SPE Library

The two key processability characteristics of low-density polyethylenes (LDPE) in extrusion coating applications are principally concerned with the degree of neck-in (NI) and the drawdown ability (DD). Molecular structures like long chain branching (LCB) and molecular weight distribution (MWD) have strong influence on these processability parameters, which, in turn, can be correlated with the melt elasticity of the polymer at processing conditions. These molecular structures effects on melt elasticity can suitably be studied by shear and extensional rheologies. A range of extrusion coating LDPE grades have been rheologically characterised and two models relating their extrusion coating processability parameters (NI and DD) and their melt elastic properties have been established. These new rheological parameters and models enable rapid quality controls and evaluation of potential materials, from catalyst development to products scale-up, for extrusion coating applications, with the desirable processability in terms of NI and DD.

The objectives of this work are three-fold: 1) to validate that an open profile product can be made using simulation alone to design a die (design rules of thumb are not applied), 2) assess the role of the calibrator in shaping the free surface of the extrudate, and 3) quantify the heat transfer in a vacuum calibrator and assess how such data can be used to design calibrators for other products. The scope of this paper includes: 1) three-dimensional flow simulation to design a die to make a U-shaped extrusion, 2) experimental trials to validate the mass flow balance through the die and the cooling performance of the calibrator, and 3) comparison of the heat transfer data obtained during calibration trials to published data for profile calibrators.

The performance of Boron Nitride, and its combinations with fluoropolymer as polymer processing aid in the extrusion of Ziegler-Natta polyethylenes is studied by using a capillary rheometer fitted with capillary and crosshead die. Two different techniques of compounding the polymer with the additives are used. In the first technique, both additives (Boron Nitride and fluoropolymer) are compounded together directly into the polymer. In the second technique, the additives are added separately. Improved performance via the second technique is observed.

During the product change from colored into colorless especially at blown film extrusion, long purging times arise because of the long continuing colour haze. The purging behavior has been investigated in low density polyethylene (LDPE) films under variation of the process parameters mass flow rate and temperature. Our analysis shows that the improved purge process control and variation of the process parameter modifications during the purging process reduce the purging time and substandard goods compared to the conventional arrangements. Also the study examines the influence of different color pigments into the purging behaviour.

Thermal translators, along with the connected hardware and software controls, represent the core of automatic control of flexible lips in flat die extrusion. The thermo-mechanical efficiency of said translators has a direct impact on the process for ensuring proper gauge uniformity in film extrusion, thus affecting the quality and the economy of the manufacturing process. This paper will investigate the potential advantages and disadvantages related to translators based on different materials of construction and their basic thermal and mechanical performance. The work presented is the result of analytical and FEA computer simulations.

Cable jackets containing carbon black (CB) are one of the most important components in communication and power cable constructions. This jacketing provides protection for the underlying layers from physical abuse, chemical attack, and UV degradation. There are two ways of manufacturing these types of cable jackets: either by fully compounding the product or by blending a combination of a resin and a black master batch (MB). In the latter approach, a main requirement is to have excellent CB dispersion in the MB. Poor dispersion in the MB results in unacceptable mechanical properties and poor UV-resistance in the finished jacket product.This paper describes a series of single screw extrusion experiments which were carried out with let-down resins of different structures, and master batches (MB) to investigate the effects of the resin structure, MB, and extrusion conditions on acceptability of the extrudates for jacketing applications. The MBs employed for this investigation are commercial products of which dispersion is acceptable based on the manufacturing QC testing. Under certain extrusion conditions with these MBs, extrudates had unacceptable surface quality and UV absorption coefficient (termed as ABS, < 400nm ) for a given range of CB content in the finished product (2.5 to 2.7 wt%). The unacceptable properties resulted from poor melt homogenization of the MB with the letdown resin at the extrusion conditions used. Good homogenization was obtained by conducting the extrusion/compounding at higher shear stress. The high shear operation can be accomplished by several methods: selection of lower MI let-down resin (within resin spec); narrow the molecular weight distribution (MWD); lower temperature profile; and higher screw speed. The employment of tighter screen packs also showed some improvement in homogenization. A possible mechanism for the good homogenization (CB dispersion) of the MB will be given.

The effects of multiple-extrusions, up to eleven cycles, on the structure and properties of virgin and nanoclay-filled nylon-12 were investigated. X-ray diffraction and transmission electron microscopy studies showed that the degree of clay exfoliation improves with each successive extrusion sequence up to the seventh cycle. The impact strength of the nanocomposites were enhanced while that of the virgin nylon-12 deteriorated. Regrinding of the nanocomposite resin prior to subsequent extrusion was shown to further improve the clay exfoliation and mechanical performance.

Nylon-12 and nylon-12/clay nanocomposite were recycled by up to eleven times using the melt-extrusion process. Changes in thermal and rheological properties were investigated using DMTA, DSC, TGA, and capillary rheometry techniques. Both materials showed a gradual decrease in phase transition temperatures and storage modulus following repeated extrusions. In addition, the materials melt viscosity increased in response to successive reprocessing. Relative to the nylon-12, the melt viscosity of nanocomposites was reduced by more than 20% and their glass transition temperature was elevated by about 2.0 to 6.5degC depending on the number of extrusion cycles.

Polymer melt Rheology data is often represented as a power law function or as a 6 constant polynomial when used in various flow or extrusion simulation packages. Both representations are inaccurate at various shear rates due to the functionality of the representation. With the introduction of more polymers showing broad Newtonian regions, the need for better constitutive representation is needed to accurately estimate viscosity as a function of shear rate and temperature in simulation packages. This paper will compare the fit of various equations and constitutive relations to capillary rheometer data for polymer melts to determine an efficient representation of the Newtonian and power law regions

Scaling up a polymerization process in the fully filled screw elements of co-rotating twin-screw extruders was investigated with a 3-D Finite Volume Method. The simulation results show that, with identical mean residence time in the screw elements, the polymerization progression is accelerated when the screw diameter is increased. Moreover, the difference in conversion based on 1-D and 3-D models becomes more significant with increasing screw dimension, indicating that 1-D model can not predict the polymerization progression correctly in large extruders. The polymerization requires short screw length (L/D) to complete when the screw diameter is increased. This may be due to the fact that the heat loss through the barrel surface is more difficult to occur, and the non-uniformity in temperatures at the cross section of screw elements becomes more pronounced after scaling up.

The deformation and breakup of a single polymer drop with and without compatibilizer (pre-made or insitu) inside a polymer matrix at high temperatures under simple shear was visualized in a specially designed transparent Couette mixer. The uncompatibilized polymer systems studied were polyethylene matrix/polystyrene drop (PE/PS) and polyethylene matrix/polystyrene oxazoline drop (PE/PSOX). The compatibilized systems were PE/PS with polyethylene-block-polystyrene copolymer (P(E-b-S)) and PSOX with polyethylene maleic anhydride (PEMA).Two main kinds of breakup are observed: (a) with or without compatibilizer, the drop elongates in the vorticity direction and then breaks up in the same direction, and (b) in the presence of compatibilizer, a thin cylinder stretches and ruptures from the elongated mother drop in the vorticity direction.

In multi-layer coextrusion, interfacial instability is a commonly encountered issue, which can limit the process operating windows. There are many factors that affect interfacial instability including material properties and die geometry. A Brampton Engineering 7-layer Stacked Coextrusion Die, called Streamlined Coextrusion Die (SCD), with three different die geometries was used to study the onset of interfacial instability. Several commonly used PE/tie combinations were investigated. The PE materials used included LDPE, HDPE and LLDPE of different viscosities. Elongational viscosity was found to be important in detecting the onset of interfacial instability. An attempt was also made to correlate the experimental results to computer simulations.

This work shows results of experiments on a coextrusion cast film line and is focused mainly on studying conditions for the onset of zig-zag interfacial instabilities. The measurements were performed for a wide range of LDPEs polymerized by different polymerization methods. It is shown that for a group the materials prepared by a similar polymerization method there exists a critical shear stress value at the interface that determines the onset of the instability. The values determined for different material groups are different and it seems that there is a correlation between the molecular weight distribution width and the critical shear stress value. The broader is the MWD the lower is the critical shear stress value.

Multilayer structures have been prepared by coextrusion technology using a feedblock with layer multiplying elements (LME) to split the melt stream and then layer the two streams together. Polycarbonate (PC), polypropylene (PP), and a cyclic olefin polymer were extruded to form alternating structures by repeated layer multiplications using the layer multiplying elements. The materials were characterized by microscopy to study the effect of polymer properties and process conditions on layer instabilities. Instabilities were found at the layer interfaces, predominantly in the center of the samples, resulting from high interfacial shear stresses. This could be reduced by increasing the temperature of the LMEs and decreasing the total flow rate of both materials.

Experiments with a co-rotating twin-screw extruder show that a considerable throughput increase can be achieved by feeding the extruder with preheated pellets. The use of preheated pellets is also advantageous in that, for a given throughput, the extrusion of preheated pellets requires a lower torque. Thus, when shifting from cold to preheated pellets, the screw speed can be lowered until the original torque is again reached. As a result, the temperature of the exiting melt is lower too. The use of preheated pellets thus gives an extra degree of freedom in terms of the maximum throughput of a given extruder, and control of degradation-sensitive polymers.

By application of different unique microphotometric sensors it is possible to detect and quantify disturbing particles (gels, unmolten resins, black spots, additive agglomerates, bubbles) within a flowing polymer melt in realtime during extrusion processing. These particles result in disadvantageous optical and mechanical properties of a final polymer product. Sensors can be adapted inline and online to different extruders at various positions. This seems to give technical and economic benefit to quality control and process optimisation in polymer processing.

In this study, we performed RTD measurement at the die exit of co-rotating twin screw extruder using a non-destructive ultrasonic device. The ultrasonic device was composed of a steel buffer rod and 10 MHz longitudinal piezoelectric ultrasonic transducer. Steel buffer rod is a safety implement for the protection transducer against hot operating temperature. This in-line method is based on the ultrasonic response of a filled polymer, where the solid particles act as a tracer. To determine RTD, calcium carbonate (CaCO3) was used as a tracer. The RTD measurements involved the use of ultrasonic tracer and the measurement of the variation of ultrasonic signal strength with time caused by the tracer concentration change. The ultrasonic tracer, pellet types of compounded CaCO3 in polymer were used in this study. The effects of CaCO3 concentration on RTD and flow patterns were studied in the extrusion of a thermoplastic resin. Experiments on the residence functions of different screw speed, feeding rate and screw configurations were also carried out.

Polytetrafluoroethylene (PTFE) is a remarkable material having high melting temperature, high chemical resistance, low frictional and dielectric coefficients, etc. Due to its high melting point, PTFE cannot be processed using the conventional methods such as the injection molding, extrusion and blow molding, etc. In this research, PTFE is processed by a number of techniques including paste extrusion, rolling and calendering. It is necessary to preform the PTFE powder-lubricant mixture before extrusion to ensure paste densification. The processing behavior of extruded PTFE pastes was first studied. The length of extension zone was changed to investigate the variation of extrusion pressure profile. Two-colored preform paste packed sequentially was used to observe the flow behavior in extrusion process. It was found that the extrusion pressure increases in the reduction zone and decreases after the paste passed the extension zone. Increasing the packing time in the performing will result in a more steady extrusion pressure. Higher extension length would raise the required pressure for paste extrusion. Furthermore, it was also found that an increase in the lubricant content increases the extent of density uniformity. The paste flow exhibits laminar behavior of viscous fluid. However, highly non-Newtonian characteristics and slip boundary also occur.

Polymeric flow in intermeshing co-rotating and counter-rotating twin-screw extruders is simulated. Effect of the elongational viscosity of the polymer on the flow in the two extruders is included by using independent Carreau models for the shear and elongational viscosities of the polymer. It is found that for similar screw cross-sections and rotational speed, axial velocity as well as degree of mixing is higher in the co-rotating extruder, whereas pressure build up is higher in the counter-rotating extruder. In contrast to the flow in the co-rotating extruder, where the velocity was always maximum at the screw tips, in the counter rotating extruder the velocity was higher in the intermeshing zone.

In previous studies1-3, both rheological and extrusion characteristics of a 50% wood-HDPE composite as well as its virgin HDPE resin were investigated. In this paper, the extrusion characteristics of the blends made up of these two materials were studied. It was found that for a screw which has the higher compression ratio, for all the blends, the pressures increased with increasing RPM with the exception of 50% HDPE-wood composite at 50RPM; whereas for a screw with lower compression ratio, for the 36% and 50% wood-HDPE blend, there was no pressure generation at any RPM even though the output increased in a nearly linear fashion. The effect of temperature on pressure generation was also looked at and will be presented here. Similar to the previous studies, the experimental results were compared to those simulated using a commercially available computer program, Flow 2000™.