HDPE Samples Research Articles

Extrudate swell behavior of polyethylenes: Capillary flow, wall slip, entry/exit effects and low-temperature anomalies

This paper describes a comprehensive experimental study of the extrudate swell behavior of high-density (HDPE) and linear low-density polyethylenes. The contributions from bulk melt elasticity, entry extensional flow, and exit boundary discontinuity are each explicitly investigated as a function of molecular weight and its distribution. The elusive apparent molecular weight dependence of the transient extrudate swell ratio (ESR) is elucidated to arise from the different molecular relaxation rates. A master curve is obtained at a fixed stress for the time-dependent ESR of different molecular weights when represented in terms of the rescaled time, t/τ, where t is the actual time allowed for growth of ESR and τ is related to the overall molecular relaxation time of each HDPE. The intricate influence of the exit boundary discontinuity on the extrudate swell is illustrated on the basis of the recently accumulated precise knowledge of wall slip behavior of linear polyethylenes (LPE). Wall slip induced by fluoropolymer coatings localized at the die exit wall is demonstrated to reduce the ESR of LPE under all stresses below an interfacial stick–slip transition. The entry effect is characterized in two ways: by comparing the ESR values below and above the stick–slip transition of the HDPE at the critical stress, and by comparing the ESR from a long die of aspect ratio L/D=15 with that from an orifice die of L/D=1/4. The HDPE samples are found to exhibit an anomalously larger ESR above a critical stress at low temperatures (from 150 to 160 °C).

Comparative study on the lamellar crystal structure of high and low density polyethylenes

A comparative study was undertaken on well characterized HDPE and LDPE samples in order to obtain a better understanding of the morphology of the crystalline phase. Thickness of the lamellae was determined by WAXS. The melting points were calculated by empirical and Gibbs-Thomson equations, considering either folded or extended chains, and compared to DSC data. The results confirm that in HDPE the chains are folded, the segment between folds containing about 100 carbon atoms, while in LDPE, the chains are extended, with segments of about 73 carbon atoms in the crystalline core, linked to the paracrystalline layer and amorphous zones, which contain long folds and chain ends, as proposed in the 3-phase model by Vile et al.

A study of brittle‐ductile transition in polyethylene

AbstractThe brittle‐ductile transition temperatures (BDTT) of polyethylenes with different degrees of crystallinity, such as LDPE, LLDPE, MDPE, and HDPE, were evaluated using Charpy impact test. The effect of the specimen cooling condition on the BDTT was a main concern in the study. A rise in the cooling plate temperature (Tp) for high density resins brought about a fall in BDTT and an increase in the Charpy impact value Et. However, embrittlement was observed during a tensile test for HDPE samples with a rise in Tp. A crack shielding effect by microvoid formation is proposed as the toughening mechanism for HDPE.

Comparison of crystallinities of high density polyethylene determined by different techniques

The differences in the crystallinities of HDPE samples conditioned by three different procedures with different cooling rates were followed by use of FTIR, DSC and X-ray scattering techniques. From FTIR, relative values of crystallinities are obtained and if absolute values obtained from DSC and X-ray scattering are compared, the latter yielded higher values. All three groups of results showed the expected trend of increase of crystallinities with decrease of rate of cooling.

微分型構成方程式の比較検討

Experimental data were compared with the predictions of differential constitutive equations proposed by Larson, Giesekus and Phan-Thien and Tanner (PTT). Applicapability of these equations was investigated with steady shear flow, start up of uniaxial steady elongational flow and elastic recovery of shear and uniaxial elongation of three high density polyethylene having broad distribution of molecular weight. The following conclusions were obtained(1) Larson, Giesekus and PTT-E model (Phan-Thien and Tanner model with non-linear term of exponential type) give good fit with the transient shear viscosities and transient uniaxial elongational viscosities for three HDPE.(2) Larson model gives the best fit when non-linear parameter α equals to 0.5∼0.9 under steady shear and 0.1∼0.2 under transient uniaxial elongation. PTT-E gives the best results when α equals to 0.9 under steady shear and 0.1∼0.2 under steady uniaxial elongation. The best values of α in these two models are different for shear and elongational flow. The values of α are, however, similar in three HDPE samples which have different relaxation spectra.(3) Of three models, the Giesekus model is the best from the point of applicapability to elastic recovery in shear and elongation of three constitutive models. PTT-E is moderate, and the Larson model shows unrealistic results.

Effect of high molecular weight fraction in HDPE on its impact properties: An HTSEC study

Three commercial injection-molding-grade HDPE samples were evaluated for their properties. It was found that their impact property determined in the laboratory does not bear a simple correlation with the actual performance. The HTSEC study, undertaken to evaluate these samples, revealed that they differ significantly in z-average and z + 1-average molecular weights. This difference is attributed to the presence of a small amount of a very high molecular weight fraction and is responsible for the variation in the performance.

Small-Angle X-ray Scattering Functions in the Vicinity of Zero Scattering Angle with an Application to Polymer Blends

The small-angle X-ray scattering (SAXS) of isotropic scatterers is studied in the region near zero scattering angles, where void scattering may contribute to the scattering power Q. Since Q is evaluated by an integration, the corresponding integrands I 1 (s) (for point collimation) and I 1 (s) (for slit focus) as well as the relations between them are studied. The properties of both their moments and their Taylor series expansions are discussed. For a lamellar two-phase system, which is frequently observed in semicrystalline or microphase-separated polymers, analytical equations are presented. They can help (1) to extrapolate I 1 (s) and I 1 (s) toward zero scattering angle, (2) to detect void scattering, and (3) to improve the modeling of the paracrystalline stack even in the case of considerable roughness of the layer surfaces. An example for a set of semicrystalline HDPE samples illustrates how to quantify void scattering and the effects of interfacial statistical structure.

Micellar morphology and its plastic deformation behaviour in ultra-high molecular weight polyethylene

Highly oriented samples from ultra-high molecular weight and normal high-density polyethylene (UHMWPE, HDPE) were prepared under the same experimental conditions. The morphology of the UHMWPE, investigated by transmission electron microscopy, was found to be oriented micellar, while the HDPE samples had a shishkebab morphology. Tensile test experiments under cyclic loading conditions exhibit a strain-hardening effect (increase in stress for the onset of plastic deformation) for HDPE, while the UHMWPE shows a decrease in Young's modulus, which is attributed to similar molecular mechanisms as for the Mullins effect in particle-filled elastomers.

Water vapor diffusion in model tablet systems: I. Design of a diffusion apparatus

A diffusion apparatus was constructed to investigate the diffusion of water vapor through model tablet systems. The apparatus consisted of three main components; a humidification system; a flow-through diffusion cell; and a modified gas Chromatographie system which served as the detector. The system was qualified using compressed high density polyethylene powder as an ideal non-reactive porous barrier. Results from preliminary investigations suggested that the donor and receptor compartments of the diffusion cell were well mixed. Results from studies examining the effect of surface area, compact thickness, water vapor concentration in the donor compartment and particle size of the HDPE powder were consistent with a model of Fickian Diffusion through a porous barrier. Subsequently, the pore structure of the HDPE samples was determined using mercury intrusion porosimetry. The pore size distribution of the samples was in the range where normal gaseous diffusion was expected. Thus, the apparent diffusivity of water vapor through the HDPE samples was controlled exclusively by sample porosity and was independent of pore size.

Simulation of the extrusion of HDPE and HDPE/UHMWPE blends

A Criminale-Ericksen-Filbey (CEF) constitutive equation was used to simulate the behaviour of HDPE samples of different molecular weights and blends with UHMWPE. Shear rate dependent viscosity and first normal stress coefficient data were obtained by capillary rheometry. The extruder operation point was calculated by using the central finite differences method and the result was compared with the experimental and power law values. At low temperatures, the CEF gave better approximations.

Effect of chromium on the oxidative pyrolysis of gas-phase high-density polyethylene as determined by dynamic thermogravimetry

The oxidative degradation of HDPE samples containing various chromium compounds was studied by chemiluminescence and thermogravimetry. The importance of the valence state and solubility of chromium in relation to its pro-oxidant behaviour is reported. Chemiluminescence experiments confirm that the effectiveness of chromium compounds in promoting thermal oxidation is dependent on their solubility in the polymer. In order to study the effect of chromium catalyst residues on the stability of HDPE, a chromocene/silica polymerization catalyst was deactivated by exposing it to air and was then added to the polymer. Although the deactivated catalyst is quite insoluble in the polymer, it exhibits strong pro-oxidant behaviour at 125°C due to the high effective surface area of its silica substrate. Thermogravimetry shows that the onset of thermal oxidation of the virgin HDPE occurs at about 280°C whereas rapid oxidative pyrolysis commences at 360°C. The deactivated catalyst increases the temperature of onset of pyrolysis by up to 70°C. The increase in the degradative resistance of HDPE containing the deactivated catalyst is attributed to the ability of chromium ions to complex macroalkylperoxyl radicals, thereby terminating the oxidative chain process. This termination occurs most efficiently under conditions where the polymer hydroperoxide concentration is very low. Oven ageing of HDPE in air leads to oxygenated groups which are eliminated in the early stages of thermogravimetric analysis to produce conjugated sequences in the polymer backbone. The presence of conjugation in the polymer suppresses the rate of volatilization during the analysis and thus retards the rate of oxidative pyrolysis.

Crystallinity of polyethylene measured by density, DSC, and raman spectroscopy. A comparison

AbstractIn this work three different techniques were used to determine the crystallinity of polyethylene. After different pretreatment HDPE, LLDPE, and LPDE samples were investigated by density measurements, differential scanning calorimetry, and Raman spectroscopy. The results were compared and relationships were estabilished between the values measured by the different methods.

Influence of low extents of degradation on the processing behaviour of high density polyethylene

Processing operations may induce low extents of degradation in high density polyethylene which cannot be determined by chromatographic measurements and does not change significantly the rheological behaviour in shear flow. On the other hand, elongational viscosity is strongly enhanced. This is due to the formation of small numbers of long chain branches. The influence of this small amount of degradation on the processing behaviour is thus limited, for HDPE samples, to the operations in which elongational flow is involved.

Effect of temperature and chemical treatment on the rate of the reaction of high density polyethylene with ozone

A break was detected on the temperature dependance of ozone oxidation of isotropic and oriented HDPE samples between 40–60°, in connection with a relaxation transition. Previous annealing at T < 90° and chemical treatment merely reduce the rate of the reaction with ozone and only weakly affect the character of the temperature dependence of the reaction rate. Annealing at T<90° with simultaneous ozonization lead both to a reduction of the reaction rate, and to a change in the character of the temperature dependence of the rate.

Elongational Flow of HDPE Samples and Bubble Instability in Film Blowing

Abstract In assisting product development the processing behaviour of similar and different samples of polyethylene film grades were compared with laboratory data. Bubble stability of the samples in film blowing on a commercial production line is related to rheological properties in elongation. Strain hardening behaviour enhances stability as is well known. The influence of even small differences can be shown, but in these cases the linear viscoelastic stress growth, the absolute value of the viscosity, and the activation energy of flow become important as well. The experimental results are discussed confirming the pulsations to be a kind of draw resonance.

Two separate ranges for shear flow instabilities with pressure oscillations in capillary extrusion of HDPE and LLDPE

In polymer melt extrusion at constant piston rates, it is well known that linear polymers show a characteristic type of flow instability within a single range of flow rates. In this range, pulsations of the extrusion pressure occur in connection with typical distortions of the extrudate. When several HDPE and LLDPE samples were tested, it was found that one HDPE and one LLDPE sample showed two separate ranges of pulsating pressure in connection with characteristic distortions of the extrudate.

Effect of oxidation on the dynamic and structural parameters of oriented polyethylene in the deformed state

ESR spectroscopy using a radical probe and X-ray analysis have been employed to study the behaviour under a load of oriented HDPE samples with narrow and wide MD first oxidized with an ozone-oxygen mixture. Loosening of the amorphous phase on reversible loading of the polymer films increase with rise in the degree of oxidation. The elasticity modulus E in the initial stage rises but falls for deeper degrees of oxidation. For oxidation of short duration no appreciable changes were found in the orientation; for longer oxidation times the orientation considerably diminishes. Oxidation leads to a sharp increase in the correlation time of the radical-probe.

Influence of the molecular structure on the melt strength and extensibility of polyethylenes

AbstractMelt strength (MS) and breaking stretching ratio (BSR) data relative to polyethylenes with different molecular structure (high density (HDPE), low density (LDPE), and linear low density (LLDPE)) are shown. HDPE and LLDPE samples show high breaking stretching ratio and low melt strength values. The LDPE samples exhibit very large melt strength values but low breaking stretching ratio values. For the last mentioned samples, differences are shown in the non‐isothermal elongational behavior between samples polymerized with tubular and vessel technology. For all the samples, MS decreases with increasing melt index while BSR increases with melt index.

Influence of Anisotropy on the PTC Effect in Injection Moulded Samples of CB-filled Polyethylene and Polystyrene

Abstract This paper reports on measurements of the temperature dependence of resistivity, ρ, in injection moulded samples of LDPE, HDPE, and PS filled with two grades of carbon black (CB). The CB concentration was adjusted to give a relatively small PTC-effect in isotropic (annealed) samples. It was found that the orientation of the matrix polymer that occurred during the injection moulding process highly enhanced the PTC-efficiency of all the compounds used. The magnitude of this effect and the level of RT resistivity were orientation dependent. Upon annealing (second heating run) the behaviour of the isotropic samples was recovered. In general, ρ increased with the degree of orientation in a fashion similar to the effects associated with lowering the CB content. One of the effects observed in highly oriented samples was a pronounced frequency-dependence of the resistivity (ρ decreasing at higher frequencies), and a non-ohmic I-V-behaviour. The ρ-data relate to d.c. and 1 kHz. Samples with higher and low...

High-temperature relaxation transitions in polyolefins

High-temperature relaxation transitions in oriented HDPE and PP samples have been investigated by ESR spectrometry, using a radical-probe method. Jump-like changes in mechanical properties and in molecular mobility parameters occurred at 40–50° for the PE and at 60–70° for PP. A mechanism proposed for the α-relaxation transition is based on thawing amorphous phase microregions with denser packing of extended chains.