 0.4% dispersant was approximately five times larger than in LDPE, but in LDPE containing 0.4% dispersant and 10% nanoparticles it was approximately a factor of 6-7 smaller than in LDPE. Since the conductivity of the latter composite was smaller than that of its least conductive component, it may be that the interface regions between the nanoparticles and the LDPE control the dc conductivity. The dispersant and the nanoparticles both increased ε', the real part of the relative permittivity. The ε' values for the nanoparticles, implied by the Lichtenecker-Rother equation for a three-part composite, were of order several hundred, much smaller than the value of at least 10,000 quoted by the nanoparticle suppliers. Space charge measurements were made in air using the Laser Intensity Modulation Method (LIMM), and the profiles calculated using the LIMM Monte Carlo method. The maximum space charge density adjacent to the negative poling electrode was 600 C/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , for the pure LDPE and the LDPE with dispersant, and 200 C/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> near the positive electrode. The corresponding maxima in samples with dispersant and nanoparticles were approximately an order of magnitude smaller. The Monte Carlo method is more accurate than other methods of analyzing LIMM data, and this may account for the very large densities close to the electrodes.

A small amount of phenolphthalein, acting as a nucleating agent, was added to low density polyethylene (LDPE) to modify the size and number of spherulites. The crystalline morphology was observed and analyzed with the aid of a chemical etching method. It was found that the growth of water trees is faster in LDPE samples in which spherulites are larger and the number of the spherulites is lower. In contrast, water trees grow more slowly in other LDPE samples in which spherulites are smaller in size and greater in number. On the other hand, three different kinds of additives, weak electrolyte and hydrophilic or hydrophobic substances, were added to the LDPE materials to investigate the influence of typical spherulite boundary regions on the growth of water trees. It was verified experimentally that during the process of crystallization, these three different additives are rejected and finally settled near the spherulite boundaries. Therefore, three kinds of boundary regions with different characteristics are formed. It has been concluded that hydrophillic boundary regions can strongly inhibit the growth of water trees, while the other two boundary regions (weak electrolyte, hydrophobic) have little influence on the water tree growth.

Physico-chemical aspects of polyethylene processing in an open mixer 4. Comparison of PE-LLD and PE-HD with PE-LD

There has been considerable controversy over whether one can extract the primary normal stress difference (N1) for polymer melts from hole pressure (PH) and exit pressure (Pex) data. In this paper values of PH and Pex are obtained for three polymer melts in the same slit‐die apparatus under the same operating conditions. It is found that there is a good correlation between values of N1 obtained by means of a cone‐and‐plate rheometer and those obtained by means of PH data and the Higashitani‐Pritchard theory. On the other hand, values of N1 obtained by means of Pex data for polystyrene were two to five times higher than those obtained by means of the cone‐and‐plate device. For the lower molecular weight low density polyethylene (LDPE) sample, values of Pex were found to be both positive and negative. For the higher molecular weight LDPE sample, values of Pex were found not to increase monotonically with with σw. Hence, it was difficult to obtain unequivocal values of N1 for LDPE.

Dahi is widely used fermented milk product in India. Low Density Polyethylene (LDPE) is the most extensively used packaging material for Dahi in India. The present study was conducted to develop the analytical methods for extraction and migration of chemical additives from LDPE into dahi. Characterization of dahi packaging materials collected from five different firms was done by Fourier Transform Infrared Spectroscopy. Focused ultrasound solid liquid extraction method was observed to be better as compared to solid liquid extraction method as the former extracted maximum additives from the LDPE. Out of total 76 chemical additives extracted from LDPE, only eight (10.52%) matched with the existing positive list of polyolefins prescribed by Bureau of Indian Standads (BIS). The overall migration of chemical additives from all the LDPE samples was below their maximum limit as given by BIS standards. Chemical additives which migrated into the simulants included the antioxidants, fatty acids and their derivatives, unreacted hydrocarbons, plasticizers, lubricants and surfactant etc.

Low density polyethylene (LDPE) has been widely applied on high voltage transmission cables due to its excellent insulating properties, good processability and chemical stability. Large numbers of experimental and theoretical studies have shown that space charge plays an important role in aging and breakdown in LDPE, and the applied voltage can significantly impact charge transport characteristic and space charge formation and accumulation in LDPE. By solving Schottky thermal emission equation, charge continuity equation and Poisson's equation, simulate space charge injection and accumulation phenomenon in LDPE under different applied voltages (0.4 kV-8 kV). The results show that: in a 1×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> m LDPE sample, space charge accumulation starts at the applied voltage of about 2 kV. The number of trapped electrons, free electrons and total amount of electrons all increase as the applied voltage goes higher. The higher the applied voltage, the faster the trapped electrons and total amount of electrons reach their stability. The space charge accumulation rate is fast before the injected electrons reach the anode, and slows down significantly after injected into the anode. The model has an important significance for in-depth studies to research and assess the LDPE insulating properties.

Long chain branching in low density polyethylene: 2. Rheological behaviour of the polymers

Effects of flame retardants on extinction limits, spread rate, and smoke release in opposed-flow flame spread over thin cylindrical polyethylene samples in microgravity

The electrical properties of nanocomposites are closely related to their nanostructures. In order to investigate the nanostructures and space charge characteristics of magnesium oxide (MgO)/low-density polyethylene (LDPE), samples of LDPE with nano-MgO from 0.1 to 2.0 wt% are prepared, as well as pure LDPE. The nanostructures of nanocomposites are investigated by the technology of synchrotron radiation small-angle x-ray scattering (SAXS), and the space charge distributions in nanocomposites and LDPE are measured by the pulsed electro-acoustic (PEA) method. Results of the synchrotron radiation SAXS experiments reveal that there is obvious interphase between the nano-MgO particle and the LDPE matrix, the thickness of which increases along with the increase in nano-MgO concentration. The agglomeration and mass fractal exist in all nanocomposites, and become more obvious at higher concentration. Results of space charge measurements indicate that the apparent trap-controlled mobilities and the apparent trap depths are respectively larger and smaller than those in LDPE during the initial phase of depolarization when the concentrations are 0.1 and 0.5 wt%, and are respectively smaller and larger than those in LDPE when the concentrations are 1.0 and 2.0 wt%. The space charge amount decreases after the addition of nano-MgO. This is mainly due to the introduction of shallow traps which promote the charge recombination in 0.1 and 0.5 wt% MgO/LDPE nanocomposites, and the introduction of deep traps which restrict the charge transportation in 1.0 and 2.0 wt% MgO/LDPE nanocomposites. The space charge characteristics of MgO/LDPE nanocomposites are explained combined with the nanostructures obtained from the synchrotron radiation SAXS experiments.

Polymers play a very important role in electrical systems as insulation materials. The microscopic failures occurring in the polymer under working conditions will be important in its required characteristics. In this work we focused on the effects of accelerated aging on the electrical properties of low density polyethylene (LDPE) and crosslinked polyethylene (XLDPE). The samples of LDPE were prepared in sheet form with the thickness of 0.6 ± 0.1 mm and were irradiated under the 10 MeV electron beam of a Rhodotron accelerator at the dose range of 70 to 370 kGy. Then, the samples were stored in an oven at temperature 75°C for 3, 6, 10 and 13 days. Electrical properties such as the breakdown voltage and resistance of the aged samples were measured and compared. Increasing the aging time for unirradiated samples has a serious and moderate influence on their surface resistivity and breakdown voltage, respectively. The results indicated a maximum value for the surface resistance at the optimum radiation absorbed dose, which is approximately 220 kGy for all the samples.

Exogenous gibberellic acid treatment extends green chili shelf life and maintain quality under modified atmosphere packaging

Solubility and crystallinity data for ethylene/polyethylene systems

Temperature stability is one of the most determinant factor in the design of HVDC cable insulation, since the conductivity and correlated field strength distribution are temperature-dependent. Low density polyethylene (LDPE)-based cross-linked polyethylene (XLPE) material is the prevailing material system in HVDC cable insulation. Here, we report a method on improving the temperature stability for the DC performance of XLPE by changing the starting material into linear low density polyethylene (LLDPE). In order to understand the role of crosslinking agent-dicumyl peroxide (DCP), we investigate DC breakdown characteristics of LLDPE-based materials with different crosslinking degrees under different temperatures. The results show that DC breakdown strength for LLDPE-based materials varies smaller compared with LDPE-based materials. Further differential Scanning Calorimetry (DSC) results suggest that the temperature-stable DC performance for LLDPE material stems from its higher crystallinity as well as melting point than LDPE samples. And the optimal DCP content for LLDPE-based materials has been further discussed.

The temperature dependence of the thermal conductivity, κ, has been obtained from 2K to 100K on specimens of extruded high density (Rigidex) polyethylene, with extrusion ratios, R, as great as 20. The results are compared with our earlier measurements on isotropic polyethylene, and on samples of low density (Hostalen) polyethylene of appreciably smaller extrusion ratios. In all cases the values of κ below 20K are roughly comparable. Above 20K, however, the value of κ parallel to the draw direction, κ, increases markedly with increasing R. In fact, in the present measurements, for R ≃ 20, κ at 100K is greater than the value in the isotropic Rigidex material by a factor of 8. The thermal conductivity is much higher than normal for polymers or glasses and is more comparable to that of a metallic alloy such as stainless steel. These observations cannot be explained in terms of the models that we have previously used to account for the values of κ in low density polyethylene. In the present instance it is necessary to consider the role of tie molecules that are invoked to explain the exceptionally high mechanical stiffness of these same materials.

Polyethylene characterization by analytical temperature rising elution fractionation with evaporative light scattering detector