The SPE Library contains thousands of papers, presentations, journal briefs and recorded webinars from the best minds in the Plastics Industry. Spanning almost two decades, this collection of published research and development work in polymer science and plastics technology is a wealth of knowledge and information for anyone involved in plastics.
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Melting of Polymer Blends in a Shear Field - Experimental Investigation
The melting of polymers is one of the most difficult problems of the modeling of the process behavior in screw machines. Because it is a complex task to analyze and visualize this phenomenon directly in the extruder, experimental studies are often inadequate. Therefore a model apparatus with parameters close to processing conditions, which can generate a shear flow, was developed. With this apparatus, it is possible to analyze optically the structural modifications during the melting of polymer pellets in a surrounding melt. The melting process can be observed directly with a CCD camera attached to a microscope. In order to quantify the influence of the different parameters, investigations have been performed. For the melting, the most important factors are the material properties of the melt and the granule, the temperature field in the pellet as well as the environment and the flow characteristics.
Real-Time Crystallinity Measurements in a Multilayer Blown Film
Online measurement of crystallinity development in a multilayer blown film using Raman spectroscopy is discussed. The Raman spectrum of a multi-layer film consists of superimposed spectra from the individual layers. For polyethylene (PE) and polypropylene (PP), some peaks are distinctly different but some overlap (1295-1350 cm-1). Offline Raman experiments were used to measure the contribution of polypropylene from the total integral intensity calculated in the range 1295-1350 cm-1, so that crystallinity evolution can be calculated for the two components. Preliminary results suggest that Raman spectroscopy is a useful technique to monitor crystalline growth of PE and PP in multi-layer blown films.
Stability Analysis of the Nonisothermal Film Blowing Process
The stability of film blowing process has been investigated using the governing equations taking care of nonisothermal nature of the process. In this study, on top of the linear stability analysis, employing a newly-devised numerical scheme, the hitherto unavailable transient solutions of film blowing dynamics have been obtained for the first time to produce temporal portrayal of draw resonance instability strikingly close to experimentally observed profiles. Many interesting aspects of the film blowing stability have also been revealed including multiple steady states and their diverse stability characteristics.
Effect of Crystallization Kinetics on the Morphology of Linear Low-Density Polyethylene Blown Films
Previous real time studies during the fabrication of a blown film have reported that the imparted molecular orientation is dependent not only on the stresses acting at the freeze line but also on the crystallization process that takes place along the axial distance. In this study, the relationship between the crystallization kinetics, as estimated using real-time Raman spectroscopy measurements, and the film morphology was investigated for linear low-density polyethylene (LLDPE) blown films. Crystallization half-time (t0.5), defined as the time taken for the polymer to reach 50 % of its equilibrium crystallinity, was proposed as a single parameter to relate the processing conditions with the orientation of the films. The results showed an asymptotically decreasing relationship between crystallization half-time and the a-axis orientation factor for a range of processing conditions.
Rheological Evaluation of the Processability of Polyethylenes for Extrusion Coating
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.
Computational Design of U-Profile Die and Calibrator
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.
Performance of Boron Nitride and its Combinations as Processing Aid in the Extrusion of Ziegler-Natta Polyethylenes
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.
Optimizing Purging Time at Product Change in Blown Film Extrusion by Selective Modification of Process Parameters
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.
Lip Actuator Performance in Automatic Profile Control Systems for Flat Dies Extrusion
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.
Factors Affecting the Performance of Carbon Black Master Batches in Wire and Cable Applications
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.
Recycleability of Polymer-Clay Nanocomposites: Part 1 - The Influence of Multiple-Extrusions on Structure and Mechanical Properties
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.
Recycleability of Polymer-Clay Nanocomposites: Part 2 - The Influence of Multiple-Extrusions on Thermal and Rheological Properties
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.
Representation of Polymer Melt Rheology Data
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
Scale-Up Consideration for Polymerization in a Twin-Screw Extruder using 3-D Numerical Simulation
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.
Effect of Pre-made Compatibilizer and Reactive Polymers on Polymer Drop Breakup
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.
Eliminating Interfacial Instability at PE/Tie Interface Using a Multi-Layer Stacked Coextrusion Die
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.
Coextrusion of LDPEs in the Cast Film Process
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.
Investigation of Interfacial Instabilities in Nanolayer Extrusion
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.
The Effect of Using Preheated Pellets in Co-Rotating Twin-Screw Extruders
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.
Realtime Process and Quality Control in Polymer Extrusion - Detection and Sizing of Particulate Inhomogeneities in Flowing Polymer Melts
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.
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