High-density Polyethylene Melt Research Articles

Effect of vibration extrusion on high‐density polyethylene

AbstractA new type of extrusion apparatus with a hydraulic vibration field was self‐developed to study the effects of vibration on the rheological behavior, mechanical properties, and morphology of high‐density polyethylene (HDPE) during the process of extrusion. In the research, the morphology and mechanical properties of specimens produced by conventional extrusion molding were used for comparison with specimens obtained via vibration extrusion molding. The research showed that the extrusion property of the HDPE melt was improved under a vibration field, and the apparent viscosity decreased. The maximum decrement of the apparent viscosity was 37.98%. The change in the apparent viscosity of the HDPE melt corresponded to the vibration frequency and amplitude, die temperature, and rotational speed of the extruder screw. Both the longitudinal and transverse yield strengths of the HDPE specimens increased under the vibration field. The increment of the longitudinal yield strength (11.5%) was bigger than that of the transverse yield strength (6.9%). According to scanning electron microscopy micrographs, the vibration field made the crystallites orient and become smaller, and the crystal size became more homogeneous. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007

Probing nucleation and growth behavior of twisted kebabs from shish scaffold in sheared polyethylene melts by in situ X-ray studies

By utilizing synchrotron rheo-WAXD (wide-angle X-ray diffraction) and rheo-SAXS (small-angle X-ray scattering) techniques, the nucleation and growth behavior of twisted kebabs from the shear-induced shish scaffold in entangled high-density polyethylene (HDPE) melts were investigated. The evolution of the (110) reflection intensity in WAXD at the early stages of crystallization could be described by a simplified Avrami equation, while the corresponding long period of kebabs determined by SAXS was found to decrease with time. The combined SAXS and WAXD results indicate that the kebab growth in sheared HDPE melts consists of two-dimensional geometry with thermal (sporadic) nucleation. The WAXD data clearly exhibited the transformations of (110) reflection from equatorial 2-arc to off-axis 4-arc and of (200) reflection from off-axis 4-arc to meridional 2-arc, which can be explained by the rotation of crystallographic a-axis around the b-axis during twisted kebab growth. This observation is also consistent with the orientation mode changes from ‘Keller/Machin II’ to ‘intermediate’ and then to ‘Keller/Machin I’.

Effects of melt‐extension and annealing on row‐nucleated lamellar crystalline structure of HDPE films

AbstractThe row‐nucleated lamellar crystalline structure of high‐density polyethylene (HDPE) films was prepared by applying elongation stress to HDPE melt during T‐die cast film extrusion and subsequently annealing the extruded films. This unusual crystalline structure was analyzed in terms of lamellar crystalline orientation, long‐period lamellar spacing, crystallite size, and degree of crystallinity. The contribution of melt‐extension represented by draw‐down‐ratio (DDR) to the overall orientation was found to be most noticeable than other processing variables. Meanwhile, the long‐period lamellar spacing, the crystallite size, and the degree of crystallinity were influenced predominantly by the annealing temperature. Finally, the processing (melt extension and annealing temperature) – structure (lamellar crystalline structure) – property (hard elasticity) relationship of HDPE films was investigated. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3326–3333, 2007

Lamellar crystalline structure of hard elastic HDPE films and its influence on microporous membrane formation

The development of hard elastic high-density polyethylene (HDPE) precursor films and its influence on the microporous membrane formation have been investigated. As a first step, the HDPE precursor films with ‘row-nucleated lamellar crystalline’ structure were prepared by applying elongation stress to the HDPE melt during T-die cast film extrusion and subsequently annealing the extruded films. This unusual crystalline structure was analyzed in terms of lamellar crystalline orientation, long-period lamellar spacing, crystallite size, and degree of crystallinity. The processing (melt extension and annealing temperature)-structure (lamellar crystalline structure)–property (hard elasticity) relationship of HDPE precursor films was also investigated. The uniaxial stretching of hard elastic HDPE precursor films induces the bending of crystalline lamellae, which leads to the formation of micropores between them. The observation of morphology and air permeability for the HDPE microporous membranes have revealed that the well-developed porous structures characterized by superior air permeability were established preferably from the precursor films prepared by the high stress levels and the high annealing temperatures. Finally, the relationship between the hard elasticity of HDPE precursor films and the air permeability of corresponding microporous membranes was discussed.

Interfacial stick-slip transition in hydroxyapatite filled high density polyethylene composite

Effect of filler addition and temperature on the stick-slip transition in high density polyethylene melt was studied. Results showed that shear stresses corresponding to stick-slip transition increases with the addition of filler. Increase in temperature also increases the shear stresses for stick-slip transition. The features of the flow curves of composites and that of unfilled system remain identical. Filler addition lowers the shear rate at which the transition occurs. The composite extrudate did not show characteristic extrudate distortions associated with the unfilled polymer.

Numerical simulation of large amplitude oscillatory shear of a high-density polyethylene melt using the MSF model

We study the flow response in large amplitude oscillatory shear of the molecular stress function (MSF) model that has recently been proposed by Wagner et al. [M.H. Wagner, P. Rubio, H. Bastian, The molecular stress function model for polydisperse polymer melts with dissipative convective constraint release, J. Rheol. 45 (2001) 1387–1412]. The MSF model is derived from molecular theory and has only two parameters to describe the non-linear material response. The model predictions are analysed in both the frequency and time domain. It shows good agreement with experimental data for a linear high-density polyethylene melt. At low and medium strains, MSF model predictions are in excellent agreement with experimental data and predictions of a six-mode Giesekus model which has six parameters to describe the non-linear material response. At medium strains, the basic Doi–Edwards model, which has no non-linear parameters, already underpredicts the data. At high strains, the MSF model predictions agree slightly better with the experimental data than the Giesekus model. Surprisingly, however, it is the Doi–Edwards model that shows excellent agreement with experimental data at high strains. For the linear melt we consider, it outperforms the models that have non-linear parameters, both in the time and frequency domain.

Estimation of Melt Shear Modulus from Extrudate Swell Ratio and Exit-pressure Drop Data

Obvious viscoelastic phenomena, such as die-swell and exit-pressure losses, appear when a viscoelastic fluid leaves a die. It is generally believed that the die-swell ratio (B), the melt shear modulus (G), and the exit-pressure drop (Δ Pex) are important parameters for the characterization of the elastic behavior of viscoelastic fluids during die flow. There may be, therefore, some interrelations between them. In the present article, relationships among G, B, and Δ Pex are discussed. On the basis of previous work, some expressions for describing relationships between B and G, Δ Pex, and B, as well as Δ Pex and G during flow of viscoelastic fluids in a long circular die are proposed. The values of G are estimated from the measured B during extrusion of a carbon black and calcium carbonate-filled rubber compound, and predictions of G by using the measured data of Δ Pex are compared with its calculations from the B data during capillary extrusion of a high-density polyethylene (HDPE) melt. The results showed that G increases linearly with increasing shear stress. Furthermore, a good agreement was obtained among these theoretical calculations of G at high shear stress level, also being consistent with predictions by using the Bagley-Duffey equation.

Effect of the high‐density polyethylene melt index on the microcellular foaming of high‐density polyethylene/polypropylene blends

AbstractThe effect of high‐density polyethylene (HDPE)/polypropylene (PP) blending on the crystallinity as a function of the HDPE melt index was studied. The melting temperature and total amount of crystallinity in the HDPE/PP blends were lower than those of the pure polymers, regardless of the blend composition and melt index. The effects of the melt index, blending, and foaming conditions (foaming temperature and foaming time) on the void fractions of HDPEs of various melt indices and HDPE/PP blends were also investigated. The void fraction was strongly dependent on the foaming time, foaming temperature, and blend composition as well as the melt index of HDPE. The void fraction of the foamed 30:70 HDPE/PP blend was always higher than that of the foamed 50:50 HDPE/PP blend, regardless of the melt index. The microcellular structure could be greatly improved with a suitable ratio of HDPE to PP and with foaming above the melting temperature for long enough; however, using high‐melt‐index HDPE in the HDPE/PP blends had a deleterious effect on both the void fraction and cell morphology of the blends. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 364–371, 2004

Reversible and irreversible crystallization in high-density polyethylene at low temperature

Reversible and irreversible crystallization and melting of high-density polyethylene at low temperature has been re-evaluated and is discussed in terms of the concept of the specific reversibility of a crystal. The concept of the specific reversibility links reversible and irreversible melting of a specific crystal such that reversible melting occurs only at slightly lower temperature than irreversible melting. In this study evidence for irreversible crystallization at low temperature in high-density polyethylene is provided, non-avoidable by primary crystallization and extended annealing at high temperature. The simultaneously observed reversible crystallization and melting at low temperature can be attributed to lateral-crystal-surface activity in addition to the well-established reversible fold-surface melting, dominant at high temperature, and evidenced by small-angle X-ray data available in the literature.

Mechanochemical degradation kinetics of high-density polyethylene melt and its mechanism in the presence of ultrasonic irradiation

In this paper, the effect of ultrasonic intensity on the degradation of high-density polyethylene (HDPE) melt, degradation mechanism, ultrasonic degradation kinetics of HDPE melt as well as the development of molecular weight distribution of HDPE melt during ultrasonic degradation were studied. In the initial stage, the ultrasonic degradation of HDPE melt shows a random scission process, and the molecular weight distribution broadens. After that, the ultrasonic degradation of HDPE melt shows a nonrandom scission process, and the molecular weight distribution of HDPE melt narrows with ultrasonic irradiation time. The average molecular weight of HDPE decreases with the increase of ultrasonic intensity and increases and trends forward that of undegraded HDPE with the increase of distance from ultrasonic probe tip, indicating that attenuation of ultrasonic intensity in HDPE melt is very quick. Ultrasonic degradation kinetics of HDPE melt obeys the equation: M t = M ∞+ Ae − kt . The theoretic calculation by this equation accords well with the experimental results. The plausible ultrasonic degradation mechanism of polymer melt based on molecular relaxation was also proposed in this paper.

Melt flow indexer evidence of high‐temperature transitions in molten high‐density polyethylenes

AbstractA melt flow indexer (MFI) was used to investigate high‐temperature transitions in melts of high‐density polyethylene (HDPE). The MFI data were obtained in the range 190–230°C. These transitions were found in the MFI at about 210 and 225°C and reproduced in a Haake melt blender. Polystyrene was used in the blender experiment to demonstrate typical amorphous behavior. For HDPE melts, the MFI–temperature behavior and the torque–temperature data of the blender were found to be alternative images of the same anomalous temperature dependency in the range 210–225°C. Also, the Haake melt blender was able to reproduce the 150°C transition observed by Kolnaar and Keller in the extrusion of HDPE. Regardless of the simplicity of the MFI device, results are in agreement with our previous DSC findings. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1309–1313, 2004

Effect of wall slip on blown film thickness distribution

AbstractOne of the most important materials for blown film is high‐density polyethylene (HDPE) with wide molecular weight distribution. First, we computed a wall stress at the entrance of a spiral groove in a particular die during blown film processing on a particular condition, to which a similar condition is widely utilized in a film works. The computed value is about 170 kPa, while the HDPE melt slips at die wall at stresses above approximately 50 kPa. The stress of 170 kPa is sufficiently large for the slip occurrence of the melt. Then, we investigated the effects of wall slip and melt visosity on film thickness distribution in the circumferential direction; the distribution tends to decrease with decreasing wall slip and melt viscosity. This tendency is explained by considering flow distribution in a spiral mandrel die and polymer melt flow characteristics.

Interfacial tension measurements and evidence of order in high-density polyethylene melts

Polyethylene (PE) is a widely used product commercially. However, our knowledge is incomplete about the properties of high-density polyethylene (HDPE) at temperatures above its melting point, where solid crystals disappear. Recently, there has been increasing evidence from rheological, differential scanning calorimetry, and NMR studies that suggests the presence of microstructural order in the bulk of PE melts. In this study, the interfacial tension of HDPE melts in contact with silicone oil was measured with a spinning drop tensiometer in the same temperature range in which phase transitions have been observed in the bulk HDPE. Anomalous temperature dependence of interfacial tension was found between 200 and 230°C. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 4061–4067, 2003

Use of ultrasound to determine variation of carbon dispersion in carbon black filled HDPE melts

An ultrasonic technique was used to determine whether differences in the degree of dispersion of carbon filler in high density polyethylene (HDPE) melts could be identified. Ultrasonic transit time measurements were used to identify differences between three grades of HDPE with different degrees of filler dispersion (designated high, medium and low dispersion). Tests were carried out on static melts (off line) over a range of temperatures and pressures, and during extrusion (inline). Tests carried out on samples of static melt showed a significant increase in ultrasonic transit time for the poorly mixed (low dispersion) sample, compared with the medium and high dispersion samples. Differences measured during off line tests were not observed during extrusion tests.

Interfacial slip between polymer melts studied by confocal microscopy and rheological measurements

Interfacial slip between high density polyethylene (HDPE) and polystyrene (PS) melts was studied. The HDPE/PS layered structure was subjected to steady shear in a Cambridge shearing system. Through thickness melt velocities of the HDPE/PS layered structure were measured in situ by a confocal microscope. Velocity profiles of both HDPE and PS layers were obtained. Velocity discontinuity was observed at the HDPE/PS interface from the velocity profiles and indicated interfacial slip. The slip velocity obtained from velocity discontinuity is in good agreement with that deduced by independent rheological measurements.

Polymer melting and polymer powder sintering by thermal analysis

Sintering of polymeric powders is a peculiar characteristic of many processing technologies, including rotational moulding and selective laser sintering (SLS). During polymer sintering, viscosity reduction in the melt state promotes densification of polymer powders, through a double stage mechanism, involving powder coalescence and bubble removal. In particular, sintering of semi-crystalline polymers is strongly influenced by the melting behaviour. Nevertheless, melting itself in absence of pressure is not necessarily accompanied by powder sintering, unless low viscosities are achieved. In this work, the melting and sintering behaviour of recycled high density polyethylene (rHDPE) have been analysed through differential scanning calorimetry (DSC) and Thermomechanical Analysis (TMA). Efficient models capable of describing the melting temperature distribution and rate of sintering of rHDPE powders have been developed, highlighting the inherent differences between the two distinct processes.

1208 ポリマーメルトの不安定性の GPC-光散乱計-連続粘度計による解析

We studied the relation between flow instable phenomenon and molecular structure of polymer chain with some commercial HDPE (High Density Polyethylene) melts. In this study we analyzed molecular structure through using gel permeation chromatography and multi-angle light scattering equipping with continuous viscometry (GPC-MALS-[η]). We showed that one of the origins of the flow instable phenomenon could be ploymer chains' structure.. (High Molecular Weight Molecules or Long-Branched Molecules)

Nonaffine network structural model for molten Low-Density polyethylene and High-Density Polyethylene in oscillatory shear

We propose molten polymer’s entanglement network deformation to be nonaffine and use transient network structural theory with the revised Liu’s kinetics rate equation and the revised upper convected Maxwell constitutive equation to establish a nonaffine network structural constitutive model for studying the rheological behavior of molten Low-Density Polyethylene (LDPE) and High-Density Polyethylene (HDPE) in oscillatory shear. As a result, when the strain amplitude or frequency increases, the shear stress amplitude increases. At the same time, the accuracy of the nonaffine network model is higher than that of affine network model. It is clear that there is a small amount of nonaffine network deformation for LDPE melts which have long-chain branches, and there is a larger amount of nonaffine network deformation in oscillatory shear for HDPE melts which has no long-chain branches. So we had better consider the network deformation nonaffine when we establish the constitutive equations of polymer melts in oscillatory shear.

Viscoelastic computations of polymeric wire‐coating flows

This study considers both a single and multi‐mode viscoelastic analysis for wire‐coating flows. The numerical simulations utilise a finite element time‐stepping technique, a Taylor‐Petrov‐Galerkin/pressure‐correction scheme employing both coupled and decoupled procedures between stress and kinematic fields. An exponential Phan‐Thein/Tanner model is used to predict pressure‐drop and residual stress for this process. Rheometrical data fitting is performed for steady shear and pure extensional flows, considering both high and low density polyethylene melts. Simulations are conducted to match experimental pressure‐drop/flowrate data for a contraction flow. Then, for a complex industrial wire‐coating flow, stress and pressure drop are predicted numerically and quantified. The benefits are extolled of the use of a multi‐mode model that can incorporate a wide‐range discrete relaxation spectrum to represent flow response in complex settings. Contrast is made between LDPE and HDPE polymers, and dependency on individual relaxation modes is identified in its contribution to overall flow behaviour.

Predictions of normal stress difference during circular duct flow of polymer melts

Obvious viscoelastic behaviour appears during die extrusion flow of viscoelastic fluids, such as die-swell, end pressure losses, and unsteady flow or melt fracture. The first normal stress difference ( N 1), die-swell ratio ( B) and exit pressure drop (Δ P ex) are very important parameters for characterization of the elastic behavior for viscoelastic fluids in die flow. There should be, therefore, some interior correlation between them. In the present paper, the relationship between the first normal stress difference, die-swell ratio and exit pressure drop is discussed. On the basis of previous work, some expressions for describing the relationship of B and N 1, B and Δ P ex, and Δ P ex, and N 1 during long circular die flow of viscoelastic fluids are proposed. The predictions of N 1 by using the measured data for Δ P ex were compared with the calculations from the die-swell ratio data and the measured N 1 during capillary die extrusion of a high-density polyethylene (HDPE) melt and two low-density polyethylene melts (LDPE) published in the literature. Good agreement was shown among these theoretical calculations of N 1 at high shear stress level, and was consistent with the predictions by using the Bagley–Duffey equation.