Commercial High Density Polyethylene Research Articles

Rheological Investigation of Molecularly Characterized Commercial High Density, Low Density and Linear Low Density Polyethylene Melts

Rheological Investigation of Molecularly Characterized Commercial High Density, Low Density and Linear Low Density Polyethylene Melts

Ideas about reptation, tube renewal and tube relaxation applied to the description of the non-linear viscoelastic behaviour in shear of some polydisperse polyethylene melts

In melts of polydisperse linear polymers like high-density polyethylene (HDPE) the dynamics of a given macromolecule are affected by the renewal of the confining tube owing to movements of surrounding molecules, the more so according as the matrix contains more shorter molecules. At large strains, the relaxation of the tube due to the retraction of matrix molecules also contributes to stress relaxation. For long-chain branched molecules in a polydisperse melt like that of low-density polyethylene (LDPE), the dynamics are dominated by tube renewal and tube relaxation effects. It is argued that for melts of HDPE as well as LDPE the combination of these ideas leads to a rheological constitutive equation consisting of the sum of two integrals. Both integrands are factored into the product of a strain-independent time function and a time-independent nonlinear strain tensor. Using the relaxation spectrum from oscillation experiments and the nonlinear strain measure from first normal stress growth data, this equation satisfactorily describes the decay of the stresses after step strain or after cessation of steady shear flow for melts of two commercial HDPEs and of two LDPEs. For the HDPE samples one integral suffices, but for the LDPE melts the additional time and strain dependences of the integral relating to tube relaxation are indispensable for a good fit of the experimental curves.

Melt rheology and extrudability of polyethylenes

AbstractCommercial high density polyethylene (HDPE), low density polythylene (LDPE), and linear low density polyethylene (LLDPE) resins were tested at 150, 170, and 190°C in steady state, dynamic, and extensional modes. Within the low rates of deformation \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \gamma $\end{document} = ω ≤ 0.3, the steady state and dynamic functions agreed: η = η′ and N1 = 2G′; at the higher rates, the steady state parameters were larger. The elongational viscosity, ηe, was measured under a constant rate, \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \varepsilon $\end{document}, or stress, σ, condition. In the first case for LLDPE, the transient η reached an equilibrium plateau value, ηe. For HDPE, η increased up to the break point. For LDPE, stress hardening was recorded. Under constant stress the ηe, could always be determined; its value, within experimental error, agreed with the maximum value of η determined in a constant \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \varepsilon$ \end{document} experiment. The maximum strain at break was only ε = 1.5 for HDPE and 3, to 4 for LDPE and LLDPE. The rate of deformation dependence of the η (or η′) and ηn may be discussed in terms of the Trouton ratio, RT = ηe/3η at \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \gamma $\end{document} = ω = \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \varepsilon$ \end{document}: RT ≤ 1.2 for LLDPE, RT ≤ 2.5 for HDPE, and RT ≤ 15 for LDPE. The PE resins were extruded at 190°C through a laboratory extruder equipped with a slit or rod die. The rotational speed of the screw varied from 0 to 90 rpm. Extrusion pressure, output, and energy were measured and correlated with the rheological parameters of the resins.

Crystallinity as a selection criterion for engineering properties of high density polyethylene

AbstractThe Differential Scanning Calorimeter (DSC) was used to discriminate among 25 commercial high density polyethylenes (HDPE) on the basis of their degree of crystallinity and melting temperature. The area under the melting endotherm correlated directly with density and inversely with creep and thermal expansion measurements. Since high crystallinity was related to the design required properties of density, creep, and thermal expansion, DSC studies readily identified eight of the more promising polymers from the group of 25. The overall crystallization kinetics of polyethylenes with 75 percent crystallinity were analyzed by the Avrami and Fischer‐Turnbull equations. Results indicate small disk‐like spherulites (Avrami n = 2) following nucleation‐controlled growth kinetics. These conclusions are in reasonable agreement with polarizing microscope observations. An equilibrium melting temperature between 141 and 142°C was estimated from Hoffman‐Weeks plots. Processing thick parts from highly crystalline polyethylene is difficult because of the 14 percent volume change on crystallization. Higher degrees of crystallinity are associated with moderate molecular weight, so the viscosity range of these polyethylenes is not especially suited for processing by extrusion. These caveates necessitate tradeoffs between optimal design properties and processing requirements for HDPE parts.

Dynamic Viscoelastic and Steady Flow Properties of High Density Polyethylene Melts

Shear storage (G′) and loss moduli (G″), and steady shear (σ) and first normal stress difference (N1) of a commercial high density polyethylene (HDPE) melts and the fractions have been measured with a Weissenberg Rheogoniometer. Pseudo-equilibrium modulus (GeN0) and average molecular weight between entanglement points (Me) were found to be 1.32 × 106 Pa and 2.18 × 103, respectively. The value of Me is approximately half of the literature value reported for Mc, the critical molecular weight for entanglement coupling estimated from molecular weight dependence of viscosity. The value of zero shear viscosity (η0) obtained from both dynamic and steady shear measurements are in good agreement and found to be proportional to 3.8 power of the weight- average molecular weight. Cox- Merz rule does not hold so well at high rate of shear for both the whole polymer and its fraction. Steady state compliance (Je0) of the fractions is much higher than expected value and is remarkably dependent on molecular weight. Storage compliance (J′) obtained from dynamic measurement and steady shear compliance (Js) from steady flow measurement agree with each other in the value and the behavior for each fraction. It was found that J′ (and JS) of the fractions at low frequencies (low shear stresses) increase as increasing molecular weight and molecular weight distribution.On the contrary, J′ at higher angular frequencies decreases with increasing molecular weight and molecular weight distribution. It is suggested that Js, at high rates of shear decreases with increasing molecular weight and molecular weight distribution supporting Mendelson's results.

Orientation and Structure of Polyethylene Crystallized under the Orientation and Pressure Effects of a Pressure Capillary Viscometer

A commercial high-density polyethylene has been crystallized in the Instron Capillary Rheometer under the combined influence of pressure and orientation. A transparent high-modulus, and highly oriented filament structure was produced. In order to determine quantitatively the degree of orientation, azimuthal x-ray scans of the 002 reflection plane were used to obtain the orientation function of the orthorhombic unit cell relative to the flow axis in the rheometer (identified with the filament axis). A value of +0.996±0.002 was measured for this orientation function indicating that the backbones of the crystalline polyethylene chains were virtually aligned perfectly parallel to the flow axis of the rheometer. This report is concerned with the high-resolution techniques required to measure such highly oriented samples. In conjunction with the x-ray study, birefringence measurements are included which permit the determination of the orientation of the amorphous component as well as the crystalline component.

Molecular Weight Distribution and Melt Rheological Properties for High Density Polyethylenes

High density polyethylene (HDPE) has each its distinctive features in the rheological properties according to its manufacturing process. The present work proposes to present a quantitative relationship between its molecular weight distribution and the melt viscoelastic properties for some samples of HDPE.The samples have been fractionated by both the elution column technique and the precipitation technique. The number-average, weight-average and Z-average molecular weights have been calculated from the smoothed molecular weight distribution curve for each sample by the method of summation.The viscoelastic properties of some series of the commercial HDPE have been measured by means of a concentric cylinder type rheometer at different temperatures (150∼230°C). The angular frequency (ω) ranges from about 10-2 to 50sec-1. The ratio of flow rate at shear stress 106dyne/cm2 to at 105dyne/cm2 (=flow ratio; FR), which gives a good indication of the non-Newtonian property of samples, have been measured by means of a melt indexer type rheometer.The results are as follows;(1) The linear relationship is found between Mw/Mn and Mz/Mw for only a series of samples made by the same manufacturing process.(2) FR is found to be related to Mz/Mw, not to Mw/Mn. log FR=0.137Mz/Mw+1.04(3) Dynamic viscosity (η') and dynamic regidity (G') can be superposed according to the temperature-time superposition principle, and shift factors aT from η' and G' are practically the same.Agreement with constancy of shift factor for various molecular weights (Mv=5∼10×104) and distributions is good, and the apparent activation energy (ΔHa) calculated from the shift factor is 6-7kcal/mol. ΔHa of copolymer is also of the same value.(4) Master curves also are constructed with respect to the molecular weight for measurements of η' and G' versus ω for only those samples in which Mz/Mw are equal to each other.Shift factors aM from η' and G' are the same.The relationship must also be right in the steady flow viscosity, and confirms that HDPE with the same Mz/Mw have equal FR.(5) The relationship between η0 and Mw is effected by the molecular weight distribution.log η0=3.5log Mw+log (Mz/Mw)-12.7