Low-density Polyethylene Specimens Research Articles

Injection Molding of Low-Density Polyethylene (LDPE) as a Model Polymer: Effect of Molding Parameters on the Microstructure and Crystallinity.

Due to its relatively simple structure, low-density polyethylene (LDPE) can be considered as a model polymer for the study of its properties. Herein, the effect of processing variables on the microstructure and crystallinity of injection-molded LDPE specimens was quantitatively determined. The polymer was injected at different temperature conditions in the barrel and the mold. The specimens were characterized by scanning electron microscopy and X-ray diffraction. With the data obtained, an analysis of variance (ANOVA) was carried out, and response surface graphs (SRP) were constructed to quantify and to observe the behavior of the processing variables, respectively. Different models were obtained to predict the effect of the experimental factors on the response variables. The results showed that the interaction of the two temperatures has the greatest effect on the size of the spherulite, while the temperature of the mold affects the crystallinity. The SRP showed different behaviors: for the spherulite, the size increases with the mold temperature, while for the crystallinity, higher values were observed at an intermediate mold temperature and a low melt temperature. The results presented herein are valuable for setting empirical relations between the microstructure, crystallinity, and the molding conditions of LDPE.

Assessment of Mechanical Properties of Recycled HDPE and LDPE Plastic Wastes

Most plastic materials can be recycled, and turning them into some useful products can be one of the options in reducing environmental pollutions. In this project, High Density Polyethylene (HDPE) and Low Density Polyethylene (LDPE) were used, shredded into smaller pieces and were hot-pressed to become flat plates. Both materials were then mixed with industrial waste sawdust with an aim to investigate their material properties and their suitability to be used in domestic applications such as bricks, pavement and roof tiles. Three destructive and one non-destructive methods have been conducted: tensile, impact, water absorption and ultrasonic testing. It was found that the tensile properties and impact resistance of the recycled HDPE are better than the recycled LDPE, but 10% lower than their virgin polymers. Higher stiffness and lower impact resistance were observed from specimens with additional sawdust filler in comparison to their original polymers. From the ultrasonic measurement, higher wave velocity and lower attenuation in HDPE specimens were observed, compared to the LDPE specimens, attributed by a higher modulus and density of the HDPE specimens. The ultrasonic velocity, which reflects the elastic modulus, and the attenuation that reflect the microstructure inhomogeneity are respectively decreased and increased when more fillers were added to the original specimens. This study reveals that the mechanical properties of the recycled HDPE and LDPE are closer to their virgin materials, and they have huge potential for plastic recycling.

Evaluation of Weld Line Strength in Low Density Polyethylene Specimens by Optical Microscopy and Simulation

Evaluation of Weld Line Strength in Low Density Polyethylene Specimens by Optical Microscopy and Simulation

Accelerated water tree aging of crosslinked polyethylene with different degrees of crosslinking

The effect of crosslinking degree on accelerated water tree aging in crosslinked polyethylene (XLPE) was investigated. The peroxide-crosslinking process was adopted to make XLPE specimens with different degrees of crosslinking by controlling the doping content of dicumyl peroxide (DCP) in low-density polyethylene (LDPE). The water blade electrode method was applied to accelerate water-tree aging of LDPE and XLPE specimens (hereafter referred to as the specimens), and their morphologies were observed using an optical microscope. The variation of crystalline morphology and anti-cracking performance of the amorphous region in the specimens were analyzed by differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and an electronic universal testing machine. Based on the experimental results, it was found that XLPE has great anti-water-treeing performance compared to LDPE. In addition, the higher the crosslinking degree, the better the anti-water-treeing performance. Although crystal growth is inhibited due to the crosslinking reaction, the density of tie molecular chains greatly increases in the amorphous region and exhibits significantly tighter lamellar stacking, which is the reason that water tree growth is restrained with increasing crosslinking degree.

Effect of Beta Low Irradiation Doses on the Micromechanical Properties of Surface Layer of LDPE

The experimental study deals with the effect of modification of the surface layer by irradiation cross-linking on the micromechanical properties of the low-density polyethylene (LDPE) tested using the instrumented nanohardness test. The surface layer of LDPE specimen made by injection technology was modified by irradiation cross-linking using beta irradiation, which significantly influences micromechanical properties of the surface layer. Compared to the heat and chemical-heat treatment of metal materials (e.g. hardening, nitridation, case hardening), cross-linking in polymers affects the surfaces in micro layers. These micromechanical changes of the surface layer are observed in the instrumented microhardness test. Our research confirms the comparable properties of surface layer of irradiated LDPE with highly efficient polymers. The subject of this research is the influence of irradiation dosage on the changes of micromechanical properties of surface layer of LDPE.

Anti-Algal Performances for Biocide-Enhanced Low-Density Polyethylene Film

In this work, low-Density Polyethylene (LDPE) films incorporated with 2-methylthio-4-ethylamino-6-tert.butylamino-triazin-1,3,5, or Terbutryn (TT) was used as anti-algal agent, at different concentrations were evaluated for anti-algal performances. Clear zone test and chlorophyll a measurement method were used for anti-algal evaluations against Chlorella vulgaris (TISTR 8580) and Phormidium angustissimum (TISTR 8979) as testing algae. Mechanical properties and surface color changes of LDPE were also observed for the effect of algaecide addition. It was found that addition of TT in the LDPE specimen did not significantly alter the tensile strength, tensile elongation and toughness properties of materials. For anti-algal test, it should be noted that the greater clear zone area (clear zone test) or smaller chlorophyll a content (chlorophyll a measurement method) the better anti-algal activities. It was found that the clear zone of the TT filled LDPE sample was observed with C. vulgaris but was not seen with P. angustissimum. When the TT concentration in LDPE was greater than 600 ppm, the C. vulgaris did not grow.

The Effect of Nano Titanium Dioxide on the Physicomechanical Properties and Heat Ageing of Low-Density Polyethylene

An examination was made of the effect of titanium dioxide (nano-TiO2, “Showa Denko”) on the physicomechanical properties of low-density polyethylene (LDPE). It was established that LDPE specimens filled with industrial titanium dioxide TiO2 of micron size and with nano-TiO2 “Showa Denko” exhibit different physicomechanical properties after composite preparation and thermooxidative ageing. In spite of a certain reduction in strength and elasticity after milling and pressing, the composite material containing nanopigment exhibits higher heat ageing resistance than material containing standard industrial titanium dioxide.

Apparatus for Testing the Combined Effects of Deformation and Chemical Exposure on Polymeric Liners

A new experimental apparatus was developed for determining the combined effects of mechanical stress and chemical exposure on the long-term durability of polymeric liners, which are widely used in civil and environmental engineering applications. The apparatus is capable of bending and holding up to 12 coupon-type specimens at predetermined levels of deflection while simultaneously exposing the specimens to a chemical solution bath. The apparatus was used to study the effects of three levels of deformation of 0.0, 0.03, and 0.08 mm/mm normalized deflection at the midspan of high-density polyethylene (HDPE) and low-density polyethylene (LDPE) specimens (3.2×12.7×63.5 mm or 0.13×0.50×2.50 in.). The bent specimens were immersed in deionized water, chlorine (3 or 800 mg/L) or trichloroethylene (TCE) (3 or 1,000 mg/L) solution baths. The specimens were then stored at a constant temperature (23, 50, or 80°C/73, 122, or 176°F) for a predetermined period of 1,250, 2,500, 3,750, and 5,000 h. Results of the tests indicate that neither TCE nor chlorine affect the modulus of elasticity of the tested HDPE and LDPE materials, whereas deformation at 8% normalized deflection (in deionized water and the chemical solutions) reduced the flexural modulus of elasticity of HDPE. In addition, elevated temperature (80°C) combined with deformation had a notable effect on the ultimate strength of HDPE and elevated temperature only (80°C) made all of the LDPE specimens fail.

Mechanical Properties of Blends of Low-Density Polyethylene with Chlorinated Polyethylene

The results of experimental investigation into the mechanical properties of blends of low-density polyethylene (LDPE) with chlorinated polyethylene (CPE) in tension are presented. The specimens of pure LDPE, CPE, and nine types of LDPE/CPE blends, with different content of components at 10 wt.% intervals, were examined. Data on the influence of blend composition on the tensile stress-strain diagram, elastic modulus, yield stress, breaking stress, and ultimate elongation are obtained. The results of investigations of creep are also reported. It is found that the creep compliance (the total current compliance minus the elastic compliance) obeys the power law of creep.

Dielectric behavior of aged polyethylene under UV radiation

The present paper reports the results of a study of dielectric aging of low-density polyethylene (LDPE) by UV radiation. The extruded polymer contained carbon black particles, approximately 3% in weight. The aging studies were made for polymer without and with carbon black. LDPE flat specimens were aged under UV radiation for up to 300 h, resorting to dielectric spectroscopy measurements in the frequency range from 10/sup -5/ Hz to 10/sup 5/ Hz. The dielectric susceptibility behaviors indicate the presence of relaxation peaks obeying the asymptotic forms of Dissado and Hill susceptibility functions. As the aging progresses, quasi-DC and flat loss processes may be observed in the low frequency range, for the LDPE specimens without and with carbon black, respectively. In addition, dielectric spectroscopy results have been compared with other results obtained with additional techniques such as Fourier transform infrared spectroscopy and scanning electron microscopy.

Nonisothermal Melting and Crystallization of Foamed Crosslinked Polyethylene

Foamed crosslinked specimens of low density polyethylene are produced by hot mould injection moulding. Dicumylperoxide, as crosslinking agent and azodicarbonamide as blowing agent, were used. The three components are blended and granulated in advance in an extruder-granulator at temperatures of 100-125°C. The contents of blowing and crosslinking agents in the composites studied are 5-15 and 0.08-0.6, respectively, in percents by weight. The ability for maximum foaming was studied by determining the overall density of the specimens. The crosslinking degree was calculated from the equilibrium strain value defined at temperature 140°C and pressure stress 0.045 MPa. The same parameter was estimated also by the melt flow index. The LDPE morphological parameters were determined on the basis of DSC-graphics obtained by a Perkin-Elmer DSC-7 differential scanning calorimeter at heating and cooling rates of 10°C/min. The activation energy for transport through phase boundary crystal melt, the surface energy of the crystal, the long period and the degree of the crystallinity were calculated by computer programs using formulae of non-isothermal melting and crystallization kinetics theory. It is found that the overall density of the specimens increases with the increase of the crosslinking agent concentration in the range of 0.08-0.6% by wt. At low concentrations of dicumylperoxide, the increase of the blowing agent concentration results in a decrease of the density, while at high crosslinking agent concentration the opposite trend is observed. The crosslinking degree of the foamed polyethylene increases with the increase of the concentrations of dicumylperoxide and azodicarbonamide, and the flowability of the foamed polyethylene melt decreases correspondingly. The crosslinking of the foamed LDPE affects the non-isothermal phase transitions. With the increase of the crosslinking degree, the phase transition temperatures and the degree of crystallinity decrease. The values obtained for the energies of activation for transport through the phase boundary, the surface energy of the crystal and the long period are comparable with the values obtained by other authors.

Influence of radiation environments on space charge formation in γ-irradiated LDPE

In the present paper the formation of space charge in irradiated low-density polyethylene (LDPE) was investigated using the pulsed electroacoustic (PEA) technique. Specimens of LDPE, /spl sim/100 /spl mu/m thick, were irradiated to various doses at room temperature in a /sup 60/Co /spl gamma/ source in one of three different environments: ambient room air, oxygen-free nitrogen and vacuum. The results obtained indicate that the space charge distribution is dependent on the irradiation environment. There is a large concentration of positive charge evident in the sample irradiated in air compared to very little for material irradiated in either vacuum or nitrogen under similar conditions. This indicates that the presence of oxygen during irradiation, and thereby oxidation itself, has a major influence on the mechanism by which space charge evolves. Thus the monitoring of space charge may in itself be an indicator of the degree of oxidation and of aging in a material. The evolution of space charge and its decay are discussed.

Space charge formation in γ-irradiated low density polyethylene

Energetic ionizing radiation can alter the chemical structure of polymeric materials and also may give rise to the presence of trapped charge within the material, the trapping characteristics of which may be influenced by these radiation-induced structural alterations. In the present work, the formation of space charge in /spl gamma/-irradiated LDPE (low-density polyethylene) was investigated using the technique of a LIPP (laser induced pressure pulse). Specimens of LDPE, /spl sim/350 /spl mu/m thick, were irradiated in a /sup 60/Co /spl gamma/-source in room air to various doses and the results indicate that space charge distributions are dependent on both dose and electric stress, including the length of time during which the stress is applied. At low doses (/spl les/10 kGy), there is a large amount of positive charge adjacent to the cathode, leading to stress enhancement at the interface. Higher applied stress serves to extend the positive charge towards the anode. At high doses (/spl ges/50 kGy), the charge distribution is more complicated than that for low doses. Here there is initially negative charge present adjacent to the cathode and a substantial positive charge adjacent to the anode. With the passage of time, however, there is a polarity change from negative to positive at the cathode and a decrease in the density of the positive charge at the anode, with negative charge in the middle of the sample.

Space charge behavior in XLPE cable insulation under 0.2-1.2 MV/cm dc fields

This report deals with space charge behavior in PE (polyethylene) under dc fields. Direct observation of time-dependent space charge profiles in 3-mm thick XLPE (crosslinked low-density polyethylene) cable insulation under dc electric fields was performed using the pulsed electroacoustic method. Stable hetero charges were formed when the field was as low as 0.2 MV/cm, and intermittent generation of packet shaped space charges and their propagation through the insulation were observed when the field was as high as 0.7 MV/cm. These phenomena were reproduced in sheet specimens of XLPE and LDPE (low-density polyethylene). It was found that hetero charges resulted from heat treatment of the XLPE specimen containing antioxidant and acetophenone, which is one of the crosslinking by-products, suggesting dissociation of the antioxidant through solvation at high temperature by acetophenone. The packet charges were easily detected when acetophenone was diffused into the LDPE specimen. However, uniformity of acetophenone distribution prevented the packet charge generation. It is suggested on the basis of several experimental results that local ionization of impurities in the insulation through solvation by acetophenone takes place assisted by high field and leads to the packet charge generation. A numerical simulation was carried out based on the above model.

Thermal Decomposition Process of a Polyethylene Pellet in a Hot Stagnation Flow

A low density polyethylene specimen fixed at the stagnation plate of high temperature air jet was measured on the time histories of thermal decomposition rate and temperature, and it was verified that the convective heat flux at the surface of specimen controlled the thermal decomposition rate. Moreover, from another series of experiment using a collum shape specimen whose cud surface was always fixed at the stagnation plate and exposed to the hot jet, the temperature distribution inside the polyethylene specimen was obtained and also the thickness of melting layer was estimated from the distribution.

Oxidative degradation of polymers under load. Ozone-oxygen action on oriented polyethylene

A study has been made of the influence of external loads on the rate of ozone oxidation of oriented high- and low-density polyethylene specimens, under regimes of constant stress and strain. It is shown that tensile stress has an accelerating effect on the kinetics of H atom abstraction. The higher sensitivity to loading of low density PE specimens compared with high density specimens is noted, and a relationship between loading sensitivity and MWD is observed for PE.

A method for the identification of crack-front location during fatigue fracture in polymers

An attempt has been made to accurately locate the crack-front position at intervals during fatigue cycling of low density polyethylene specimens by means of markers. Scanning electron microscopy has been used to inspect the fracture surfaces and the markers have been identified by means of the X-ray image produced by the signal from a fully focusing crystal spectrometer.

Influence of high injection pressures on the internal stress level in injection moulded specimens

Internal stresses in high and low density polyethylene specimens, injection moulded at pressures ranging from 100 MPa to 450 MPa have been measured by a stress relaxation method. The internal stress parameter ( σ i ), which is an average value of the internal stress distribution in the samples, changes from a negative value (compressive stresses) at normal injection pressures to a small positive value (frozen-in tensile stresses) at the highest pressures used. The yield stress increases in approximately the same way with the pressure, while the mould shrinkage in the flow direction decreases. It is suggested that the decrease in the absolute value of the internal stress parameter originates from an increase of the melting temperature with pressure, resulting in a more homogeneous solidification.