High-density Polyethylene Samples Research Articles

Influence of the polymer structure and nanotube concentration on the conductivity and rheological properties of polyethylene/CNT composites

A series of multi-walled carbon nanotube /polyethylene (MWNT/PE) composites with several concentrations (0.5, 1, 2.5, 5, 7 wt%) of chemical vapour deposition (CVD)-grown carbon nanotubes (CNTs) have been investigated. High-density polyethylene (HDPE) and low-density polyethylene (LDPE) have been chosen as matrix. The nanocomposites were prepared by melt mixing; a good dispersion in the matrix and a good CNT–polymer interfacial adhesion have been verified by scanning electron microscopy (SEM). In Raman measurements the characteristic bands of the MWNTs are shifted to slightly higher wavenumbers when increasing the CNT content in the nanocomposite, indicating an effective interaction between MWNTs and polymer matrix. Melt rheological investigations in dynamic mode indicate the dispersion degree and the percolation state of the CNTs within the polymer matrix. The rheological percolation threshold of the nanocomposites is between 1 and 2.5 wt%. For HDPE/CNT as well as for LDPE/CNT composites, we found a six orders of magnitude increase in electrical conductivity from 1 to 2.5 wt%, that is the same percolation threshold as determined by rheology. Below percolation threshold we found reproducible diode-like behaviour with different conductivity in forward and reverse bias direction for HDPE sample.

Proton Spin Diffusion in Polyethylene as a Function of Magic-Angle Spinning Rate. A Phenomenological Approach

Starting from the phenomenological Bloembergen-Purcell-Pound equation a relation between magic-angle spinning (MAS) rate and spin diffusion is derived. The resulting model equation was fitted to observed spin diffusion versus MAS rate data obtained at 298 K on an high-density polyethylene sample, revealing a reduction in the effective spin diffusivity by (65 + 5)% when increasing the MAS rate from 2 to 12 kHz. The same model equation enabled the rigid-lattice diffusivity to be estimated and was found to be only slightly higher, by approximately 10%, compared to the spin diffusivity observed at the lowest MAS rate applied (2 kHz). Moreover, the model equation predicts a reduction in the effective spin diffusivity by more than 90% when increasing the MAS rate to more than 30 kHz.

Biological evaluation of partially stabilized zirconia added HA/HDPE composites with osteoblast and fibroblast cell lines

In the present study, the biocompatibility of partially stabilized zirconia (PSZ) added hydroxyapatite (HA)--high density polyethylene (HDPE) composites was evaluated by proliferation and cell attachment assays on two osteoblast cell lines (G-292, Saos-2) and a type of fibroblast cell isolated from bone tissue namely HBF in different time intervals. Cell-material interactions on the surface of the composites were observed by scanning electron microscopy (SEM). The effect of composites on the behavior of osteoblast and fibroblast cells was compared with those of HDPE and Tissue Culture Poly Styrene (TPS) (as negative control) samples. Results showed that the composite samples supported a higher proliferation rate of osteoblast cells in the presence of composite samples as compared to the HDPE and TPS samples after 3, 7 and 14 days of incubation period. It was showed that an equal or in some cases an even higher proliferation rate of G-292 and Saos-2 osteoblast cells on composite samples in compare to negative controls in culture period (P < 0.05). The number of adhered cells on the composite samples was equal and in some cases higher than the number adhered on the HDPE and TPS samples after the above mentioned incubation periods (P < 0.05). Adhered cells presented a normal morphology by SEM and many of the cells were seen to be undergoing cell division.

Changes in mechanical properties due to gamma irradiation of high-density polyethylene (HDPE)

This paper presents an experimental analysis of the effect of dose and dose rate parameters during gamma irradiation of high-density polyethylene (HDPE) samples. Considerations concerning the influence of these parameters on HDPE mechanical strength properties as a result of the predominance of oxidative degradation or of cross-linking are presented. The experimental results show an improvement of HDPE mechanical strength as dose increases, indicating the predominance of cross-linking over oxidative degradation and that lower doses are necessary to obtain a similar change in resistance parameters when radiation is applied at lower dose rates, showing that gamma radiation affects the HDPE in a more efficient way at lower dose rates.

Characterization of high molecular weight polyethylenes using high temperature asymmetrical flow field-flow fractionation with on-line infrared, light scattering, and viscometry detection

High temperature asymmetrical flow field-flow fractionation (HTAF4) coupled to infrared (IR), multi-angle light scattering (MALS), and viscometry (Visc) detection is introduced as a tool for the characterization of high molecular weight polyethylenes. The high molecular weight fraction strongly affects the rheological behaviour and processability of polyethylene materials and can often not be accurately resolved by current technology such as high temperature size-exclusion chromatography (HTSEC). Molecular weight ( M), radius of gyration (Rg), and intrinsic viscosity [ η] of linear high density polyethylene (HDPE) and branched low density polyethylene (LDPE) samples are studied in detail by HTAF4 and are compared to HTSEC. HTAF4 showed a better separation and mass recovery than HTSEC for very high molecular weight fractions in HDPE and LDPE samples. As no stationary phase is present in an HTAF4 channel, the technique does not show the typical drawbacks associated with HTSEC analysis of high molecular weight polyethylenes, such as, exclusion effects, shear degradation, and anomalous late elution of highly branched material. HTAF4 is applied to study the relation between the molecular weight and the zero shear viscosity η 0 for high molecular weight HDPE. It was found that the zero shear viscosity values predicted from HTAF4 results are in good qualitative agreement with measured values obtained from dynamic mechanical spectroscopy (DMS) experiments, whereas η 0 values predicted from HTSEC do not show a strong correlation. The low molecular weight cutoff of HTAF4 is approximately 5 × 10 4 as a result of relatively large pores in the HTAF4 channel membrane. HTAF4 is, therefore, currently not suited to analyze low molecular weight materials.

Influence of Molecular Conformation on the Constitutive Response of Polyethylene: A Comparison of HDPE, UHMWPE, and PEX

The current work presents the characterization and comparison of the mechanical response of three different industrial forms of polyethylene. Specifically, high-density polyethylene (HDPE), ultra high molecular weight polyethylene (UHMWPE), and cross-linked polyethylene (PEX) were tested in compression as a function of temperature (−75 to 100°C) and strain-rate (10−4 to 2,600 s−1). The responses of UHMWPE and PEX are very similar, whereas HDPE exhibits some differences. The HDPE samples display a significantly higher yield stress followed by a flat flow behavior. Conversely UHMWPE and PEX both exhibit significant strain hardening after yield. The temperature and strain-rate dependence are captured by simple linear and logarithmic fits over the full range of conditions investigated. The yield behavior is presented in terms of an empirical mapping function that is extended to analytically solve for the mapping constant. The power-law dependence on strain-rate observed in some polymers is explained using this mapping function.

Catalytic degradation of polyethylene: An evaluation of the effect of dealuminated Y zeolites using thermal analysis

Commercial samples of high density polyethylene were degraded over Y zeolites with different Si/Al: ultrastable Y zeolite (USY) and three dealuminated Y zeolites (HY(4), HY(12) and HY(20)). Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) was used aiming to evaluate the apparent activation energy of polymer cracking. The results obtained show that the efficiency of different zeolites was due to their acidity. A high Si/Al ratio gave a greater acidity to zeolites and the onset degradation temperature of the polymer and related activation energy decreased. The sample residues obtained after an isothermic TGA, were analysed by DSC and it was observed that the crystallinity decreases with the acidity of zeolites. HY(20) zeolite was the most active catalyst due to the isolation of the residual Brønsted acid sites which promote their strong acidity.

Volume Variation Process of High-Density Polyethylene During Tensile and Creep Tests

Samples of high-density polyethylene have been subjected to tensile tests and creep experiments by means of a video-controlled testing system (VideoTraction ©). The evolution of specific volume in this semi-crystalline polymer is determined versus true strain. In the elastic stage, we measure a hydrostatic expansion and then, in the plastic stage, we observe a competition between a compaction effect and a dilatation phenomenon. Although compaction is probably overestimated in the present testing technique, it represents a pertinent mechanism that is ascribed to the orientation of the amorphous chains during stretching. This phenomenon is characterized by X-ray diffraction measurements that show a reduction of average distance between amorphous chains. Dilatation process is explained by the diminution of crystallinity and by the formation, growth and coalescence of crazes inside and between spherulites. Electron microscopy reveals these defects. The competition between compaction and dilatation, controlled by the mobility of the amorphous phase, depends on temperature and time.

Structure, thermal stability, and photocrosslinking characterization of HDPE/LDH nanocomposites synthesized by melt‐intercalation

AbstractStructure, thermal properties, and influence of layered double hydroxide (LDH) fillers on photocrosslinking behavior of high‐density polyethylene (HDPE)/LDH nanocomposites have been studied in the present article. The X‐ray diffraction and transmission electron microscopy analysis demonstrate that the completely exfoliated HDPE/LDH nanocomposites can be obtained by controlling the organomodified LDH loading via melt‐intercalation. The data from the thermogravimetric analysis show that the HDPE/LDH nanocomposites have much higher thermal stability than HDPE sample. When the 50% weight loss was selected as a comparison point, the decomposition temperature of HDPE/LDH sample with 5 wt % LDH loading is ∼40 °C higher than that of HDPE sample. The effects of UV‐irradiation on the HDPE/LDH nanocomposites show that the photoinitiated crosslinking can destroy the completely exfoliated structure to form the partially exfoliated structure, which decreased the thermal stability of the nanocomposites. However, the thermal stability of photocrosslinked samples can increase with increasing the UV‐irradiation time. The effect of LDH loading on the gel content of UV‐irradiated nanocomposites shows that the LDH materials can greatly absorb the UV irradiation and thus decrease the crosslinking efficiency. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3165–3172, 2006

Effects of acetophenone‐formaldehyde resin on the degradation of high‐density polyethylene by UV‐irradiation

PurposeProposes to examine acceleration of photo‐oxidative degradation of high‐density polyethylene (HDPE) by using photosensitive acetophenone‐formaldehyde resin (AFR).Design/methodology/approachDegradation of HDPE by UV light was investigated in the presence of photosensitive AFR on natural weathering. The experiments were done at constant temperatures (40, 65 and 90°C). The results were determined by FT‐IR spectrophotometric and viscometric methods. Measurement of the rate of formation of carbonyl groups on the FT‐IR showed the evidence of degradation. The carbonyl indices of photo‐oxidation of HDPE with/without AFR were determined by FT‐IR spectroscopy. The molecular weights of the samples (Mη values) were measured by viscometry.FindingsThe amount of carbonyl present in the AFR containing HDPE samples and the changes in their molecular weights were found to depend on the irradiation period, temperature and amount of AFR in the mixture. The improvements in UV performance have been observed by using 1 per cent photosensitive AFR in the mixture. Photo‐oxidative degradation also appeared to be accelerated by heat.Research limitations/implicationsThis study can be focused on using photosensitive resins for the polymer degradations just as powder mixture, but the HDPE sample used did not contain antioxidants. From this point of view, commercial HDPE and AFR must be mixed as a film‐former and the AFR concentration will be higher than those of this work.Practical implicationsThis work provides technical information for the application of photosensitive resins for easy degradation of HDPE packaging materials.Originality/valueThe method in which a photosensitive resin is used in the polymer degradation may be a reference for other relevant studies.

The hierarchy structure and orientation of high density polyethylene obtained via dynamic packing injection molding

The evaluation of microstructure and crystal morphology in injected-molded bar becomes much complicated because of the existence of a shear gradient and a temperature gradient from the skin to the core of the samples. To understand the relationship between shear rate-molecular weight and oriented structure of injection molded bar, in this work, the hierarchy structure and the effect of molecular weight on the formation of shish-kebab structure were investigated by examining the lamellar structure of injection molded samples of high density polyethylene (HDPE) with different melt flow index (MFI), layer by layer, along the sample thickness. To enhance the shear effect, so-called dynamic packing injection molding (DPIM), in which the melt is firstly injected into the mold and then forced to move repeatedly in a chamber by two pistons that move reversibly with the same frequency as the solidification progressively occurs from the mold wall to the molding core part, was used to obtain the molded bar. Furthermore, a small amount of ultra-high molecular weight polyethylene (UHMWPE) was added into HDPE to explore the effect of UHMWPE on the crystal morphology and orientation. Our results indicated (1) that the overall orientation in the molded bar increased with decreased MFI, and a small amount of UHMWPE could enhance substantially HDPE orientation; (2) at the skin, there existed intertwined lamellae constituting an interlocked lamellar assembly, a typical shish-kebab structure gradually developed from the subskin-layer to the core, with increased shish content toward the center, but in the core was a spherulite-like superstructure with randomly distributed lamellae; (3) UHMWPE played an important role not only in the formation of shish, but also in the transformation from spherulite to shish-kebab oriented structure for HDPE with a low molecular weight (high MFI).

Orientation-Insensitive Spectra for Raman Microspectroscopy

Separating effects due to molecular conformation from those due to orientation in the spectra of oriented samples obtained by Raman microspectroscopy is a complex issue. To solve this problem, we propose a procedure to calculate an orientation-insensitive spectrum (so-called isotropic spectrum) from polarized spectra obtained by Raman microspectroscopy that is valid for systems that exhibit a uniaxial symmetry. The method has first been tested on highly oriented samples of high-density polyethylene (HDPE). Polarized and isotropic spectra of a highly oriented HDPE cylindrical rod and an isotropic HDPE sample have been compared. The differences in the relative intensities, which occur in the polarized spectra and are due to orientation of the polyethylene chains, are nearly cancelled in the isotropic spectra, showing that the orientation-insensitive spectrum adequately represents the molecular conformation without contributions of orientation. Second, spectra of silk fibroins have been compared in the amide I region for Bombyx mori cocoon silk fibers and methanol-treated regenerated fibroin films. The similarity of the shape of the amide I band of the isotropic spectra indicates that the secondary structure of the fibroins is very close in both samples. These experimental results support the conclusion that the molecular conformation can be efficiently characterized from the intensity and the shape of Raman bands in the orientation-insensitive spectrum.

Vibration-Induced Self-Reinforcement of Injection Molding Samples of High-Density Polyethylene

ABSTRACT A pressure vibration injection machine with vibration frequency of 0–1.5 Hz and vibration pressure of 0–75 MPa was developed to investigate self-reinforcement of high-density polyethylene (HDPE). The effect of vibration frequency and vibration pressure on tensile strength and elongation of HDPE DGDA6098 vibration molding samples, which were obtained at different melt temperatures, was studied, and SEM and WAXD measurements were conducted. Experimental results showed that vibration changed the crystal structure of vibration samples and enhanced their orientation. Instead of spherulites of static samples, crystal structure of vibration samples was lamellae that was orientated along melt flow direction. When vibration frequency was high, lamella size was small and orientation degree was low. When vibration pressure was high, lamella size was large and orientation degree was high. Therefore, vibration samples were self-reinforced with increasing vibration frequency and vibration pressure, where the maximum increment of tensile strength was 41.0 %.

Thermogravimetric Study of Polyethylene Catalytic Degradation by Zeolites

Commercial samples of high density polyethylene were decomposed over faujasites (FAU) type zeolites (Y-type) using thermal gravimetric analysis (TGA). The catalytic activity and behavior of the FAU-zeolites catalysts were determined. It was found that the degradation temperature of polymer strongly depends on the catalytic acidity of the zeolites. HY zeolite was identified as the most active catalyst due to its strong acidity. The results showed that the activation energy decrease with the amount of catalyst added.

Characterization of volume strain at large deformation under uniaxial tension in high-density polyethylene

The intensity and origin of volume changes under uniaxial tension is investigated at room temperature in high-density polyethylene samples with a large initial degree of crystallinity. At the macroscopic scale, volume strain is defined as the trace of the finite strain tensor whose components are recorded in situ by means of a 2D video extensometer within a representative volume element situated at the center of the neck. At the microscopic scale, volume strain is ascribed to the competition of cohesive mechanisms and non-cohesive mechanisms. The former are associated both to the packing of oriented chains in the amorphous phase (compaction) and to the decrease of crystallinity (dilatation), as characterized by wide-angle X-ray diffraction analysis. The latter are due to the development of crazes and voids (dilatation) while the spherulitic morphology is progressively transformed into a highly fibrillated structure, as revealed by scanning electron microscopy. Detailed evaluation of the relative importance of these two classes of mechanisms shows that they compensate nearly exactly at moderate strains, so that volume strain is very small in the first stage of the tensile tests. By contrast, the effect of cavitation becomes prominent at large deformation, so that the overall dilatation reaches more than 30% before rupture. It is demonstrated that volume strain measurements obtained from mechanical testing and from microstructural investigation agree fairly satisfactorily.

A sensitive method to detect very low levels of long chain branching from the molar mass distribution and linear viscoelastic response

We have developed a sensitive method to detect very low levels of long chain branching in sparsely branched polymers and applied it to high-density polyethylene samples with broad molar mass distribution obtained by Ziegler-Natta, Phillips, and metallocene catalysis. We compare experimental dynamic moduli with predicted values. The predicted moduli, which are only valid for truly linear chains, are computed from a known molar mass distribution by the application of a modified time-dependent diffusion reptation model described in [van Ruymbeke et al., Macromolecules 35, 2689 (2002)]. Discrepancies between experimental and predicted moduli are observed at branching levels below the usual detection limit Of C-13 nuclear magnetic resonance (about I branch per 10000 carbon atoms). Our method is easier to implement and the results more clearcut than those obtained by the zero-shear viscosity method. The sensitivity is comparable with the activation energy spectrum method described in [Wood-Adams et al., Macromolecules 33, 7489 (2000)]. The use of our method is however not restricted to thermorheologically complex polymers. The interest of polymer fractionation is also demonstrated for the detection of extremely low levels of long chain branching. (c) 2005 The Society of Rheology.

Plastic Deformation of Crystalline Polymers: The Role of Cavitation and Crystal Plasticity

Plane strain compression in a channel die is kinematically very similar to drawing; however, the possibility of void formation is limited due to a compressive component of stress. In drawing, voids were detected by small-angle X-ray scattering (SAXS) and density measurements in poly(methylene oxide) (POM), polypropylene (PP), and high-density polyethylene (HDPE), but no voiding was found in polyamide 6 (PA 6), low-density polyethylenes (LDPEs), and ethylene−octene copolymer (EOC). The slope and shape of the initial elastic part of true stress−true strain curves are similar in tension and in channel die compression. When drawn samples of POM, PP, HDPE, and PA 6 already show yielding, the channel die compressed samples still undergo elastic deformation to a much larger deformation and respond with a much larger stress. Channel die compressed POM, PP, HDPE, and PA 6 exhibit strong and rapid strain hardening up to 400 MPa in contrast to their behavior in tension. The difference in strain hardening is related ...

Absorption of 1-MCP by Nontarget Materials during Storage

We tested the sorptive capacity of a number of nontarget materials found in apple storage rooms on their capacity to remove 1-MCP from the storage atmosphere and thereby compete with the fruit for the active compound. Furthermore, we evaluated the impact of temperature and moisture. Nontarget materials included bin construction materials [high density polyethylene (HDPE), polypropylene (PP), weathered oak, nonweathered oak, plywood, and cardboard] and wall construction materials (polyurethane foam and cellulose-based fire retardant). Each piece had an external surface area of 76.9 cm2. We placed our “nontarget” materials in 1-L mason jars and added 1-MCP gas to the headspace at an initial concentration of ≈30 μL·L-1. Gas concentrations were measured after 2, 4, 6, 8, 10, and 24 hours. The concentration of 1-MCP in empty jars was stable for the 24-hour holding period. Little to no sorption was detected in jars containing dry samples of HDPE, PP, cardboard, polyurethane foam, or fire retardant. Inclusion of plywood, nonweathered oak, and weathered oak lead to a loss of 10%, 55%, and 75% of the 1-MCP after 24 hours, respectively. Using dampened materials, no sorption resulted from the inclusion of HDPE, PP, polyurethane foam, or the fire retardant. However, the rate of sorption of 1-MCP by dampened cardboard, plywood, weathered oak, and nonweathered oak increased markedly, resulting in a depletion of ≈98%, 70%, 98%, and 98%, respectively. The data suggest that there are situations where 1-MCP levels can be compromised by wooden and cardboard bin and bin liner materials, but not by plastic bin materials or typical wall construction materials.

Adsorption of natural dyes on clay fixed on polymers

The main purpose of this work was to produce a pellet to minimize undesirable effects in fixed beds, such as colmatation, through clay fixation on the surface of polymeric particles. Preliminary experiments were carried out by heating the clay, namely Tonsil Terrana 580FF, in order to observe variations on its capacity to adsorb anthocyanins derived from red cabbage. Clays was fixed on five samples of high density polyethylene (HDPE) at 180°C -210°C for two hours. The morphological analyses of the resulting particles were accomplished through Scanning Electron Microscopy (SEM). The experiments demonstrated an increase on clay's adsorptive capacity of 15.65% at 120°C for 30 hours and 16.80% at 170°C for two hours. The SEM analysis showed that the clay particles adhered on the external surface of the pellets .These results show that it is possible to obtain polymeric pellets using HDPE coated with clay.

Investigating the role of anionic surfactant and polymer morphology on the environmental stress cracking (ESC) of high-density polyethylene

The influence of a commercial grade and a pure anionic surfactant (dodecylbenzenesulfonic acid sodium salt, DBS) in aqueous solutions on the environmental stress cracking (ESC) of high-density polyethylene (HDPE) has been investigated. Injection moulded HDPE samples in “dog-bone” shape were exposed to the aqueous solutions of surfactant (0.14 M) at 80 °C under strain ( ɛ = 1.2%) for 5 days. Both the commercial and pure surfactants caused cracks under the experimental conditions. The change in surface chemistry of the HDPE was followed using attenuated total reflectance Fourier transform infrared (ATR-FTIR) and Raman spectroscopy. The thermogravimetric analysis (TGA) did not show any significant differences in thermal stability of the samples studied. Differential scanning calorimetry (DSC) analysis, however, revealed an increase in crystallinity of the exposed samples. Wide Angle X-ray Scattering (WAXS) indicated that the HDPE sample has a boundary of both preferred and random (isotropic) crystallite orientations close and far away from the injection moulding inlet, respectively. The structure of the surfactants in solution was investigated by light scattering (LS). This showed that the commercial grade surfactant forms large aggregates (e.g., 500 nm) both at room temperature and at the exposure temperature of 80 °C, whereas the pure surfactant has a critical micelle temperature (CMT) of around 45 °C. Below the CMT aggregates are formed, and above the CMT the surfactants are unimeric. Matrix-assisted laser desorption ionisation time-of-flight mass spectrometry (MALDI-TOF-MS) analysis revealed that the commercial surfactant has a wide distribution of hydrocarbon chains from C-10 to C-13, whereas the pure one contains predominantly hydrocarbon dodecyl (chain of C-12).