Additive-free Low-density Polyethylene Research Articles

Effect of heat treatment on morphology and dielectric properties of PE cable insulation material

It is known that the heat treatment process in theproduction of extruded cables, affects the electricalproperties of polymeric insulation materials; this effectis via two main phenomena; first, it leads to diffusionand removal of polar substances from the system;second, it affects the polymer morphology by increasingthe polymer chain relaxation and creation of crystallites,hence increasing the crystallinity of the material.In this study, thick pressed samples with additive freelow density polyethylene (LDPE) insulation materialwere heat treated at different conditions for differentperiods of times. Differential scanning calorimetry(DSC) measurements confirmed that heat treatmentleads to higher crystallinity and higher lamella thicknessin LDPE. Dielectric properties of the material wasstudied by conduction current measurement anddielectric spectroscopy.Conduction current of samples with different heattreatment times was measured at high electric fieldswhich shows that the conduction current of LDPE isaffected by heat treatment. Generally, the apparentconductivity decreases by heat treatment.Dielectric spectroscopy measurements show that heattreatment does not have a significant effect on real andimaginary permittivity of the polymer. Interestingly, anincrease in the sample thickness via heat treatment, wasobserved both by sample thickness related to thememory effects in the polymer which needs to be takeninto consideration for accurate capacitancemeasurements.

The trapping characteristic of low density polyethylene in the presence of crosslinking by-products

The by-products of dicumyl peroxide (DCP) from the crosslinking process such as acetophenone, cumyl alcohol and α-methylstyrene are said to be the sources of space charge formation in XLPE cable due to deep traps in the chemicals. However, by using space-charge-experimental approach, it appeared that these chemicals show a different trapping nature. This paper is intended to present this approach. Additive-free low density polyethylene (LDPE) was used as base material so that each chemical can be tested individually. Space charge measurement was done using the pulse electroacoustic (PEA) method. All results were compared to the clean LDPE to identify the contribution of the chemicals to the trapping characteristic. The data collected supported that although the chemicals introduce charge in the insulator, the charge decay is extremely fast especially in the presence of α-methylstyrene. It is believed that the chemicals modify the trapping characteristic of LDPE so that more shallow traps are formed in the insulator.

Space Charge Measurement as a Diagnostic Tool to Monitor Ageing in Polymeric Materials

Charge dynamics in polymeric materials after aged under ac electric field using the pulsed electroacoustic (PEA) technique is reported. The emphasis is placed on charge decay. The charge dynamics of the ac aged additive free low density polyethylene (LDPE) samples under dc bias differ from the sample without ac ageing, indicating changes brought in by ac ageing. It is believed that a slow decay rate of charge in the ac aged sample is related to the formation of deep traps in the material. However, chemical analysis by infrared spectroscope (FTIR) and Raman microscope reveals no significant chemical changes taken place in the bulk of the material after ac ageing. Further experiments on irradiated LDPE have revealed a similar behaviour, i.e. the charge decay is slower in irradiated samples than that of fresh sample. The findings presented clearly indicate that space charge measurement can be used as a diagnostic tool to monitor ageing in polymeric materials.

Phase resolved electroluminescence measurements in thin films of low density polyethylene using a charge coupled device camera

Electroluminescence (EL) produced by a commercially available additive free low density polyethylene film has been investigated under a 50 Hz AC electrical stress. The spatial distribution, spectral characteristics and the phase angle relationship of EL with respect to the 50 Hz applied AC voltage were studied using a sensitive Peltier cooled charge-couple device (CCD) camera. The experimental results from several test samples show that the EL can be divided into two different groups according to the EL spatial distribution: (1) homogeneous EL and (2) inhomogeneous EL. At the same applied voltage, the spectra of the homogenous and inhomogeneous EL were found to be significantly different. Homogeneous EL had maximum emission intensity in the red and near infrared (>700 nm), whereas the inhomogeneous emission had maximum intensity at shorter wavelengths (550-600 nm). Phase resolved measurements showed that homogeneous EL always produced equal emission intensity in the positive and negative half-cycles (symmetric distribution), whereas, inhomogeneous EL produced an asymmetric intensity distribution. To explain these results, it is suggested that homogeneous emission is associated with charge injection and bipolar recombination in the surface states on both sides of the sample, whereas inhomogeneous emission is associated with field enhancement at defects on one of the two sides of the film which allow charge to be injected into states more representative of the bulk material.

Chemical group in crosslinking byproducts responsible for charge trapping in polyethylene

It is known that space charge is formed in cables insulated with crosslinked polyethylene. It is probable that the crosslinking byproduct or the crosslinked polymer morphology is responsible for the space charge. In order to examine the former possibility, an additive-free low-density polyethylene sheet was soaked in various liquid chemicals including the crosslinking byproducts and space charge distribution inside the sheet was measured under DC voltage application. As a result, space charge appears in the vicinity of the injecting electrode only in the case that the sheet was soaked in the liquids with at least one hydroxyl group in their structures. Therefore, it is concluded that cumyl alcohol, which is the only byproduct with a hydroxyl group in its structure, is responsible for the formation of space charge in polyethylene on the assumptions that chemical functions of a given molecule act separately from each other and that other contributions such as the crosslinked polymer morphology are negligible.

The influence of primary ion bombardment conditions on the secondary ion emission behavior of polymer additives

The secondary ion (SI) emission behavior of pure polymer systems is meanwhile well understood. However, common plastics not only consist of the polymer host material but also contain a variety of additives normally present in low concentrations only. In order to better understand the parameters governing the SI emission of these trace compounds we performed a systematic study on the influence of the analysis parameters (primary ion (PI) type, PI energy, electron bombardment for charge compensation, etc.) using model systems. Samples were prepared by spin coating (sub)monolayers of Irganox 1010 onto additive-free low density polyethylene (LDPE). The SI parameters yield, disappearance cross-section and efficiency (yield per damaged area) were determined for PI bombardment with Ga +, Cs +, and SF 5 +. Furthermore the damaging influence of electron bombardment for charge compensation on the organic surface layers was investigated.

Compatibility of paclitaxel in 5% glucose solution with ECOFLAC® low-density polyethylene containers–stability under different storage conditions

The compatibility of paclitaxel with low-density polyethylene containers (ECOFLAC®) was studied under different temperature and light conditions. Solutions of 0.4 and 1.2 mg/ml of paclitaxel in 5% glucose solution were prepared, put into ECOFLAC® containers and stored: (i) at ambient temperature (20–25°C) and in ambient light; (ii) at ambient temperature in the dark; and (iii) at +4°C in the dark. Paclitaxel was assayed by high-performance liquid chromatography after visual inspection of the solutions. The results show that solutions of TAXOL® in 5% glucose should not be stored for more than 5 days in glass or ECOFLAC® containers because a whitish precipitate tends to form, lowering the paclitaxel concentration. The decrease in the paclitaxel concentration observed after chromatographic analysis ranged very widely (from 12 to 83% of the initial concentration). However solutions of TAXOL® diluted in 5% glucose was stable for 5 days in ECOFLAC® containers under all the storage conditions tested. These additive-free low-density polyethylene containers offer the advantage of not releasing DEHP into the paclitaxel solutions.

Preliminary results of combined thermo-luminescence and thermally stimulated current measurements on an additive free polyethylene

Samples of additive free low density polyethylene have been irradiated at − 180 and − 130 C with 1 keV electrons. The sample arrangement was such that simultaneous measurements of thermally stimulated discharge currents (TSDC) and luminescence (TL) were possible and these techniques were used to observe effects of surface charge relaxation. The resulting spectra were similar in some respects to those obtained by other workers using either higher electron energies, which create bulk charges, and higher irradiation temperatures, or using X- or γ-radiation to create internal charge. In particular some of the TL peaks in the spectra obtained by the present authors were observed in similar regions and a current reversal seen in the TSDC spectrum obtained at the higher irradiation temperature (− 130°C) was also observed by another worker but for material irradiated at above room temperature. The current reversal is most likely to be associated with charge movement in the bulk rather than across the sample surface.

The influence of ions on the thermally stimulated discharge current spectra of water-treed additive-free low-density polyethylene

The effects of water treeing on steady state dc currents and thermally stimulated discharge current spectra (TSDC) of additive-free low density polyethylene (LDPE) were studied using heavily water-treed material in solutions of LiCl, LiCl plus HCl, MnCl/sub 2/ and FeCl/sub 3/. Significant changes in the dc conductivity of treed samples are attributed to the effects of ions. For such samples, graphs of the logarithm of current versus inverse temperature show three distinct linear regions. The transition temperatures demarcating these regions appear to correlate with those reported for thermal relaxations. TSDC spectra of water-treed samples show a dependence on both the ionic solutions used and on the poling conditions. For LiCl treed samples, twin peaks in the TSDC spectra were produced only under specific poling conditions. These peaks may be correlated with thermal relaxations observed using differential scanning calorimetry (DSC). The melting point temperature and the melting peak area derived from DSC spectra were reduced by treeing. >

Thermally-stimulated creep spectroscopy in the study of physicochemical ageing of LDPE

In polymeric power cables, the physicochemical ageing of the insulator is an important limiting factor of its lifetime. The present work reports a comparative study performed on additive-free low-density polyethylene (LDPE) aged in different electrolytes under an electric field. The influence of the electric field, ageing temperature and ionic species on LDPE was investigated by thermally stimulated creep (TSCr) spectroscopy. The TSCr spectrum of reference LDPE shows the existence of three mechanical retardation modes, labelled α, β and γ in order of decreasing temperature. The α-mode has been associated with mechanisms involving the rigid amorphous domains stressed by crystallites, while the β- and -γ-modes have been attributed to the amorphous phase. The experimental data deduced from the analysis of TSCr spectra have been plotted on a compensation diagram: they define the structure of the disordered phases of reference LDPE. The comparative study of various aged LDPE samples shows that the para-crystalline phase or the rigid amorphous phase is affected first. In a second step, ageing mechanisms hinder the relaxational process of the amorphous phases and the material becomes fragile and brittle at a rate that is a function of the ionic solution used.

The effects of gamma-irradiation on thermally stimulated discharge current spectra in low-density polyethylene

An investigation has been made of permanent effects of γ-radiation on additive-free low-density polyethylene (LDPE) using thermally stimulated discharge current techniques (TSDC) over the temperature range -100–100°C. In the low temperature regime (-100–0°C) a peak occurring around -30°C was introduced by irradiation in air but not in oxygen-free nitrogen. It was found that this peak is due to dipole orientation of oxidation products such as carbonyl groups. The movement of the peak towards higher temperatures with increasing radiation absorbed dose, is explained in terms of radiation-induced crosslinking which increases with increasing dose. In the high temperature region, (0–100°C) a peak observed at approx. 45°C in pristine samples, is due to the release of charge carriers injected from the electrodes. This peak was marked in irradiated samples by charge carriers created by the radiation. In the latter case, a peak occurred in the temperature range 60–100°C depending on dose and polarisation voltage. This behaviour can be explained by assuming that irradiation in air introduces shallow traps within the material which can be filled from deeper traps by a field assisted process during repeated polarisations associated with successive measurements. The process does not occur for irradiation in nitrogen and it is suggested that the shallow trapping levels are the result of oxidation, particularly with the production of carbonyl groups.