Unfilled Low Density Polyethylene Research Articles

Carrier transport in LDPE and its nanocomposites

The mobility of positive and negative charge carriers has been separately measured independently of carrier concentration in low-density polyethylene (LDPE) and its nanocomposites using space charge measurements in a bilayer system. It was found that the carrier mobility of positive charge carriers was larger than that of negative charge carriers for both LDPE and its nanocomposites and that the carrier mobility was one to two orders of magnitude less in the nanocomposite than unfilled LDPE under the same conditions. The mobility is interpreted in terms of a field-assisted hopping model whose estimated activation energies were verified by measurements at different temperatures. The form of the charge traps and carrier transport is interpreted in terms of known features and electron and hole traps and the effect of nano-particles upon these features is discussed.

Accelerated aging of pine wood waste/recycled LDPE composite

The production of composites from recycled thermoplastic polymers and natural fibers has been an interesting alternative to generate polymeric materials of low cost, with sufficient properties for application and environmentally adequate. The aim of the present study was to evaluate the effects of the accelerated photochemical aging on properties of a composite produced from recycled low-density polyethylene (LDPE) with pine wood waste (PWW) in the presence of coupling agent. The oxidation, performance of mechanical properties and surface changes of the LDPE and LDPE/PWW composite were monitored at different times of aging. It was observed that LDPE/PWW composite presented a higher resistance to the accelerated photochemical aging than unfilled LDPE, mainly due to the better performance of the mechanical properties of elongation at break and tensile strength, showing that the presence of natural fibers from PWW lead to the stabilization of the polymeric matrix of LDPE.

Electrical Conductivity and Moisture Uptake Studies of Low Density Polyethylene Octylnanosilica Composite

In this paper, we report on the time dependent DC-conductivity response at constant electric field of unfilled low density polyethylene (LDPE) and its composites containing untreated and octylsilane treated silica in ambient and dry atmospheres. The conductivity of both composites decrease with time under dry conditions and increase with time in ambient atmosphere. The dielectric response of dry and wet silica polyethylene composites is studied by dielectric spectroscopy.

AC and DC pre-stressed electrical trees in LDPE and its aluminum oxide nanocomposites

Resistance of pure low density polyethylene (LDPE) and its aluminum oxide nanocomposites (up to 3.0 wt%) to degradation by electrical treeing under AC stress and DC pre-stress is analyzed. The experiments were carried out on wire-plane electrode specimens before and after exposure to thermal and DC electro-thermal ageing at 80 °C. The obtained results showed enhanced resistance of the nanocomposites to electrical tree inception under AC stress and the tree inception voltage (TIV) increased with nanoparticles content. It has been shown that there was an improved partial discharge (PD) resistance in the nanocomposites compared to the unfilled LDPE. The results also showed that the AC TIV in the nanocomposites consistently increased with the ageing and especially the DC electro-thermally aged specimens had about 30% higher the AC TIV as compared to the unaged material. This effect is attributed to significantly reduced mobility of charge carriers in the nanocomposites. The DC pre-stressed electrical trees generated in the investigated materials were of filamentary-branch structure and the branch channels content increases with the addition of nanoparticles. The mean tree number of the DC pre-stressed electrical trees decreased in the LDPE and its nanocomposites while the mean maximum tree length increased with the ageing treatments. It is postulated that material recrystallization and a very high electric field level on the wire electrode during the DC pre-stressed electrical tree test are the main reasons for the observed effects.

Study of the factors that suppress space charge accumulation in LDPE nanocomposites

Two mechanisms have been proposed to explain the suppression of space charge in polyethylene by the addition of nano-fillers, i.e. an interface change that reduces charge injection and a bulk modification that affects charge migration and recombination. The relative importance of each mechanism in Low Density Polyethylene (LDPE) nanocomposites is investigated by the measurement of space charge in samples of double-layer and triple-layer structures. The experimental results show that the nano-fillers reduce charge injection, but do not entirely eliminate the space charge. Bulk changes also play an important role in space charge suppression. The space charge near the interface between the unfilled LDPE and nanocomposites shows a surface charge at 20 °C, but at 60 °C a bipolar blocking phenomenon is observed in some circumstances. It is shown that these space charge distributions can be reproduced using a band theory approach with the assumption that the introduction of deep traps by nano-fillers raises the Fermi level of the nanocomposite towards the conduction level.

Effect of morphology on electrical treeing in low density polyethylene nanocomposites

Influence of polymer morphology on the inception and the growth of electrical trees in unfilled low density polyethylene (LDPE) as well as LDPE filled with 1, 3 and 5% by weight nanoalumina samples stressed with 50 Hz ac voltage has been studied. It is seen that there is a significant improvement in tree inception voltage with filler loading in LDPE filled with nanoparticles. Tree inception voltage increased with the filler loading up to 3% by weight nanoalumina loading and showed a reduction at 5% by weight loading. Change in tree growth patterns from branch to bush as well as a slower tree growth with increase in filler loading in LDPE alumina nanocomposites were observed. The degree of crystallinity and change in crystalline morphology induced by the presence of alumina nanoparticles in LDPE was studied using differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). DSC results show a similar melting behaviour for both unfilled LDPE and LDPE nanocomposites. However, there is a reduction in the degree of crystallinity for LDPE filled with 5% by weight nanoalumina. An increase in lamellae packing with increase in filler loadings and a highly disordered spherulitic structure for LDPE filled with 5% by weight nanoalumina was observed from the SEM images. The slow propagation of tree growth as well as reduction in tree inception voltage with increase in filler loadings were attributed to the morphological changes observed in the LDPE nanocomposites.

Electrical treeing and the associated PD characteristics in LDPE nanocomposites

Treeing in low density polyethylene (LDPE) filled with alumina nanocomposite as well as unfilled LDPE samples stressed with 50 Hz ac voltage has been studied. The tree inception voltage was monitored for various samples with different nano-filler loadings and it is seen that there is an increase in tree inception voltage with filler loading in LDPE. Treeing pattern and tree growth duration for unfilled and nano-filled LDPE samples have also been studied. Different tree growth patterns as well as a slower tree growth with increase in filler loading in LDPE nanocomposites were observed. The observed slow propagation of tree growth with filler loading is attributed to the changes in the polymer crystalline morphology induced by the presence of nano-particles and the greater ability of the nanoparticles to resist discharge growth. SEM studies carried out to determine the morphology of unfilled and nano-filled LDPE showed an increase in lamellae packing in LDPE nanocomposites and this increased lamellar density leads to a reduction in the tree propagation rate. Partial discharge activities were also monitored during the electrical tree growth in both the unfilled and the nano-filled LDPE samples and were found to be significantly different. PD magnitude and the number of PD pulses per cycle were found to be lower with electrical tree growth duration in LDPE nanocomposites as compared to unfilled LDPE. The same trend was seen with increased filler loading also.

LDPE/Mg–Al layered double hydroxide nanocomposite: Thermal and flammability properties

Layered double hydroxides (LDHs) are new nanofillers which exhibit improved thermal and flammability properties in various kinds of polymer matrices. These materials have certain advantages over conventional metal hydroxides and also layered silicates so far as the flame retardancy is concerned. In this article, flammability and thermal properties of the nanocomposite based on low density polyethylene (LDPE) and Mg–Al based layered double hydroxide (Mg–Al LDH) are reported in detail. The nanocomposites containing different LDH concentrations were prepared by melt-compounding using a tightly intermeshing co-rotating twin-screw extruder. The morphological analysis reveals an exfoliated/intercalated type LDH particle morphology in these nanocomposites. The thermogravimetric analysis (TGA) shows that even a small amount of LDH improves the thermal stability and onset decomposition temperature in comparison with the unfilled LDPE. The heat release rate (HRR) and its maximum (PHRR) during cone-calorimeter investigation are found to be reduced significantly with increasing LDH concentration. The nanocomposites not only exhibit reduced total heat released (measure of propensity to produce long duration fire), but also lower tendency to fast fire growth (measured by the ratio of PHRR and time of ignition). The limited oxygen index (LOI) and the dripping behavior are also improved with increasing LDH concentration.

Determination of filler concentration in magnesium hydroxide filled LDPE during extrusion, using ultrasound

The relative effects of changes in temperature, pressure and weight percentage filler on ultrasonic velocity through static samples of magnesium hydroxide filled low density polyethylene (LDPE) melt have been determined for a range of temperatures and pressures. Ultrasonic velocity was found to be most sensitive to change in temperature and least sensitive to change in pressure. Using these data and data obtained during extrusion of unfilled LDPE, the amount of magnesium hydroxide present (weight percentage) in LDPE was determined. Values of weight percentage calculated from extrusion data were compared with values obtained from the thermogravimetric analysis of extrudate. Variations in weight percentage of filler between the two sets of data followed the same trend with time of extrusion.

Effects of pressure and temperature on the melt density and the melt flow rate of LDEP and glass bead-filled LDPE composite

The effects of pressure and temperature on the melt density (ρm) and the melt flow rate (MFR) of the low-density polyethylene (LDPE) and a glass bead-filled LDPE composite (LDPE/GB) have been investigated using a capillary plastometer. It was discovered that the MFR of the melts increased as an exponential law function with a rise in temperatures and increased as a quadratic function with increasing pressure; the ρm of the melts increases non-linearly with increases in pressure and decreased approximately linearly with increases in temperature. For the same temperature, the values of the MFR for LDPE/GB are somewhat lower than those of the unfilled LDPE when the pressure is not too high. The relationship between the composite melt density, the glass bead density and the unfilled LDPE melt density obeys a simple mixing rule.