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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The Melting Behavior of Polyethylenes Synthesized with Ziegler-Natta and Constrained Geometry Catalysts
The melting flux and shear stress values during melting of five polyethylenes were measured using a Screw Simulator. Four materials were produced using constrained geometry catalyst Technology (CGCT), while the fifth was produced with a traditional Ziegler-Natta (ZN) catalyst. All of the materials have nearly the same melt index value, but the solid density of each material varied. The melting flux was measured over a range of sliding velocities, temperatures and pressures typical in single screw extrusion. Melting flux values for all materials increased with velocity, temperature and applied pressure. The materials produced with CGCT had a higher melting flux than the ZN material under the same experimental conditions, in all cases. Experimental data were compared to existing melting flux and shear stress models.
The Melting Characteristics of Polycarbonate Resins
Melting of a polymer inside a single-screw plasticating extruder is described by a dissipative melting mechanism of the solid bed rubbing on the barrel surface. Both the melting rate and the shear stress of a solid bed are expected to increase with decreasing melt index (i.e., with increasing viscosity). However, the melting rates of polystyrene (PS) in controlled Screw Simulator tests were found to decrease with decreasing melt index even though the shear stresses increased. Such unexpected results were reported previously (1).This paper will report the results of controlled Screw Simulator tests for polycarbonate (PC) resins with varying melt indices and also on the melting phenomena in single-screw plasticating extrusion experiments.
Melting Model for Co-Rotating Twin-Screw Extruders
A comprehensive model describing the last stage of the melting process in co-rotating twin-screw extruders is proposed. At this stage, the melting is governed by viscous dissipation and heat transfer from the melt to the unmelted particles. The residence time in the melting section mainly determines the final melt temperature and the remaining fraction of unmelts.The effects of screw speed, polymer viscosity and particle size on the melting process are shown, leading to implications for the new high-speed, high-torque machines. The model is supported by experiments using a screw configuration in which the melting section is placed at the downstream end of the screw.
Melting of High-Heat Polyamide in a Co-Rotating Twin Screw Extruder
Experimental work is presented on the melting behavior of high-heat polyamide in co-rotating twin screw extruders. Screw designs were tested with different melting sections located near the exit of the extruder, to isolate the melting performance. A slit-die provided a melt ribbon that was drawn down for visualization of unmolten particles that together with melt temperature quantify melting performance. This method proved to be efficient in scanning an entire process window. Evident are effects of screw speed, degree of fill, screw configuration, and low melting additives. These experimental directions, along with model simulations are the basis to optimize compounding screws covering modern hi-speed, hi-torque process windows.
Melting Regimes in Modular Intermeshing Co-Rotating Twin Screw Extrusion: Effect of Barrel Temperature Screw Speed and Feed Rate
The melting of polyolefins in a modular co-rotating twin screw extruder was investigated as a function of (i) barrel temperature (ii) screw speed and (iii) feed rate. Carcasses of the melting region were removed from the twin screw extruder and sliced into sections.Our observations suggest three different mechanisms of melting; (i) Hot barrel induced melting, (ii) Screw surface induced melting, and (iii) Homogeneous bulk melting. The mechanism of (i) is favored by high barrel temperatures, the mechanism of (ii) by lower barrel temperatures and slower screw speeds, and the mechanism of (iii) by lower barrel temperatures and higher screw speeds.
Micro Molding - A New Way""
Small parts less than one quarter of a pellet require a complete rethink by molding engineers and mold designers. Total part cost justification becomes more challenging based upon the amount of material wasted and tooling costs. Micro machinery is helping to meet the requirements of accuracy through simplicity. There is no need for complicated multiple profile closed-loop servo controlled injection and metering functions.The two-stage plunger over plunger injection unit is capable of processing basic polyolefins to high temperature LCP materials. Shot pot injection provides accuracy of volumetric control by eliminating variation caused by slider or check ring shutoff inconsistency. The minimum number of components used in the design achieves simplicity.
Microcellular Foaming of Polypropylene Containing Wood Fibre in an Injection Moulding Process
Microcellular foams of polypropylene containing wood fibres, cell sizes on the order of 10 to 50?m were produced in an injection moulding process. The relationships of processing/structure/property were investigated for wood fibre-thermoplastics composites foaming with a chemical blowing agent. Wood polypropylene composites (WPC) of different wood content (30%,40%,50% and 60% by weight) have been prepared using maleic anhydride-polypropylene copolymer (5% relative to the wood fibre content) as a coupling agent. Measurement of density, cell size, void content, tensile and flexural test of the prepared WPC were carried out. The shape and distribution of the voids were investigated by optical photo examination of longitudinal sections of specimens, using polishing technique and reflected light microscopy. Density of foamed composites decreased about 24%. The cell morphology and foam properties showed improvement when the coupling agent was added. Water absorption and scanning electron microscope of the composites also investigated.
Micro-Scale Extrusion for the Accelerated Development of New Polymeric Materials
Polymeric materials exhibit complex degradation mechanisms during their processing and end use. Rapid development of new polymer formulations requires new methods of processing. We have investigated increasing the throughput of such experiments by using a micro-scale (4.5-cm3 volume) twin-screw extruder. A method has been developed for evaluation of stabilizer performance that significantly reduces the amount of material and experimental time when compared to traditional methods. The method employs a micro-extruder and specific processing conditions. Validation of the method was performed on multiple polymer/additive combinations with different oxidative stabilities. The rank order of degradation of the materials in the micro-extruder correlates well (R2 = 0.982) with the results of multiple pass extrusions in traditional scale equipment.
Microstructure of Blow Molded Bottles from Polyolefin Nanocomposites Prepared by Melt Compounding
Polypropylene-organoclay and high density polyethylene-organoclay nanocomposite pellets were extruded by melt compounding using an industrial-scale co-rotating twin screw extruder. The as-extruded pellets were then made into bottles by using an industrial-scale single screw extrusion blow molding machine outfitted with a screw involving shearing sections. The microstructure of bottles as investigated by wide-angle X-ray diffraction and transmission electron microscopy exhibited partial exfoliation with minor regions containing locally exfoliated clay platelets and major regions of intercalated clay.
Miniaturized Sensor for the Measurement of Temperatures in a Mold Cavity
The prediction of shrinkage and warpage of injection molded parts, especially from semi-crystalline polymers, still poses a major problem to the polymer processing industry.Attempts have been made to incorporate crystallization kinetics into simulation programs. The heat released by crystallization in combination with the low heat conductivity of plastics strongly influences the temperature profile in the cavity and thereby the cooling and cycle times. This could be shown with a special apparatus, which has been developed recently, for the fast cooling of polymers in connection with computer simulations (1, 2).For the measurement of the temperature distribution in the mold cavity a miniaturized sensor has been developed. The sensor design and measured temperature profiles are presented here.
Mixing and Structure Development in 3D Chaotic Mixing Flows
This paper addresses mixing, interfacial area generation and structure developement in Kenics static mixers. A statistical description of the microstructure development is obtained using the extended mapping method. This method is adopted to the special flow conditions in spatially periodic flows, of which a static mixer is an example. The efficiency of the interface generation for different mixer layouts is compared and additional attention is given to the distribution of the interfacial area across the mixer. It is shown that the extended mapping method enables us to find the blade configuration that optimizes the mixing performance, in accordance to the standard mapping method, but now including much more details concerning the microstructure development in this chaotic flow.
Mixing Behavior of Model Miscible Polymer Systems Having Extremely Low Viscosity Ratio
Hydrocarbon based oils can be used to plasticize styrenic block copolymers. At high levels (30%), the method of oil addition and the properties of the oils used will affect mixing time. This becomes very important in twin screw compounding processes where increased throughput reduces residence time (available mixing time). This paper describes the investigation of factors affecting mixing of several model polymer systems having a very low viscosity ratio (well below 0.001) using a batch internal mixer. Similar to the findings of Ratnagiri, Scott, Joung, Shih & Burch (1-5) on morphological development during mixing of immiscible and miscible polymers, we have observed Phase Inversion (PI) during mixing of miscible polymer systems of block copolymers with hydrocarbon oils (6). The time to reach high torque after addition of the hydrocarbon oil, i.e., the Phase Inversion (PI) time as defined by Ratnagiri and Scott (2), decreased with increasing viscosity and hydrocarbon oil molecular weight. It was shown that splitting of the oil addition could decrease total PI time. It was also shown that an unequal split, with the lowest amount first, led to the fastest PI times. This emphasized that a slight lowering of the major component viscosity with small additions of the plasticizing agent was the most advantageous process for decreasing total time for mixing. In addition, it was shown that part of a lower viscosity (or MW) oil could be substituted with a higher viscosity oil thereby reducing overall Phase Inversion time. Of course, it would be important that the substituted hydrocarbon oil be compatible in the final product.
Mixing in Extrusion - Parts One, General Considerations
Introduction Mixing is one of the important functions of a plasticating extruder. Other functions are solids conveying, melting (or plasticating), melt conveying, and, in vented extruders, degassing (or devolatilization). It is well recognized that mixing is important when different plastics are blended or when fillers are added to the plastic in an extruder. However, it is not widely understood that mixing is equally important when a single plastic is extruded. In this case mixing is necessary to achieve a thermally homogeneous melt at the end of the extruder. Plastics have very low thermal conductivity resulting in large differences in melt temperature in the absence efficient mixers along the extruder screw. When the extruder discharges a melt into the die with non-uniform temperatures the flow in the die and the extruded product quality will be adversely affected.
Model of Isothermic Laser-Sintering
In the product development of plastic components, increasing use is made of laser-sintering (LS) processes [1, 2, 3, 4]. To improve properties of prototypes, the main goals of development are reproducible density, to maintain edge sharpness, and to prevent uncontrolled shrink-age. Today, R+D mainly focuses on laser-technology, development of scan strategies, and LS process optimization.Another approach to make LS even more effective for product development is to identify the most important material properties of possible raw materials (polymer powder). The knowledge of significant material proper-ties could be an important tool for the choice of the correct material as well as for the development of new raw materials (structure- and chemistry synthesis).Thus, the current research of our group summarized in this paper mainly focuses on formulating requirements for LS raw materials . Therefore, the theoretic model of isothermic laser-sintering was developed. Based upon this model it can be shown that the most important requirements for raw materials refer to crystallization and melting behavior as well as surface tension and melt viscosity.
Model Studies in Enzyme Catalyzed Transesterification Reactions
Lipases are now known to also function as polyester synthases. In a previous report, we showed the ability of Lipase B from Candida antartica (Novozyme-435) to catalyze transesterification reactions between preformed polymer chains. To further study the kinetics and mechanism of these reactions, model reactions were performed using benzyl alcohol as the nucleophile and various aliphatic polyesters as substrates. Effects of the reaction temperature, time, nucleophile concentration, and the structure of the polymeric substrates on the rate of transesterification are reported. We also report the extent that these transesterification reactions occur with selectivity with respect to the site on chains that is cleaved.
Model-Based Control of Material Distribution in Thermoformed Parts
The thermoforming industry has achieved a good understanding of the process, which has been in large scale operation since the 1950's. Consequently, control of machine settings such as heater band temperatures, plug position, plug and mould temperatures is quite advanced. However, to date, little work has been done to address the control of state parameters describing material behaviour during processing, such as sheet temperature and material distribution in the part. Control of state parameters is essential as material property changes, environmental factors and machine operating drifts can significantly change the dynamic operating point of the machine.
Modeling of the Temperature Profile in Film Coextrusion
The question of temperature in film coextrusion is an important issue for the quality of the product. Since a direct measure of the temperatures in the melt is difficult, the modeling gives the values of the local temperatures, and the velocity profile as well. Important is the determination of the temperature at the interfaces of the layers, since this allows to compute the viscosities and the stresses at the interfaces. The ratio of the stresses is important in order to predict if the flow is stable, and to avoid instabilities. The knowledge of the temperature makes possible to avoid local overheating. We present a software running on a PC, a good tool to predict local peaks of temperature.
Modeling Peroxide Crosslinking in Polyolefins
By combining sol-gel analysis and curemeter testing with a Monte Carlo simulation, the 160°C dicumyl peroxide initiation efficiency and scission/crosslinking ratio of a metallocene catalyzed ethylene 1-octene elastomer was determined to be 48% and 0.24, respectively. Using a calibrated simulation analysis, the network structural evolution during crosslinking was determined. Commonly used methods of curemeter interpretation were found to be severely flawed due to the combined effects of nonlinear evolution of elastically active chains, trapped entanglements, and slowly relaxing chain structures. A framework for correct interpretation of cure behavior is described.
Modeling Stress-Strain of Glassy Polymers up to Yield
Based on a picture of a polymeric glass as a mosaic of nanoscale clusters of differing viscoelastic characteristics, we propose a new model for glassy polymers that accurately captures the stress-strain dependence at different rates and temperatures from small strain up to yield for polycarbonate. The model allows one to interpolate and extrapolate limited experimental data (it requires three stress-strain curves as input). The model also provides insight into the fundamental issue of glassformer fragility" in the glassy state and a practical means to assess dynamic inhomogeneities within polymeric glasses."
Modeling the Temperature Variation of Intrinsic Viscosity Using a Temperature Dependent Scaling Exponent
Intrinsic Viscosity measurements are usually analyzed using the empirical Mark-Howink-Sakurada equation, which gives a power-law dependence of the intrinsic viscosity to the molecular weight of the polymer. Variation of the scaling exponent, a" with temperature is only poorly understood necessitating individual measurements at each temperature for every polymer/solvent system. The temperature dependence of "a" is shown to fall on a single universal curve under appropriate rescaling of the temperature of the solution. A method to obtain "a" from the static exponent is also described."
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