Structure Of Low Density Polyethylene Research Articles

Effect of phenol-formaldehyde resin on mechanical durability and structure of low-density polyethylene

Relevance. The development of technology for producing new polymer modifications with specific properties, which remain stable even when exposed to external factors, is a key area of focus for researchers in the field of high-molecular compounds. Purpose. The purpose of this study was to create new composite materials based on low-density polyethylene. Methodology. The extrusion blowing method on an industrial URP 1500 unit was used for processing low-density polyethylene (LDPE) and its modified films. Electron microscopic images of the surface were captured using a S-570 scanning microscope (Japan) at X1000 magnifications. Results. The composition and quantity of a low-molecular organic additive that alters the electrical characteristics of low-density polyethylene grade 10803-020 was ascertained. The modified low-density polyethylene is noteworthy for its minimal usage of additives and their compatibility with technology. Based on experimental data, it was found that the inclusion of 0.05 wt% phenol-formaldehyde resin in low-density polyethylene increases its mechanical strength to the highest level when compared to both the unaltered low-density polyethylene and low-density polyethylene with other additives. Furthermore, the study found that the addition of phenol-formaldehyde resin in small proportions (0.05 wt%) substantially enhances mechanical strength at varying temperatures. Conclusions. The electrophysical characteristics of low-density polyethylene and its modified versions were comprehensively investigated. The adequately altered low-density polyethylene exhibits considerably improved mechanical durability. It was indicated that the additives used, at optimal levels, influence the physical framework of low-density polyethylene, highlighting their technological compatibility differences

Simulations mapping the influence of oxygen, extruder residence time, and mechanical shear on low-density polyethylene structure during recycling

We present a computational study of the development of structural properties for recycled low-density polyethylene (LDPE). First, we apply the population balance method together with the method of moments to study the mutual influences of scission and crosslinking reactions. Then, we study the combined effects of extruder residence time and oxygen concentration as the initiator of thermo-mechanical degradation in the recycling process of LDPE in a twin-screw extruder. Further, we extend the models to study the influence of mechanical scission. We explore various case study scenarios to incorporate the effect of extruder residence time and mechanical shear. It is concluded that the competition between random scission and crosslinking, as well as the presence of oxygen, play important roles in the degradation of LDPE during recycling. Random scission is a key factor in terms of altering molecular weight distribution (MWD) during a recycling extrusion process. The various explored scenarios with mechanical scission demonstrate the sensitivity of molecular structure to the combination of competing mechanisms during recycling.

Graphene impact of the LDPE characteristics

Graphene addition to low-density polyethylene prolonged oxygen penetration in low-density polyethylene (LDPE), deferred embrittlement effect of polymeric compound, developed storage modulus, electrical conductivity and enhanced viscosity of LDPE nanocompounds. The presence of graphenes inhibited movement of polymer chains, which affected increasing toughness and capability of LDPE compounds. Continuity of carbon–carbon connection threshold of graphene compound took place with about 0.5 wt% graphene inclusion in LDPE structure. The impenetrability of oxygen over the surface of LDPE compounds achieved with 0.5 wt% graphene inclusion, which made severe perfections and decreased 37% fuel penetration if it is compared to pristine LDPE.

Reversible crosslinked low density polyethylenes: structure and thermal properties

In the present study, low density polyethylene (LDPE) has been crosslinked at 170 °C with three different systems by a) using peroxide, b) peroxide and accelerator and c), peroxide, accelerator and sulfur. The effect of chemical crosslinking on LDPE structure has been investigated using torque measurements, Fourier transform infrared spectroscopy (FTIR), wide angle X-ray diffraction (WAXS), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Therefore, effects of each crosslinking system on the structural and thermal properties of the material in terms of crystallinity, thermal transitions and stability have been discussed. The reversible crosslinking of LDPE allow the recyclability of polyolefins, increasing the thermal properties.

Synthesis and characterization of low density polyethylene with multiferroic bismuth ferrite nanocomposite

A novel low density polyethylene (LDPE) with multiferroic bismuth ferrite (BiFeO3) nano-dielectric was successfully prepared via a solution-melt blending method. The structure, thermal, magnetic, as well as dielectric properties of the nano-dielectric were examined. The microstructures of LDPE/BiFeO3 were characterized by Fourier transform infrared, X-ray diffraction, and scanning electron microscopy. Microstructural analysis indicated that BiFeO3 nanoparticles were well dispersed in the LDPE matrix and the structure of LDPE was unchanged even after doping with BiFeO3 nanoparticles. The differential scanning calorimetry data of the thermal properties revealed that melting temperature has decreased and crystallinity has increased. The reason was that amount of nucleation agents was increased and crystal propagation rate was changed. The corresponding magnetic and dielectric properties were measured using vibrating sample magnetometer and broadband dielectric spectrometer. The results showed that the magnetization behaviour of diamagnetic LDPE was transformed due to the antiferromagnetism of BiFeO3. In addition, the dielectric permittivity and loss of LDPE/BiFeO3 increased with increasing BiFeO3 filler concentrations in the LDPE matrix due to different response mechanisms.

Bionanocomposites with enhanced antimicrobial activity and photodegradability based on low density polyethylene and nano TiO2/organoclay

AbstractBionanocomposite materials with enhanced photodegradability and bactericidal activity, as well as improved gas barrier properties, were manufactured by incorporating silicate nanolayers into the structure of low density polyethylene (LDPE) filled with nano titanium dioxide (TiO2) via melt compounding. Effects of interfacial compatibilization upon developed microstructure were studied by incorporating maleated LDPE into the nanocomposites formulation. Field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and rheo-mechanical spectroscopy (RMS) techniques were conducted to characterize the microstructure of the nanocomposites. Interfacially compatibilized TiO2/organoclay (OC) based nanocomposites exhibited shorter induction time for the onset of photodegradation, and an acceptable inactivation of Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) microbe cells upon UV light irradiation, indicating enhanced photoactivity of the hybridized TiO2/OC nanosystem. TiO2/OC-based nanocomposites exhibited increased melt viscosity and pseudo solid like characteristics in melt linear viscoelastic behavior. Moreover, TiO2/OC-based nanocomposites presented improved barrier properties, which make such materials applicable for packaging applications.

Extrusion Foaming System for Fabricating Open-Cell Structure of Low Density Polyethylene (LDPE)

Low Density Polyethylene (LDPE) foam of open-cell type was manufactured through extrusion foaming system using a physical blowing agent for the application of sound absorption. For the purpose of enhancing the open cell content, salt powder was blended with LDPE changing the content and particle size. Scanning electron microscopy (SEM) analysis was carried out in order to observe the cell morphology. With a view to characterizing the open-cell structure, open-cell content and expansion ratio were measured with extruded foams. Finally the effect of formulation and processing parameters on cell morphology was investigated.

Sorption of PBDE in low‐density polyethylene film: Implications for bioavailability of BDE‐209

The coefficients of partitioning (K(pew) ) between low-density polyethylene (LDPE) film (50-µm thickness) and water for 23 polybrominated diphenyl ether (PBDE) congeners were determined based on a regression analysis of sorption kinetics over an extended exposure period (up to 365 d). A curvilinear relationship between log K(pew) and log K(OW) (octanol-water partition coefficient) was obtained for the target BDE congeners with the turning point at log K(OW) approximately 8. Previously obtained dietary uptake efficiencies of BDE congeners in common carp (Cyprinus carpio) were also found to relate curvilinearly to log K(OW) . In addition, field-measured relative abundances of BDE-209 compiled from previous investigations conducted in the Pearl River Delta of South China were significantly (p < 0.001) higher in abiotic samples (n = 79 from 11 matrices) than in biotic samples (n = 73 from 12 matrices), suggesting the likelihood for reduced bioavailability of BDE-209 in certain biota. Finally, a molecular-scale analysis indicated that the curvilinear relationship between log K(pew) and log K(OW) can be attributed to the energy barrier that a molecule has to overcome as it attempts to diffuse into the LDPE structure, which can become significant for larger molecules. Similarly, the reduced bioavailability of BDE-209 in many biological species can be regarded as a reflection of the magnitude of molecular interactions between cell membranes and BDE-209.

N2‐filled hollow glass beads as novel gas carriers for microcellular polyethylene

AbstractN2‐filled hollow glass beads (HGB) were first used as novel gas carriers to prepare microcellular polymers by compression molding. Dicumyl peroxide was acted as crosslink agent to control the produced microcellular structure of low density polyethylene (LDPE)/HGB. The effect of temperature, pressure and the content of gel on the embryo‐foaming, and final‐foaming structure are investigated. Scanning electronic microscopy shows that the average cell size of microcellular LDPE ranges from 0.1 to 10 μm, and the foam density is about 109–1011 cells/cm3. A clear correlation is established between preserving desirable micromorphologies of microcellular LDPE in different processing stage and tuning processing factors. The pertinent foaming mechanism of microcellular materials foamed with HGB is proposed. Because of the good mechanical strength, low density, weak water‐absorption, and excellent heat insulate ability, microcellular LDPE has great potential application in energy building materials. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Compressed ethylene phase states and their importance for the production of low density polyethylene

In the last three decades the authors have published papers on the concept of the supra-molecular organization and the phase state of compressed ethylene gas and their effects on the mechanism and kinetics of free-radical ethylene polymerization. The effects on the macromolecular structure of low density polyethylene (LDPE) were also explained. The importance of the phase state of compressed ethylene on the industrial process of LDPE production are presented in this paper: The start-up of polymerization, the peak in the polymerization rate curve, the stability of the reaction, the structure and properties of LDPE, the separation of unreacted ethylene and polyethylene and ethylene compression are discussed.

Positron annihilation studies of the effect of gamma irradiation dose in polymers

Positron annihilation lifetime (PAL) technique has been applied to study the effect of gamma irradiation dose on the structure of low-density polyethylene and polystyrene. The positron annihilation lifetime measurements were performed with a conventional fast–fast coincidence system. Gamma irradiation was carried out at room temperature with a 60Co gamma source up to a dose of 1000 kGy. The delayed lifetime spectra of irradiated and unirradiated samples were decomposed into three lifetime components. The lifetime parameters give indication of the free-volume size and concentration. It was found that the crosslinking efficiency in the polymer increases significantly with increasing irradiation dose. This appears as a decrease in the intensity of the long-lived positron lifetime components attributed to ortho-positronium formation. Measurements of the Doppler broadening energy spectra of the low-density polyethylene (LDPE) samples have also been performed using a variable mono-energetic positron beam. The trend of the Doppler broadening lineshape S-parameter as a function of the radiation dose supports the lifetime results.

Thermomechanical study of low-density polyethylene, polyamide 6 and its blends

By using thermomechanical analysis (TMA) multiple relaxation transitions in the amorphous part of semi-crystalline polymers and their blends can be found. These result from differences in the interaction energies between segments of macromolecules, and as a result, in molecular mobility. TMA shows essential changes in the structure of low-density polyethylene (LDPE) resulting from the grafting of a maleic anhydride (LDPE-g-MAH) onto this semi-crystalline polymer. The grafting process did not suppress the ability of polyethylene to crystallize. Essential changes were found in molecular weight distribution and relaxation transitions of the ternary blends LDPE/PA6 (polyamide 6, PA6)/LDPE-g-MAH studied. For a concentration of PA6 up to 30 wt.%, a single relaxation transition is visible, which testifies that the components are miscible in the amorphous region. For blends with 40 and 50 wt.% of PA6, this structure was transformed and two relaxation transitions are visible. From the results obtained in this study it is concluded that the introduction 5 wt.% of grafted polyethylene is sufficient to produce a PA6/LDPE blend only for PA6 concentration up to 30 wt.% which is homogenous on “molecular” level. The transformation of the structure of the ternary polymer blend was explained by the interaction of the components during the melt mixing and changes in the structure of its amorphous regions.

Surface structure of low density polyethylene films grafted with acrylic acid using corona discharge

Chemical composition, morphology, and crystalline structure of low density polyethylene (LDPE) films surface grafted with acrylic acid (AA) using corona discharge were studied by attenuated total reflection infrared (ATR-IR), electron spectroscopy for chemical analysis (ESCA), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and wide angle X-ray diffraction (WAXD) techniques. The grafted film surface is covered with grafted chains. After grafting for 3.0 h in 20% aqueous solution of AA, the depth of the grafted layer is more than 10 nm. A grain structure was observed on the grafted surfaces which was probably caused by the isolated dispersion of active sites generated by corona discharge, and these active sites initiated the graft copolymerization. However, surfaces of grafted films were smoother than that of ungrafted ones. DSC curves of grafted films show a small peak at about 100°C due to vaporization of adsorbed water. The longer the graft copolymerization time, i.e. the higher the graft degree of AA on LDPE, the higher the amount of adsorbed water. The position of each peak in WAXD patterns, crystal axial length, crystal plane distance and crystal grain size remain almost unchanged during the graft copolymerization time of 2.0 h. However, when the graft copolymerization time reaches 3.0 h, twin peaks at about 21.4° and 22.0° are observed, indicating that a different crystal form is formed at longer copolymerization time, i.e. at a higher graft degree.

Characterization of gamma irradiated polyethylene films by DSC and X-ray diffraction techniques

The effect of gamma radiation on the thermal behavior and crystalline structure of low density polyethylene (LDPE) has been investigated using differential scanning calorimetry (DSC) and X-ray diffraction techniques (XRD). Gamma irradiation was carried out in atmospheric air to a maximum dose of 883 kGy. DSC results of the heating run from room temperature up to 150 °C showed that the melting temperature (Tm), and onset temperature of LDPE film decrease linearly with increasing irradiation dose. However, upon cooling LDPE film from 150 °C to room temperature, the crystallization temperature (Tc) and onset temperature were found to decrease non-linearly with increasing irradiation dose up to 500 kGy and then tended to level off. The change in heat of fusion (ΔHf) with irradiation dose was found to proceed with different behaviour, two stages, with a kink at irradiation dose of about 500 kGy, being observed. This suggests the occurrence of degradation and crosslinking at low and high doses, respectively. However, parameters of the X-ray diffraction pattern such as number of diffraction patterns, peak position (2θ) and width of the diffraction pattern, support the results of DSC measurements. © 2000 Society of Chemical Industry

Effect of isometric annealing above the melting point on the structure of low density polyethylene in its oriented blends with polypropylene

Oriented blends of compositions based on polypropylene with low density polyethylene dispersed in it and exhibiting various reactions between the components are studied by X-ray diffraction analysis and differential scanning calorimetry over a wide temperature range. Isometric heating of the system with increased interaction of the components (presence of chemical crosslinks) is accompanied by transition of some of the low density polyethylene chains into the mesomorphic state. When there is no such interaction, melting and isotropization of the dispersed component are observed in the composition.

Effect of molecular weight distribution and structure of LDPE on its deformation behavior in a high‐temperature region

AbstractThe effect of molecular weight distribution (MWD) (the low‐molecular weight “tail” in particular) on the deformation behavior of low‐density polyethylene (LDPE) during uniaxial stretching was studied over the temperature range from 293 K to the melting point. Four types of film samples were used: untreated polyethylene (PE), the same PE subjected either to solvent extraction or to thermal degradation, and PE with extracted fractions replaced by a low‐molecular weight paraffin analog. Graphs of elongation at break εb versus temperature always exhibited a minimum that occurred in the neighborhood of 370 K. The shape of the curve, the amount that εb decreases with increasing temperature, as well as the absolute values of εb strongly depend on the type of sample. The εb for a degraded PE is markedly lower than that for an untreated one. On the other hand, the extraction of low‐molecular weight fractions leads to a higher εb, especially in the premelting region. The low‐molecular weight “tail” has a different effect from that of added paraffin. This appears to be due to the plasticizing effect of the latter. The effect of temperature is much more pronounced on the work of deformation (Ad) than on εb. The results obtained were interpreted on the assumption that structural (phase) inhomogeneity of LDPE is the key factor determining the deformation behavior of polymer over the studied temperature range.

Synthesis conditions and structure of low density polyethylene. II. Average molar mass and molar mass distribution

AbstractThe first part of this publication dealt with the effect of synthesis conditions on short and long‐chain branching of low density polyethylene; this second part reports on the effect on the molar mass, the molar mass distribution and the polydispersity that can be determined from these parameters. The average molar mass was measured with a membrane osmometer and the molar mass distribution by gel permeation chromatography, using the same samples as those used for the branching analysis. These samples were produced in continuously operating stirred autoclaves under widely differing polymerization conditions.The number‐average molar mass falls as the polymerization temperature and the initiator concentration rises and the pressure decreases. When the polymerization temperature is high, the molar mass distribution is narrower and less asymmetrical. When the polymerization temperature is low, there are broader, more asymmetrical distributions with a larger higher molar mass fraction. The polydispersity calculated from the molar mass distribution falls accordingly as the polymerization temperature rises. The polyethylene samples produced in a squat stirred autoclave have a higher polydispersity than those produced in a narrow autoclave. These findings have been confirmed on commercial products produced in various types of reactors.

Synthesis conditions and structure of low density polyethylene. I. Short and long chain branching

AbstractIn order to further our knowledge about the effect of synthesis conditions on the structure and properties of low density polyethylene, a large number of polymer samples was prepared at temperatures between 100 and 300°C and at pressures from 1100 to 2000 bar. A continuously operating polymerization plant with 3 agitated autoclaves each of different geometry was available for the synthesis. The aim was to use various organic peroxides and oxygen in order to find out whether the properties of the polyethylene produced also depend on the initiator.An analysis of the polymers revealed that high temperatures promote the formation of both short and long side chains which are characteristic of low density polyethylene. High pressures reduce the amount of branching. The type of initiator was not found to have any effect but a change in reactor geometry affects the long chain branching because of the accompanying change in temperature distribution.

Elastic moduli and structure of low density polyethylene

Low density polyethylene film is drawn at room temperature four times the original length and subjected to thermal annealing at 60, 80, and 100 °C keeping the film length constant. Long spacing measured by SAXS increased with increasing temperature of annealing; the increase of the long spacing is presumed to be due to the decrease of the number of micelles through relaxation during the annealing. Simultaneous measurement of the changes of the long spacing and the film length by stretching is carried out and stress-extension curves are obtained. The values of the initial moduli of the long spacingE 1 and the film lengthY are very near to each other. Elastic modulus of the crystal latticeE c is known to be 235 GN/m2 and that of the amorphous regionE a is found to be 0.15 GN/m2. When higher stress is applied than in the case of the initial modulus, the percentage of extension of film is much greater than that of the long spacing. The discrepancy is explained by the increase of the number of micelles through stress crystallization.

A study of degradation of low density polyethylene under natural weathering conditions

The effects of natural weathering on mechanical properties and chemical structure of low density polyethylene have been studied. It is found that no simple correlation exists between the development of carbonyl groups and mechanical properties such as stress or strain at break or elastic modulus. It appears that there is recovery in the mechanical properties during the winter months.