Low-density Polyethylene Melt Research Articles

Flow-induced birefringence of linear and long chain-branched metallocene polyethylene melts subject to steady start-up flow

Optical birefringence induced during steady start-up shear flow of linear and long chain-branched metallocene linear low density polyethylene (LLDPE) melts, and their blends, were investigated at 200°C. Experiments were performed on a high temperature shearing cell, with a pair of parallel quartz plate windows, mounted on a rheometrics optical analyser which employs a rapid polarisation modulation technique for optical anisotropy measurements. The influence of long chain branching (LCB) on chain orientation and stretch were examined at a constant molecular weight (Mw) and molecular weight distribution (MWD), in terms of the amount of non-linear transient response (stress overshoots) and the steady-state value of the flow-induced birefringence of these polymers. Stress overshoots (hence molecular stretch) were found to occur at lower rates in polymers containing LCB than in linear polymers.

Effect of interactions between stabilisers and silica used for anti-blocking applications on UV and thermal stability of polyolefin film 1. Adsorption studies

Interactions between stabilisers and silicas, produced by both the gel process and by precipitation, have been studied using flow micro-calorimetry (FMC) and diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS). The silicas had approximately equivalent BET surface areas. By virtue of its greater pore volume and lower water content, the gel silica was shown to have the greatest adsorption activity. For both the silicas, the adsorption activity of a stabiliser was related to its basicity and number of adsorbing groups per molecule which gave rise to multi-point adsorption, hence polymeric hindered amine light stabilisers (HALS) and large hindered phenols showed the strongest adsorption. Competitive adsorption of polymeric HALS versus hindered phenols was also investigated. The data obtained has been very useful for predicting preferential adsorption of the stabilisers on to the silicas from a linear low density polyethylene melt which in turn has been related to photo-oxidative and thermo-oxidative stability. The latter investigation will be reported separately.

Optimum Determination of Parameters in Differential and Integral Viscoelastic Models and Study on Reliability of the Models for Polymer Melts. 2. Determination of Nonlinear Parameters from Both Shear and Elongational Flow Data.

A nonlinear regression program to determine the material constants for differential and integral viscoelastic models was applied to the determination of nonlinear model parameters from both shear and elongational flow data. The availability of this program and reliability of the viscoelastic models were also discussed. The viscoelastic models employed were the Phan Thien-Tanner (PTT), Larson and K-BKZ models with damping functions of the PSM and Wagner types. The determination of the nonlinear parameters for several polymer melts with extension-thinning uni-axial elongational viscosity was successful and all models could predict the steady shear and elongational flow data well. For a low density polyethylene (LDPE) melt with extension-thickening uni-axial elongational viscosity, the K-BKZ model with a PSM damping function succeeded if we used multiple nonlinear parameters and enough number of relaxation modes in the optimization. On the other hand, the program could not determine physically realistic values of the parameters for the other models even if multiple nonlinear parameters were used. The failure of optimization by the program was due to the strong dependence on initial guesses. We confirmed by manual trial and error fitting that the PTT and the K-BKZ with Wagner damping function models could predict both the shear and elongational flow data if we successfully determined the relaxation spectrum. However, the Larson model could not predict both shear and elongational flow data.

低密度ポリエチレン溶融体の一軸，二軸，平面伸長粘度へのＫ‐ＢＫＺモデルの適応性

Measurements of uniaxial, biaxial, and planar elongational viscosity and step stress relaxation of a low density polyethylene (LDPE) melt were carried out. The description of experimental uniaxial, biaxial, and planar elongational viscosities results by the Kaye-Bernstein-Kearsley-Zapas (K-BKZ) constitutive equation was examined. Three types of damping function forms, Wagner-Demarmels (WD) model, Papanastasiou-Scriven-Macosko (PSM) model, and Luo-Tanner (LT) model, have been proposed in the K-BKZ constitutive equation. These three damping function forms were examined to see which was able to describe the experimental shear, biaxial, and planar damping functions. The experimental shear and biaxial damping functions were described by the LT and PSM models better than the WD model, however, the experimental planar damping function was not described well by any of the three models. After estimating the best parameters in each model by fitting the three experimental damping functions, the uniaxial, biaxial, and planar elongational viscosity was calculated by the estimated parameters. The uniaxial elongational viscosity was predicted best by the LT model, then by the WD, and PSM models. For biaxial and planar elongational viscosity, none of the three models predicted well, thus the lack of suitability of the biaxial and planar elongational viscosity to the K-BKZ model was suggested.

Setup and Test of a Laser Doppler Velocimeter for Investigations of Flow Behaviour of Polymer Melts

The flow behaviour of a low-density polyethylene melt is investigated in a specifically developed flow channel by means of Laser Doppler Velocimetry (LDV). The used flow channel is a slit die with a planar contraction of 14:1. The investigation of the velocity fields was performed in the steady state of flow. The optics of the LDV system as well as the used frequency analyser proved to be reliable for measurements of velocities down to 250μm/s.

Aminolysis kinetics of model and polymer-bound anhydride moieties in low- and high-viscosity media

This study examines the legitimacy of using the reaction kinetics of low molecular weight model compounds in solution to predict the chemical kinetics of polymer-bound species in a homogeneous melt. The reaction under study takes place between an aliphatic secondary amine, diisooctadecylamine (DiOA), and a 5-membered anhydride ring, saturated maleic anhydride ( MA), forming an amic acid product. The MA species was present as a pendant graft on either a model compound, dodecane-g-(maleic anhydride)(dodecane-g-MA), or a polymer chain, linear low-density polyethyl-ene-g-(maleic anhydride) (LLDPE-g-MA). Pseudo-second-order kinetics of the anhydride consumption are followed by infrared spectroscopy, either in situ in dodecane solution or by scanning frozen film samples taken from a linear low-density polyethylene melt. It was found that the LLDPE-g-MA/DiOA system reacted at a slightly slower rate than the dodecane-g-MA/DiOA system in the low-viscosity solution at 140°C. In the melt, the dodecane-g-MA/DiOA system experienced a small decrease in the overall reaction rate compared to the same reaction carried out in dodecane. However, the LLDPE-g-MA/DiOA system underwent a 65% decrease in the observed second-order rate constant on going from a solution to the melt. To explain these phenomena, the effects of diffusion, miscibility, and chain entanglements in the melt are examined here.

Elongational flow and birefringence of low density polyethylene and its blends with ultrahigh molecular weight polyethylenet

Via elongational flow opto-rheometry (EFOR), simultaneous measurements of tensile stress σ(t) and birefringence Δn(t) were conducted on a low density polyethylene (LDPE) melt and its blends with an ultra-high molecular weight polyethylene (UHMWPE) at 140°C under transient elongational flow with constant tensile strain rate ʵ0. The transient elongational viscosity nE(t)σ0 of LDPE melt first gradually increases with time t following the linear viscoelasticity rule in that ηE(t) is 3 times the shear viscosity development. 3η(t), at low shear rate γ up to a certain critical strain, beyond which ηE(t) tended to increase rapidly with t. The behaviour was often referred to as strain-induced hardening. For LDPE melt both σ(t) and Δn(t) versus tensile strain ɛ(t) (=ɛ0 t)curves were dependent on ɛ0 in such a manner that the stress optical coefficient C(t) ≡ Δ(t)/σ(t)) was independent either of ɛ0, ɛ(t), ɛ(t) or σ(t). Addition of UHMWPE up to 10 wt% to LDPE melt increased the levels of both σ(t) and Δn(t), but the tendency of strain-induced hardening was reduced. The C(t) was again independent either of ɛ0, ɛ(t) or σ(t) and also essentially independent of molecular weight (MW) and its distribution (MWD) or the blend ratio. For both LDPE and the blends the C(t) value roughly agreed with that (= 2.2 × 10−9 Pa−1) reported for shear flow experiments, thus confirming the val dity of the so far established stress optical rule.

Extrudate swell and isothermal melt spinning analysis of linear low density polyethylene using the Wagner constitutive equation

The rheological behaviour of a linear low density polyethylene melt is studied using a wide range of techniques: shear and elongational stress growth at constant strain rate, steady shear flow, extrudate swell and isothermal melt spinning. The analysis of the behaviour is based on the Wagner constitutive equation which appears to be suitable. Moreover, the extrudate swell and the isothermal spinning are simulated by taking into account the past shear and elongation deformation in the reservoir, the contraction and the die of a capillary rheometer. For this purpose, the Finger deformation tensor is evaluated using a Protean coordinate system. This approach is shown to be very useful to test the validity of the constitutive equation and to determine its characteristic parameters.

Viscoelastic flow past a confined cylinder of a low density polyethylene melt

The capabilities of the exponential version of the Phan-Thien-Tanner (PTT) model and the Giesekus model to predict stress fields for the viscoelastic flow of a low density polyethylene melt around a confined cylinder are investigated. Computations are based on a newly developed version of the discontinuous Galerkin method. This method gives convergent results up to a Deborah number of 2.5 for the falling sphere in a tube benchmark problem. Moreover, the specific implicit-explicit implementation allows the efficient resolution of problems with multiple relaxation times which are mandatory for polymer melts. Experimentally, stress fields are related to birefringence distributions by means of the stress optical rule. Three different fits, of equal quality, to available viscometric shear data are used: two for the PTT model and one for the Giesekus model. Comparison of computed and measured fringes reveals that neither of the models is capable of describing the full birefringence pattern sufficiently well. In particular it appears difficult to predict both the birefringent tail at the wake of the cylinder that is dominated by elongational effects and the fringe pattern between cylinder and the walls where a combined shear-elongational flow is present.

Effect of crystallinity on the mechanical properties of starch/synthetic polymer blends

Crystallization behaviour of starch and maleated blends was studied at 50°C over a period of 20 weeks using wide angle X-ray diffraction (WAXS). The variation of mechanical properties (tensile and flexural) and stress relaxation behaviour of the blends stored at 50°C and −10°C were studied over the same period. The starch content in the blends was 70% by weight. The synthetic polyolefins used in the blends were two grades of ethylene-co-vinyl acetates (EVA) containing 28% and 18% VA, two grades linear low-density polyethylene (melt index of 40 and 20) and high density polyethylene. An increase in the tensile properties of all the blends was observed in the first 5 weeks for samples kept at both temperature conditions. Blend samples kept at 50°C had higher tensile strengths than the ones at −10°C. Flexural strength remained constant over the duration of time. Freshly moulded specimens relaxed faster than the samples aged at either temperature. X-ray diffraction patterns showed that the starch was completely melted and had lost its crystallinity. Also, starch blends with EVA did not show any crystalline structure. The crystallinity in the starch blends with polyethylene was mainly due to the crystallinity of the synthetic polymer. The X-ray patterns of pure synthetic polymers were not found to be different from their functionalized counterparts. Crystal intensity was found to decrease for all the polyethylene blends. The effect of crystallinity on the mechanical properties is discussed.

A control volume approach for integral viscoelastic models and its application to contraction flow of polymer melts

A numerical scheme based on the control volume discretization is developed for integral-type viscoelastic models. The strain history tracking can be done more efficiently using finite volumes with staggered grids than using finite elements. The Semi-Implicit Method for Pressure Linked Equations (SIMPLE) with alternating direction implicit (ADI) iterations is employed to solve the momentum equations on staggered grids, with the non-Newtonian stress contribution integrated upstream along stream-lines and treated as a source term in each control volume. The new method gave better computing stability and solution smoothness than its finite element counterpart. The new scheme is used to simulate the 5.75:1 abrupt circular contraction flow of a low-density polyethylene (LDPE) melt with KBKZ model, for which experimental data on recirculation vortex growth with elasticity are available in the literature. A comparison of the numerical results with experimental data on the opening angle of vortex shows good agreement and some improvement over previous finite element computations. A mesh refinement study with three meshes demonstrated good mesh convergence. The present control volume scheme is found to be more than twice as fast as its finite element counterpart in dealing with integral viscoelastic models, at least in simple geometries.

The mathematical modeling of the inhibition mechanisms of sterically-hindered phenols in oxidizing low-density polyethylene melt

A kinetic investigation of the inhibition mechanisms of 2,6-di- tert-butylphenol and 2,6-di- tert-butyl-4-methylphenol in a low-density polyethylene melt oxidizing at 120, 130 and 140 °C has been carried out and the mathematical models of the mechanisms developed. At each temperature the key reactions in the mechanisms of action of the antioxidants were identified and the corresponding kinetic parameters determined. The comparison of the efficiencies of these antioxidants and 2,4,6-tri- tert-butylphenol studied earlier has been carried out.

Inhibition Mechanisms of Sterically Hindered Phenols in Oxidizing Low-Density Polyethylene Melt: Mathematical Modelling

Abstract The kinetic investigation of the inhibition mechanisms of 2,6-di-tert-butylphenol and 2,6-di-tert-butyl-4-metylphenol in low-density polyethylene melt oxidizing at 120, 130, 140°C has been carried out and the mathematical models of the mechanisms were developed. At each temperature the key reactions in the mechanisms of action of antioxidants were identified and the corresponding kinetic parameters were determined. The comparison of the efficiencies of these antioxidants and 2,4,6-tri-tert-butylphenol studied earlier has been carried out.

A numerical study of the flow of a low‐density‐polyethylene melt in a planar contraction and comparison to experiments

The flow of a low‐density polyethylene (LDPE) melt in a 10:1 slit‐die contraction at 150°C is simulated by using a two‐dimensional finite element program, and comparisons are made with the experiments of Kramer (1993) who used laser Doppler velocimetry to measure the velocity field at several flow rates. The LDPE melt is modeled by a Rivlin‐Sawyers integral constitutive equation with a spectrum of relaxation times where the parameters were determined from shear and elongational data. Evaluation of the general performance of our model and simulation at several flow rates shows that the simulation accurately predicts the vortex size but underpredicts the velocity overshoot along the centerline. Repeating Kramer’s particle tracking analysis of the flow field at one flow rate for a given set of streamlines we find good quantitative agreement for the elongation rates and relative stretch ratio, but we find our simulation generally underpredicts the shear rates and relative shear strain.

Experimental and numerical study of entry flow of low-density polyethylene melts

The present work deals with experimental and numerical features of entry flows of two polyethylene melts, namely a linear low-density polyethylene (LLDPE) and a low-density polyethylene (LDPE) in an axisymmetric converging geometry. The study also involves rheological characterization of the polymers and determination of flow parameters, at 160°C. For both fluids, the data are fed into a viscoelastic integral Wagner constitutive equation. The numerical flow simulations are performed by using a stream-tube mapping analysis. Consideration of a sub-domain of the total flow domain, the “peripheral stream tube”, close to the wall of the converging duct permits to relate the results of the numerical simulation to experimental flow characteristics as total and entrance pressure drops. The agreement is good for the total pressure losses, but, concerning LDPE, a lack of consistency remains for the entrance pressure drop.

Non-isothermal viscoelastic simulations of extrusion through dies and prediction of the bending phenomenon

Non-isothermal simulations have been undertaken for the flow of an IUPAC low-density polyethylene melt used previously in an international experimental study. First, extrusion flow from an infinitely long capillary die ( L R = ∞ ) is considered with isothermal walls and different thermal boundary conditions on the extrudate surface. Then the flow through flat dies is studied with the walls kept at different temperatures to create a non-isothermal flow. The viscoelasticity of the polymer melt is described by an integral constitutive equation of the K-BKZ type with a relaxation spectrum, which fits well experimental data for the shear and elongational viscosities and the normal stresses as measured in shear flow. The non-isothermal computations have been based on a pseudo-time method introduced earlier [X.-L. Luo and R. I. Tanner, Rheol. Acta, 26 (1987) 499–507] by applying the Morland-Lee hypothesis. The coupling between the momentum and energy equations is through the time-temperature shift factor by which the pseudo-time is defined. To avoid spurious oscillations in the temperature field due to high convection, a suitable upwind method has been used. The simulations have been performed for the full range of experimental measurements in the system before melt fracture occurs, that is, for apparent shear rates reaching 10 s −1 at 150°C, and showing a very strong viscoelastic character of the melt corresponding to a stress ratio ( S R) of about 2 and a Trouton ratio ( T R) of about 50. Stable solutions have been obtained for the whole range of experimental values. They show that the capillary die experiments were basically isothermal, due to small Nahme numbers (Na ⪡ 1), but that the Peclet numbers reach up to 44, having an influence on the cooling process occurring in the extrudate. The case of extrusion through flat dies with walls kept at different temperatures shows that the extrudate swell is strongly affected by the asymmetry in the temperature field. More importantly, bending of the extrudate occurs towards the wall with the cooler temperature. The present results aptly manifest the influence that viscoelasticity and thermal boundary conditions have on extrudate swell and are in qualitative agreement with previous experimental studies on the effect of temperature on extrusion flows.

Analysis of titanium dioxide agglomerate dispersion in linear low density polyethylene and resulting properties of compounds

AbstractThe dispersion of titanium dioxide agglomerates within linear low density polyethylene (LLDPE) melts has been investigated by using a cone‐and‐plate device installed within a temperature controlled oven. Observation of fragment evolution indicates that erosion is the predominant mechanism of dispersion. The erosion rate of titanium dioxide agglomerates in LLDPE was consistenty with the results obtained in previous dispersion studies of titanium dioxide in polydimethyl siloxane (PDMS). The effect of agglomerate dispersion on the optical, tensile, and dynamic mechanical properties of compounds incorporating titanium dioxide was also investigated. Changes in compound properties were correlated with observed morphology changes of agglomerates in order to clarify the effect of dispersion.

Polymer melt anisotropy in biaxial shear

The prediction of transient network theory is in reasonable agreement with the biaxial shear behavior of a polyisobutylene and a low density polyethylene melt in two different biaxial shear tests. Transient network theory provides a proper description of the development of melt anisotropy for highly nonlinear behavior in biaxial shear. The use of the nonaffine deformation hypothesis gives an improvement for the case of low density polyethylene, at high shear rate.

The Mathematical Model for the Inhibition Mechanism of 2,4,6-Tri-tert-butylphenol in Low-Density Polyethylene Melt Oxidizing at 120–140°C

Abstract The inhibition mechanism of 2,4,6-tri-tert-butylphenol in low-density polyethylene melt oxidizing at 120, 130, 140°C was studied using the procedures developed to study complicated processes of chain oxidation and inhibition in hydrocarbon materials. At each temperature the key reactions of antioxidant action were identified and the corresponding kinetic parameters determined. The analysis showed how the effectiveness of tri-tert-butylphenol changes as a function of temperature.

Antioxidant action in polymers: the limits of non-chemical synergism

Whilst low concentrations of a relatively inert compound octamethyl-tetrasiloxane increase the antioxidant effect of phenyl-β-naphthylamine in low-density polyethylene melt, the effect disappears at greater concentrations of this additive because the latter changes the polymer structure. The solubility and volatility of phenyl-β-naphthylamine correlate with its antioxidant effectiveness.