Cross-linked Low-density Polyethylene Research Articles

Preparation and properties of silane cross-linked polyethylene nanocomposite foams: The effect of silane and nanoclay type and content

In this study, silane grafted and moisture cross-linked low density polyethylene nanocomposite foams were prepared by melt mixing and batch foaming process and the effects of silane content, nanoclay content, foaming agent concentration and clay type (clay modified with vinyltriethoxysilane (VTES) and amino-propyltriethoxysilane on foaming efficiency and foam properties were investigated. The morphology and the efficiency of silane modification of modified clay were characterized by series of tests, including X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and thermogravimetric analysis (TGA). The results of Fourier transform infrared and thermogravimetric analysis showed that the silane modifiers were bonded to the surface of nanoclay through both physical and chemical bonds. According to XRD, these modifiers caused characteristic peak of clay to shift to lower angles, and make an increase in basal spacing. Gel content, density, scanning electron microscopy, compression, creep and rheological tests were used to determine the effect of different parameters on properties of cross-linked polyethylene foams. Results indicated that silane content has the significant effect on stress-strain behavior in compression mode. Compression set and creep deformation were affected by cell size and gel content. Furthermore, the rheological properties including complex viscosity, storage and loss modulus of unformed samples increased with the addition of silane and nanoclay content.

The effect of peroxide cross-linking on the thermal conductivity and crystallinity of low-density polyethylene

In this work, the thermal conductivity of a series of low-density polyethylene (LDPE) with different degrees of cross-linking was measured by the transient hot-wire method at different temperatures. It was found that the thermal conductivity of LDPE samples with different degrees of cross-linking decreased smoothly with increasing temperature at temperatures ranging from ‐20 to 80°C. The higher the degree of cross-linking, the smaller the negative temperature dependence of the cross-linked LDPE samples. The decrease of negative temperature dependence of LDPE was explained by the thermal vibration of molecular chains. Meanwhile, the crystallinity and melting point of a series of LDPE samples with different degrees of cross-linking were determined by differential scanning calorimetry (DSC), and the crystallization process of LDPE samples with different degrees of cross-linking was investigated using the Jeziorny method with a modification of the Avrami equation. The results show that the thermal conductivity of cross-linked LDPE is directly proportional to the crystallinity and inversely proportional to the gel content. This work provides a basis for the future design of semi-crystalline polymers with no temperature dependence of thermal conductivity.

Double-Function Oxygen Scavenger and Aromatic Food Packaging Films Based on LDPE/Polybutadiene and Peanut Aroma.

The aim of this study was to develop a double function active packaging material for nuts. The packaging solution, on the one hand, integrated polybutadiene (PB) as an oxygen scavenger and, on the other hand, it incorporated peanut aroma (PA) to improve customer’s sensorial experience. Different formulations based on low density polyethylene (LDPE), commercial PA (5 wt %) and PB at two levels (5 wt % and 13 wt %) were obtained by cast film extrusion. The obtained films were compared in terms of their mechanical, structural, optical and thermal properties confirming a plasticizing effect of PA and PB resulting in an increase in the ductility of the polymer and in a slight decrease in the thermal properties, maintaining their transparency. Regarding the oxygen capacity of the films, values of 4.4 mL and 2.7 mL O2 g−1 film were obtained for PE/PA/PB13 and PE/PA/PB5, respectively, after 6 days proving the suitability of the UV irradiation treatment in improving the oxygen absorption capacity of PB without the need of a metal catalyst. The aroma retention capacity into the polymer matrix was also evaluated in the developed formulations. The incorporation of PB in 13 wt % into a LDPE matrix improved the PA retention. This behavior was attributed to the ability of PB in enhancing cross-linking of LDPE as the concentration of PB increases. The results suggested the potential of PE/PB/PB13 films as oxygen scavenger and aromatic food packaging system to offer protection against lipid oxidation in nuts.

Nanocomposites and polyethylene blends: two potentially synergistic strategies for HVDC insulation materials with ultra-low electrical conductivity

Among the various requirements that high voltage direct current (HVDC) insulation materials need to satisfy, sufficiently low electrical conductivity is one of the most important. The leading commercial HVDC insulation material is currently an exceptionally clean cross-linked low-density polyethylene (XLPE). Previous studies have reported that the DC-conductivity of low-density polyethylene (LDPE) can be markedly reduced either by including a fraction of high-density polyethylene (HDPE) or by adding a small amount of a well dispersed, semiconducting nanofiller such as Al2O3 coated with a silane. This study demonstrates that by combining these two strategies a synergistic effect can be achieved, resulting in an insulation material with an ultra-low electrical conductivity. The addition of both HDPE and C8–Al2O3 nanoparticles to LDPE resulted in ultra-insulating nanocomposites with a conductivity around 500 times lower than of the neat LDPE at an electric field of 32 kV/mm and 60–90 °C. The new nanocomposite is thus a promising material regarding the electrical conductivity and it can be further optimized since the polyethylene blend and the nanoparticles can be improved independently.

Controlling interfacial interactions in LDPE/flax fibre biocomposites by a combined chemical and radiation-induced grafting approach

By combining chemical and gamma radiation treatments, we show that it is possible to improve the interfacial adhesion between natural fibres and a non-polar and non-reactive matrix such as low density polyethylene (LDPE), and the resulting mechanical performances of their biocomposites. To this aim, flax fibres are first functionalized with octadecylphosphonic acid (ODPA) and the quantity, effective grafting and localization of ODPA molecules on flax fibres are characterized by ICP-AES, solid state 31P NMR and SEM-EDX. ODPA molecules are effectively grafted onto flax fibres at a content of roughly 2.5 wt% and mostly localized at the fibre surface. Moreover, contact angle measurements as well as water sorption kinetics give evidence for increased hydrophobic and oleophilic character of ODPA treated fibres. Pellets of LDPE/flax fibre biocomposites obtained by melt extrusion are then submitted to gamma radiation and processed by injection moulding. Based on Soxhlet extraction experiments and SEM observations, it is shown that LDPE cross-linking rate induced by gamma radiation is locally enhanced at the fibre/matrix interface, suggesting that regio-selective cross-linking occurred between LDPE chains and the alkyl chain of ODPA grafted on flax fibre surface. Consequently, the uniaxial tensile performances of the biocomposites are enhanced by this combined chemical and radiation-induced grafting approach, especially their ultimate properties (up to 40% increase in the work of rupture). These results evidence an enhanced interfacial adhesion that is also supported by more cohesive interfacial failure and higher work of rupture through in-situ micro-mechanical tensile SEM experiments.

Polytechs focuses on polyolefin masterbatches

Robust organic thin-film transistors (OTFTs) with high temperature stability allow device integration with mass production methods like thermoforming and injection molding, and enable operation in extreme environment applications. Herein we elaborate a series of materials to make suitable gate dielectric and active semiconductor layers for high temperature stable OTFTs. We employ an anodized aluminum oxide layer passivated with cross-linked low-density polyethylene (LD-PE) to form a temperature-stable gate capacitor. As the semiconductor, we use quinacridone, an industrial organic colorant pigment produced on a mass scale. Evaporated MoOx/Ag source and drain electrodes complete the devices. Here we evaluate the performance of the OTFTs heating them in air from 100 °C in 25 °C increments up to 225 °C, holding each temperature for a period of 30 minutes. We find large differences in stability between quinacridone and its dimethylated derivative, with the former showing the best performance with only a factor of 2 decline in mobility after heating at 225 °C, and unaffected on/off ratio and threshold voltage. The approach presented here shows how industriallys calable fabrication of thermally robust OTFTs can be rationalized.

Air Plasma-Activated Crosslinking of Low-Density Polyethylene During Melt Mixing

In this work, air plasma surface treatment followed by oxidation in an atmospheric environment was used to generate activated low-density polyethylene (LDPE) with oxygen-containing functional groups and peroxide radicals. The resulting samples were then studied by using attenuated total internal reflectance–Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, and the 2,2-diphenyl-1-picrylhydrazyl method. Peroxide radicals are generally considered active substances which can initiate the crosslinking reaction. Melt mixing of the surface-treated LDPE allowed activated polymer chains with peroxide radicals to initiate the crosslinking reaction, and the maximum gel fraction obtained was 4.1%. The rheological behaviors, including viscosity, storage and loss moduli, loss tangent, and Cole–Cole plots, of the slightly crosslinked LDPE were studied, and the results of tensile experiments revealed that the formation of slightly crosslinked structures can improve yield and fracture stresses without sacrificing the breakage strain.

Enhanced through-plane thermal conductive properties of Ag/rGO nanocomposite

ABSTRACTHigh thermal conductivity of nanocomposite-based polymer matrix is one of the most important keys in developing many heat exchanger instruments. Here, we report a novel nanocomposite system based on silver-coated reduced graphene oxide (Ag/rGO) in silane cross-linked low-density polyethylene (XLPE) matrix with unprecedented through-plane thermal conductivity. Compared to the virgin rGO, Ag/rGO nanocomposite showed 67% higher thermal conductivity due to the Ag nanoparticles (NPs) decoration. The Ag NPs within the nanocomposites are believed to act as a thermal conductor among rGO nanosheets and eventually enhance the heat conduction in 3D manner.

High temperature-stability of organic thin-film transistors based on quinacridone pigments

Abstract Robust organic thin-film transistors (OTFTs) with high temperature stability allow device integration with mass production methods like thermoforming and injection molding, and enable operation in extreme environment applications. Herein we elaborate a series of materials to make suitable gate dielectric and active semiconductor layers for high temperature stable OTFTs. We employ an anodized aluminum oxide layer passivated with cross-linked low-density polyethylene (LD-PE) to form a temperature-stable gate capacitor. As the semiconductor, we use quinacridone, an industrial organic colorant pigment produced on a mass scale. Evaporated MoOx/Ag source and drain electrodes complete the devices. Here we evaluate the performance of the OTFTs heating them in air from 100 °C in 25 °C increments up to 225 °C, holding each temperature for a period of 30 minutes. We find large differences in stability between quinacridone and its dimethylated derivative, with the former showing the best performance with only a factor of 2 decline in mobility after heating at 225 °C, and unaffected on/off ratio and threshold voltage. The approach presented here shows how industriallys calable fabrication of thermally robust OTFTs can be rationalized.

Multi-scale tomographic analysis of polymeric foams: A detailed study of the cellular structure

This manuscript presents a detailed characterization of the cellular structure of two types of polymeric foams by non-destructive multi-scale X-ray computed micro-tomography. This comparative study is conducted in two different polymeric materials –rigid polyurethane foams and cross-linked low density polyethylene foams-. Based on this technique and using 3D image analysis, conventional descriptors of the foams gaseous phase (cell size distribution, mean cell size and anisotropy ratio) are characterized at a standard voxel size of 5 µm. Complementarily, 0.4 µm voxel size synchrotron X-ray tomography data sets have been used to characterize, using a new image analysis approach, features of the solid phase of the foams (fraction of mass in the struts and thickness of the different entities of the solid architecture: struts and walls). The presented methodology, based on multi-scale tomographic analysis, allows obtaining a better understanding of the connection between foaming process and cellular structure and is important to gain knowledge on the structure-properties relationships for these materials.

Photocrosslinker technology: An antimicrobial efficacy of cinnamaldehyde cross-linked low-density polyethylene (Cin-C-LDPE) as a novel food wrapper

In recent years much attention has been devoted to active packaging technologies that offer new opportunities for the food industry and food preservation. The spoilage of food products during post process handling leads to food contamination and causes life-threatening food-borne illness. The methicillin-resistant Staphylococcus aureus (MRSA) 090 is one of the food-borne pathogens associated with food poisoning that leads to an outbreak of perilous human infections worldwide. The development of resistance in bacteria and diffusion of coated synthetic preservatives into food are the major problem in food packaging industries. In the present work, we have developed a new food wrap method by cross-linking cinnamaldehyde on low-density polyethylene (Cin-C-LDPE) using novel photocrosslinker technology. The cinnamaldehyde showed potent antimicrobial activity (145mg/mL of MIC) against MRSA 090 and radical scavenging activity (RSA). The cinnamaldehyde was successfully cross-linked to LDPE and exhibited excellent antibiofilm properties against MRSA090 compared to bare LDPE. This positive interaction of developed Cin-C-LDPE against MRSA090 biofilm was confirmed by SEM and FT-IR studies, and results showed the damaged cell membrane architecture, inturn abridged adherence of MRSA090. The Cin-C-LDPE wrapped chicken, mutton, cheese, and grapes showed 2.5±0.15 log MRSA 090 reduction at the end of 10th day compared to the bare LDPE wrapped food samples. This clearly concludes that for the first time we have developed a novel Cin-C-LDPE food wrap technology effectively involved in biocidal activity against MRSA090. Applying this new strategy to develop food wrap containing nontoxic natural antimicrobial to target cell membrane components is the upcoming challenging and promising research gap remains in the food packaging industry for the future world.

Influence of strain rate and temperature on the mechanical behaviour of rubber-modified polypropylene and cross-linked polyethylene

In the present work, we investigate the effects of strain rate (e˙=0.01 s−1, 0.1 s−1, and 1.0 s−1) and low temperature (T=−30 °C, −15 °C, 0 °C, and 25 °C) on the mechanical behaviour in tension and compression of two materials: a rubber-modified polypropylene copolymer (PP) and a cross-linked low-density polyethylene (XLPE). Local stress-strain data for large deformations are obtained using digital image correlation (DIC) in the uniaxial tension tests and point tracking in the compression tests. Since both materials exhibit slight transverse anisotropy, two digital cameras are used to capture the strains on two perpendicular surfaces. Self-heating resulting from the elevated strain rates is monitored using an infrared (IR) camera. To enable the application of multiple digital cameras and an IR camera, a purpose-built transparent polycarbonate temperature chamber is used to create a cold environment for the tests. The mechanical behaviour of both materials, including the true stress-strain response and the volume change, is shown to be dependent on the temperature and strain rate. The dependence of the yield stress on the temperature and strain rate follows the Ree-Eyring flow theory for both materials, whereas Young’s modulus increases with decreasing temperature for PP and XLPE and with increasing strain rate for XLPE. Furthermore, a scanning electron microscope (SEM) study was performed on both materials to get a qualitative understanding of the volumetric strains.

Highly Insulating Polyethylene Blends for High-Voltage Direct-Current Power Cables

The insulation of state-of-the-art extruded high-voltage direct-current (HVDC) power cables is composed of cross-linked low-density polyethylene. Driven by the search for sustainable energy solutions, concepts that improve the ability to withstand high electrical fields and, ultimately, the power transmission efficiency are in high demand. The performance of a HVDC insulation material is limited by its residual electrical conductivity. Here, we demonstrate that the addition of small amounts of high-density polyethylene (HDPE) to a low-density polyethylene (LDPE) resin results in a drastic reduction in DC conductivity. An HDPE content as low as 1 wt % is found to introduce a small population of thicker crystalline lamellae, which are finely distributed throughout the material. The change in nanostructure correlates with a decrease in DC conductivity by approximately 1 order of magnitude to about 10-15 S m-1 at high electric fields of 30 and 40 kV mm-1 and elevated temperature of 70 °C. This work opens up an alternative design concept for the insulation of HVDC power cables.

Experimental set-up for determination of the large-strain tensile behaviour of polymers at low temperatures

In this study, we present a method to determine the large-strain tensile behaviour of polymers at low temperatures using a purpose-built temperature chamber made of polycarbonate (PC). This chamber allows for several cameras during testing. In our case, two digital cameras were utilized to monitor the two perpendicular surfaces of the test sample. Subsequently, the pictures were analysed with digital image correlation (DIC) software to determine the strain field on the surface of the specimen. In addition, a thermal camera was used to monitor self-heating during loading. It is demonstrated that the PC chamber does not influence the stress-strain curve as determined by DIC. Applying this set-up, a semi-crystalline cross-linked low-density polyethylene (XLPE) under quasi-static tensile loading has been successfully analysed using DIC at four different temperatures (25 °C, 0 °C, −15 °C, −30 °C). At the lower temperatures, the conventional method of applying a spray-paint speckle failed due to embrittlement and cracking of the spray-paint speckle when the tensile specimen deformed. An alternative method was developed utilising white grease with a black powder added as contrast. The results show a strong increase in both the Young’s modulus and the flow stress for decreasing temperatures within the experimental range. We also observe that although the XLPE material is practically incompressible at room temperature, the volumetric strains reach a value of about 0.1 at the lower temperatures.

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.

Thioxanthone derivatives as stabilizers against electrical breakdown in cross-linked polyethylene for high voltage cable applications

In the search of voltage stabilizers for high voltage underground cables the chemical synthesis and electrical tree inhibiting effect of a series of thioxanthone derivatives in cross-linked low-density polyethylene (XLPE) is reported. The strongest increase in electrical tree initiation field under high-voltage alternating current (HVAC) conditions was 55% compared to reference XLPE after the addition of 0.3 wt% 9-oxo-9H-thioxanthen-2-yl methacrylate. Thermal analysis, small angle X-ray scattering and gel content measurements showed that the addition of the stabilizers did not significantly influence the microstructure and gel fraction of XLPE. A comparison between the stabilizing effects of the thioxanthone derivatives and previously reported photophysical properties revealed that a short lifetime of the triplet excited state can be related to a good voltage-stabilizing effect.

Improving viscoelasticity and rebound resilience of crosslinked low-density polyethylene foam by blending with ethylene vinyl acetate and polyethylene-octene elastomer

To obtain high-rebound resilience of crosslinking low-density polyethylene (LDPE) foam and decrease the foam density at the same content of foaming agent, the melt viscoelasticity of LDPE with different compositions (ethylene vinyl acetate [EVA], polyethylene-octene elastomer, and crosslinking agent) was investigated by dynamic rheology test. Then, LDPE/EVA/(polyethylene-octene elastomer) foams with different composition ratios were produced by a continuous foaming process and investigated by the rebound resilience test. The results show that the melt viscoelasticity behavior of LDPE and its blends in the molten state possessed more melt elasticity behavior after the crosslinking was introduced. Meanwhile, the rebound resilience of LDPE foam was increased 54% at the lower foam density (0.031 g/cm3). It could meet the requirements of sports mats for high-rebound resilience (>50%) and decrease the material cost when EVA was introduced into the foaming system. J. VINYL ADDIT. TECHNOL., 22:61–71, 2016. © 2014 Society of Plastics Engineers

An electron paramagnetic resonance study of LDPE irradiated with high-energy electron beam

Cross-linking of the polyethylene was performed with a high-energy electron beam. Electron paramagnetic resonance spectroscopy was used to study the lifetime of unpaired electron in the irradiated samples. Time-dependent electrical parameters are investigated for the cross-linked low-density polyethylene. Both dielectric constant and dielectric strength almost remained unchanged, but for short times, the volume resistivity and loss factor increased and decreased, respectively. It is predicted that for lifetime longer than 48 h, the electrical parameters were constant. It is believed that the variation of some electrical properties during time is due to the effects of trapped electrons.

Crosslinking of low density polyethylene with octavinyl polyhedral oligomeric silsesquioxane as the crosslinker

We report a new strategy for the crosslinking of LDPE using octavinyl polyhedral oligomeric silsesquioxane as the crosslinker, which dramatically decreases the amount of dicumyl peroxide and thus avoids the chain scission, scorch, and production of small pores in the conventional peroxide crosslinking strategy.

Elongational rheology and cohesive fracture of photo-oxidated LDPE

It was found recently that low-density polyethylene (LDPE) samples with different degrees of photo-oxidation represent an interesting system to study the transition from ductile to cohesive fracture and the aspects of the cohesive rupture in elongational flow. Sheets of LDPE were subjected to photo-oxidation in the presence of air using a xenon lamp to irradiate the samples for times between 1 day and 6 weeks. Characterisation methods included Fourier transform infrared spectroscopy, solvent extraction method, and rheology in shear and uniaxial extensional flows. Linear viscoelasticity was increasingly affected by increasing photo-oxidation due to crosslinking of LDPE, as corroborated by the carbonyl index, acid and aldehydes groups, and gel fraction. The molecular stress function model was used to quantify the experimental data, and the nonlinear model parameter β was found to be correlated with the gel content. The uniaxial data showed that the transition from ductile to cohesive fracture was shifted to lower elongational rates, the higher the gel content was. From 2 weeks photo-oxidation onwards, cohesive rupture occurred at every strain rate investigated. The true strain and true stress at cohesive fracture as well as the energy density applied to the sample up to fracture were analyzed. At low gel content, rupture was mainly determined by the melt fraction while at high gel content, rupture occurred predominantly in the gel structure. The strain at break was found to be independent of strain rate, contrary to the stress at break and the energy density. Thus, the true strain and not the stress at break or the energy density was found to be the relevant physical quantity to describe cohesive fracture behavior of photo-oxidated LDPE. The equilibrium modulus of the gel structures was correlated with the true strain at rupture. The stiffer the gel structure, the lower was the deformation tolerated before the sample breaks.