Linear Low-density Polyethylene Melt Research Articles

Rheology of Crystallizing LLDPE.

Polymer crystallization occurs in many plastic manufacturing processes, from injection molding to film blowing. Linear low-density polyethylene (LLDPE) is one of the most commonly processed polymers, wherein the type and extent of short-chain branching (SCB) may be varied to influence crystallization. In this work, we report simultaneous measurements of the rheology and Raman spectra, using a Rheo-Raman microscope, for two industrial-grade LLDPEs undergoing crystallization. These polymers are characterized by broad polydispersity, SCB and the presence of polymer chain entanglements. The rheological behavior of these entangled LLDPE melts is modeled as a function of crystallinity using a slip-link model. The partially crystallized melt is represented by a blend of linear chains with either free or crosslinked ends, wherein the crosslinks represent attachment to growing crystallites, and a modulus shift factor that increases with degree of crystallinity. In contrast to our previous application of the slip-link model to isotactic polypropylene (iPP), in which the introduction of only bridging segments with crosslinks at both ends was sufficient to describe the available data, for these LLDPEs we find it necessary to introduce dangling segments, with crosslinks at only one end. The model captures quantitatively the evolution of viscosity and elasticity with crystallization over the whole range of frequencies in the linear regime for two LLDPE grades.

Soy‐filled polyethylene fibers for modified surface and hydrophilic characteristics

ABSTRACTBy adding soy flour (soy) to linear low‐density polyethylene (LLDPE), soy‐PE fibers with enhanced hydrophilic characteristics were developed. Blends containing only soy and LLDPE had limited draw‐down, and the resulting thick fibers showed poor mechanical properties. When monoglyceride was added as a compatibilizer, thin fibers with good properties could be successfully spun due to improved dispersion of soy agglomerates in the LLDPE melt. Fibers spun from a blend containing 23/7/70 wt % of soy‐monoglyceride‐LLDPE displayed a tensile modulus and strength of 615 ± 38 and 57 ± 8 MPa, respectively. At 30% less synthetic content, these fibers still displayed mechanical properties generally comparable to those of base polyethylene fibers. Contact angle measurements showed that the soy‐based fibers had a hydrophilic surface (contact angle of 33° ± 4°). Moisture absorption studies confirmed that soy‐PE fibers gained about 20 wt % moisture in 1 h, whereas neat LLDPE fibers did not absorb any significant amount (LLDPE is hydrophobic). This hydrophilic behavior of soy‐PE fibers mimics that of natural fibers. Presence small soy agglomerates on the fiber surface also provides a textured surface and a desired tactile feel to the soy‐PE fibers, which coupled with hydrophilic behavior indicates their potential use in disposable nonwovens. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46609.

Comparison of superimposed effects in high-shear-rate capillary rheology of polystyrene, polypropylene, and linear low-density polyethylene melts

Polymer melts exhibit unique rheological behaviors at high shear rate up to 106 s−1, which is a common phenomenon in micro-injection molding. Both online and commercial capillary rheometers, which were modified to allow regulation of back pressure, were used for measuring the melt shear viscosities of polystyrene (PS), polypropylene (PP), and linear low-density polyethylene (LLDPE) under high shear rates. The rheological characteristics of the three melts were compared through the systematical analyses for three significant effects, namely the end pressure loss, pressure dependence, and dissipative heating in capillary flow. Pronounced end effect begins to appear at the shear rates of 1.6 × 105, 8.0 × 105, and 2.8 × 106 s−1 for the PS, PP, and LLDPE melts, respectively. The significance of the end effect can be ordered as PS > PP > LLDPE. It seems that the polymers with more complex molecular structures exhibit a higher degree of divergence between the comprehensively corrected and uncorrected melt viscosity curves. Moreover, the dissipation effect begins to predominate over the pressure effect under the lowest shear rate of 105 s−1 for the PS melt among the three melts. POLYM. ENG. SCI., 55:506–512, 2015. © 2014 Society of Plastics Engineers

Simulation of Polymer Extrudsion Swelling by Using Iterative Stabilized Fractional Step Algorithm

With the introduction of the finite incremental calculus procedure a pressure stabilized mass conservation equation is reconstructed to overcome spurious oscillations of resulting pressure spatial distribution due to incompressibility of fluids. The discrete elastic viscous stress splitting method is used to retain an elliptic contribution in the weak form of the momentum equation in the absence of a purely viscous contribution or as the viscous contribution is negligible in comparison with the viscoelastic contribution. Inconsistent streamline upwinding method is employed to spatially discretize the constitutive equation of the Phan-Thien-Tanner viscoelastic constitutive model. The mass,momentum and constitutive equations are discretized and solved by the iterative stabilized fractional step algorithm based on Crank-Nicolson implicit difference scheme. The moving free surface is captured and determined in terms of the stream function. The isothermal extrusion swelling simulations for low density polyethylene and linear low density polyethylene melts are investigated. Numerical results demonstrate good agreement of numerical results obtained by the proposed algorithm with those given in the literature and experiment results.

Rheo-PIV analysis of the slip flow of a metallocene linear low-density polyethylene melt

The continuous extrusion of a metallocene linear low-density polyethylene through a transparent capillary die with and without slip was analyzed in this work by rheometrical measurements and particle image velocimetry (PIV). For this reason, a comparison was made between the rheological behaviors of the pure polymer and blended with a small amount of fluoropolymer polymer processing additive. Very good agreement was found between rheometrical and PIV measurements. The pure polymer exhibited stick-slip instabilities with nonhomogeneous slip at the die wall, whereas the blend showed stable flow. The slip velocity was measured directly from the velocity profiles and was negligible for the pure polymer before the stick-slip but increased monotonously as a function of the shear stress for the blend. The flow curves and the slip velocity as a function of the shear stress deviated from a power law and were well fitted by continuous “kink” functions. Comparison of PIV data with rheometrical ones permitted a direct proof of the basic assumption of the Mooney theory. Finally, the analysis of the velocity profiles showed that there is a maximum in the contribution of slip to the average fluid velocity, which is interpreted as the impossibility for the velocity profile to become plug like in the presence of shear thinning.

Identification of Pyrolysis Reaction Model of Linear Low Density Polyethylene (LLDPE)

Abstract This paper described how to estimate the Arrhenius parameters as well as the pyrolysis reaction model of LLDPE from isothermal kinetic data. We used a custom-made thermobalance that is capable of monitoring a weight decrease with time under pure static condition and performed six isothermal kinetic experiments at 713, 718, 723, 728, 733, and 738 K that were chosen within a temperature range (710–740 K) where main decompositions were observed from non-isothermal kinetic results. Comparing experimental reduced-time-plots (RTPs) with theoretical ones, the pyrolysis reaction model of LLDPE is accounted for by “Avrami–Erofeev” model in the investigated temperature region, allowing its functional form to be 2(1 − α)[−ln(1 − α)]1/2. It is, hence, expected that the reaction order model adopted by the previous studies without verification is inappropriate to represent the pyrolysis reaction model of LLDPE. Bubble nucleation may be a major pyrolysis reaction mechanism of LLDPE. As heat is applied, the LLDPE is melted. Volatiles may be accumulated inside the melt until reaching a critical concentration where bubble nucleation sets forth. The rates of heat, mass, or momentum transfer in the LLDPE melt and its physico-chemical properties (especially viscosity) play an important role in characterizing the thermal decomposition kinetics of LLDPE.

Extrudate swell behavior of PS and LLDPE melts in a dual die with mixed circular/slit flow channels in an extrusion rheometer

AbstractExtrudate swell behaviors of polystyrene (PS) and linear low‐density polyethylene (LLDPE) melts in a dual channel die, having mixed circular/slit flow channels, in a constant shear rate rheometer were examined. The extrudate swell ratio for PS melt was observed to be higher than that for LLDPE melt for all cases, this being associated with the differences in molecular structures that could be described in terms of power law indexes and secondary flows near the die entrance. In single channel die, the extrudate swell of both PS and LLDPE melts in circular flow channel die was greater than that in slit flow channel, whereas, in dual channel die the slit channel exhibited a higher extrudate swell ratio, the results being explained by revealing the flow patterns of the melt in the barrel and die of the rheometer. It was found that the dimensionless size of the vortex flows near the entrance, and the extent of disentanglement of molecular chains on entering the die were the important factors for the differences in the extrudate swell ratios of the melts at the die exit influenced by the die designs used. Copyright © 2003 John Wiley & Sons, Ltd.

Pressure profiles along an abrupt 4:1 planar contraction

AbstractThe ability to accurately predict the pressure profile along an abrupt 4:1 planar contraction is investigated. Predicted pressure profiles obtained using the Phan Thien and Tanner (PTT) and generalized Newtonian fluid (GNF) models are compared to experimental measurements for low‐density polyethylene (LDPE) and linear low‐density polyethylene (LLDPE) polymer melts. The results obtained for the extensional strain hardening, LDPE resin show that numerical predictions are consistently less than the experimental values by 10–15%. Furthermore, no significant difference between the PTT and GNF predictions were observed over the range of convergent solutions. On the other hand, the numerical predictions for the LLDPE melt were within 1.5–7% of the experimental values over the entire range of stable flow rates investigated. Again, the differences in accuracy of the PTT and GNF models were small. In fact, the GNF model was generally more accurate than the PTT model for the case of LLDPE. Closer investigation of the predicted streamlines patterns near the contraction clearly shows larger, more intense vortices for the extensional strain hardening LDPE than for the LLDPE melt, which is in agreement with experimental observations. However, analysis of the plane of symmetry shows that the magnitude of the planar extensional stress is mitigated by the fluid relaxation behavior as the melt briefly passes through the contraction region. Although it appears that a viscoelastic constitutive equation is not necessary for predicting the pressure profile along an abrupt 4:1 planar contraction, this is a fortuitous result of the small extensional deformations observed in the contraction region over the limited range of convergent solutions.

Ultrasonic improvement of rheological and processing behaviour of LLDPE during extrusion

AbstractThe effects of ultrasonic oscillations on die pressure, productivity of extrusion, melt viscosity and melt fracture of linear low density polyethylene (LLDPE) as well as their mechanism of action were studied in a special ultrasonic oscillation extrusion system developed in our Laboratory. The experimental results showed that, in the presence of ultrasonic oscillations, the melt fracture or surface distortion of LLDPE extrudate is inhibited or disappears. The surface appearance of the LLDPE extrudate was greatly improved. The productivity of LLDPE extrudate was increased in the presence of ultrasonic oscillations. The die pressure, melt viscosity and flow activation energy of LLDPE decreased with the rise in ultrasonic intensity. The shear sensitivity of LLDPE melt viscosity decreased due to the increase of its power law index in the presence of ultrasonic oscillations. Inducing ultrasonic oscillations into LLDPE melt greatly improved its processability. A possible mechanism for the improved processibility is proposed.© 2003 Society of Chemical Industry

Experimental studies on extrudate swell behavior of PS and LLDPE melts in single and dual capillary dies

AbstractExtrudate swell behavior of polystyrene (PS) and linear low‐density polyethylene (LLDPE) melts was investigated using a constant shear rate capillary rheometer. Two capillary dies with different design configurations were used, one being a single flow channel and the other being a dual flow channel. A number of extrudate swell related parameters were examined, and used to explain the discrepancies in the extrudate swell results obtained from the single and dual flow channel dies, the parameters including output rate and output rate ratio, power law index, wall shear rate, wall shear stress, melt residence time, pressure drop induced temperature rise, flow channel position relative to the barrel centerline, and the flow patterns. It was found in this work that the power law index (n value) was the main parameter to determine the output rate ratio and the extrudate swell between the large and small holes for the dual flow channel die: the greater the n value the lower the output rate ratio and thus decreased extrudate swell ratio. The differences in the extrudate swell ratio and flow properties for PS and LLDPE melts resulted from the output rate ratio and the molecular chain structure, respectively. The extrudate swell was observed to increase with wall shear rate. The discrepancies in the extrudate swell results from single and dual dies for a given shear rate were caused by differences in the flow patterns in the barrel and die, and the change in the melt velocities flowing from the barrel and in the die to the die exit. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1713–1722, 2003

Quantification of the slip phenomenon and the effect of shear thinning in the capillary flow of linear polyethylenes

AbstractThe slip phenomenon in the capillary flow of linear polyethylenes and its relation to their shear‐thinning behavior was studied in this work. A simple macroscopic analysis that allows a description of the trend of the slip length and the contribution of slip to the average fluid velocity as functions of the shear stress is presented. From this analysis it is shown that an increase or decrease of the slip length for increasing shear stress is determined by the polymer architecture and the competition between slip and shear thinning. A similar behavior is observed for the ratio between the slip and whole average fluid velocities. The analysis is validated using experimental data obtained during the continuous extrusion of linear low‐density polyethylene melts under strong slip conditions in a brass die.

The elastic behaviour during capillary extrusion of LDPE/LLDPE blend melts

A low-density polyethylene (LDPE) and a linear low-density polyethylene (LLDPE), with different melt flow index, were melt-blended in a screw extruder. The elastic behaviour, such as end pressure losses and die-swell, and the factors affecting it for the sample melts were investigated by means of a capillary rheometer at shear rates varying from 10 to 10 3 s −1 and a temperature range of 240–260°C. At lower wall shear stress ( τ w), the entry pressure drop (Δ P en) for LDPE and the blend melts increased quickly with increasing τ w, while for the LLDPE melt the entry effect was relatively significant, and the dependence of Δ P en on τ w was insignificant. The melt die-swell ratio ( B) increased approximately linearly with increasing τ w and Δ P en, while it decreased with increase of the die length/diameter ratio and with a rise of temperature. It was found that B achieved maximum value at a blending ratio of 50:50. This phenomenon can be attributed to the amount of elastic deformation energy stored in the melt flow and its transition due to the viscoelastic difference between the two phases.

Exploration of the slip phenomenon in the capillary flow of linear low-density polyethylene via electrical measurements

The slip phenomenon and the electrification occurring in the capillary flow of a linear low-density polyethylene melt were studied in this work using dies made up of stainless steel and brass, respectively. The experiments were carried out in a torque driven capillary rheometer at a temperature of 200 °C and spanned the different flow regimes observed in capillary flow, including stable and unstable conditions. Flow enhancement or slip was generated in the brass die, as compared to the stainless steel one, after an appropriate cleaning treatment. The melt exhibited electrostatic charge and sharkskin distortions were eliminated during the processing with the brass die in the stable flow regime prior to the stick-slip. Electrification and elimination of sharkskin distortions on the extrudates were observed when the slip velocity was a significant part of the whole average melt velocity (more than 50% in this work). The observations in this work point to a tribological origin for the electrification of the melt. Finally, an electric charge of a different sign was measured on the extrudates obtained from the stainless steel and brass dies, respectively, during the stick-slip, which suggests a detachment mechanism for slip in this flow regime.

Miscibility and isothermal crystallisation of polypropylene in polyethylene melts

Three blends were made, consisting of 20% polypropylene (PP) homopolymer and 80% high density polyethylene (HDPE), low density polyethylene (LDPE) or linear low density polyethylene (LLDPE). Isothermal crystallisation of PP, at temperatures where the PE remained molten was studied by differential scanning calorimetry (DSC) and hot-stage optical microscopy (HSOM) with polarised light. The resulting semi-crystalline morphology was studied by transmission electron microscopy (TEM). It was observed by HSOM that the PP crystallised as open armed, diffuse spherulites in the PP–LLDPE blend, similar to the crystal morphology observed in miscible blends, while in the PP–LDPE and PP–HDPE blends, PP crystallised in phase separated droplets. The crystallisation rate of PP decreased significantly in the PP–LLDPE blend, but in PP–LDPE, PP–HDPE blends, it was similar to that of the pure PP. The difference in crystallisation rates indicated that PP was much more dispersed in the LLDPE melt, either by dissolution, or greater dispersion of droplets without nuclei, or both. The DSC and HSOM results suggested that the PP was miscible with the LLDPE at elevated temperatures at a PP composition of 20%, while the PP was immiscible with the HDPE and LDPE at these temperatures. However, TEM showed that there was phase separation in the PP–LLDPE (20:80) blend as well, though the droplets were much finer. Nevertheless, the form of spherulitic growth implies a substantial amount of PP dissolved in the molten LLDPE.

EDGE EFFECTS IN FILM CASTING OF MOLTEN POLYMERS

In most analyses of the film casting process, edge effects such as neck-in and edge beading are usually neglected. In this work, we investigated the significance of these effects and their dependence on the rheological properties of the melts, the draw ratio, and the extrusion rate. Two linear low-density polyethylene melts and a low-density polyethylene melt were considered. The rheological behaviors of these melts were characterized under shear and elongational flows. Streamlines from the die exit to the chill roll, velocity profiles, film tension, neck-in profiles, thickness profile of the solidified film, and edge bead thickness profile were examined.

Temperature-dependent instabilities in the capillary flow of a metallocene linear low-density polyethylene melt

The capillary flow behavior of a metallocene linear low-density polyethylene was studied in a wide temperature range. The critical shear stress for the onset of the unstable spurt flow was found to be dependent on temperature in a nonlinear fashion and it showed a minimum value at a critical temperature, at which unusually long period pressure oscillations were observed. For temperatures above the critical one, the observed decrease of the critical shear stress with decreasing temperature is explained on the basis of an increase in the distance between entanglements. At temperatures below the critical one, the increase in the critical shear stress and the eventual suppression of pressure oscillations as the temperature is further decreased are suggested to be the result of a flow-induced phase change that ends on complete crystallization and suppression of flow. The flow-induced crystallization phenomenon and the extrudate quality were dependent on the contraction ratio. Elimination of surface extrudate distortions took place at low temperatures when using a contraction ratio of 30, this fact can be attributed to the flow-induced phase change. Finally, a decrease in the activation energy for flow or “easy flow” was observed at temperatures below the critical one. Such “easy flow” is perhaps the precursor of the “temperature window” of low-flow resistance reported by Keller and co-workers [Kolnaar, J. W. H. and A. Keller, Polymer 35, 3863–3874 (1994); Waddon, A. J. and A. Keller, J. Polym. Sci. Polym. Part B: Polym. Phys. 28, 1063–1073 (1990)].

The end pressure losses during short die extrusion of LLDPE melt

The end pressure losses and the factors affecting them during short die extrusion of a linear low density polyethylene (LLDPE) melt have been investigated under the experimental conditions of temperatures from 230 to 260°C and apparent shear rates of 12 to 120s−1, by employing a capillary rheometer and a set of dies with different diameter and length. The results show that the entry pressure loss, ΔPen, increases non-linearly with increasing shear rate, and is an index function of the wall shear stress; the dependence of ΔPen on temperature accords with a formula such as the Arrhenius equation; ΔPen decreases with increasing die diameter; ΔPen is more than 80% of the end pressure losses.

Crystallization of low-density polyethylene- and linear low-density polyethylene-rich blends

The crystallization of a series of low-density polyethylene (LDPE)- and linear low-density polyethylene (LLDPE)-rich blends was examined using differential scanning calorimetry (DSC). DSC analysis after continuous slow cooling showed a broadening of the LLDPE melt peak and subsequent increase in the area of a second lower-temperature peak with increasing concentration of LDPE. Melt endotherms following stepwise crystallization (thermal fractionation) detailed the effect of the addition of LDPE to LLDPE, showing a nonlinear broadening in the melting distribution of lamellae, across the temperature range 80–140°C, with increasing concentration of LDPE. An increase in the population of crystallites melting in the region between 110 and 120°C, a region where as a pure component LDPE does not melt, was observed. A decrease in the crystallite population over the temperature range where LDPE exhibits its primary melting peaks (90–110°C) was noted, indicating that a proportion of the lamellae in this temperature range (attributed to either LDPE or LLDPE) were shifted to a higher melt temperature. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1009–1016, 2000

Three-dimensional extrudate swell experimental and numerical study of a polyethylene melt obeying a memory-integral equation

The present work deals with the experimental and numerical features of the flow of a linear low-density polyethylene melt (LLDPE) at 160°C at the exit of a die of square cross-section. The rheological properties of the fluid are fitted by a Wagner's memory-integral constitutive equation. The characteristics of the extrudate jet are determined by optical means at different flow rates. The stream-tube analysis, already applied to two-dimensional extrudate swell problems involving rate and integral constitutive equations, is considered to simulate the flow field. The method avoids particle tracking problems related to integral models and allows computation of the unknown free surface by considering only a `peripheral stream tube' limited by the wall and the jet surface and an inner stream surface. Those boundary surfaces are determined by considering the conservation equations together with boundary condition equations, solved by the Levenberg–Marquardt optimization algorithm. The method leads to a considerable reduction in the number of degrees of freedom and the storage area. The numerical results are found to be generally consistent with the experimental data and highlight the growing importance of stress peaks due to the singularity at the exit when the flow rate increases.

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.