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SPE Library content related to rheology
Die Swell Estimation for HDPE Blow Molding Grade Resins Using the Wagner Model Constitutive Equation
A polymer flow analysis inside a concentric annular die of an extrusion blow-molding machine was conducted using momentum and continuity balances coupled with the Wagner rheological constitutive equation for six high density polyethylene (HDPE) resins and then used to determine the thickness swell. The rheological measurements included oscillatory behavior, relaxation modulus, steady state behavior, and capillary flow.
Polyethylene Powder Characteristics: Impact on Polymer Sintering and Rotational Molding
The effect of powder characteristics on the rotational molding and sintering performance was investigated. The six LLDPE resins showed comparable rheological and thermal properties. Resins with poor powder quality produced parts with lower density and impact strength. The irregularities in shape due to grinding influenced the sintering results. To eliminate this effect on the sintering experiments cylinders were used. Variations in the sintering results, however, were also observed when using cylinders.
Polylactide, A New Thermoplastic for Extrusion Coating
Polylactide (PLA), a new thermoplastic derived from corn, has been developed for extrusion coating applications. Polylactides can be rheology modified to run on high-speed commercial extrusion coating lines to produce packages with unique properties. New packaging systems will be described based on PLA's combination of adhesion, flavor/aroma barrier and heat seal/hot tack strength.
Combining Boron Nitride with a Fluoroelastomer: An Enhanced Polymer Processing Additive
The effect of a new processing additive (boron nitride powder in combination with a fluoroelastomer) on the rheology and processability of molten polymers is studied. The equipment used include an Instron capillary rheometer equipped with a special annular die (Nokia Maillefer wire coating cross-head), a twin screw extruder equipped with a blow moulding unit, and a parallel-plate rheometer. Metallocene polyethylenes with and without boron nitride (BN) and fluoroelastomer are tested in extrusion and conventional high density polyethylenes with and without BN with fluoroelastomer are tested in Blow moulding operations. First, it is demonstrated that BN is a superior processing aid compared to conventional fluoropolymer ones. Secondly, it is found that the combination of BN powders with a small amount of a fluoropolymer improves even further the processability of molten polymers (melt fracture performance).
Polyethylene Die Deposit-Measurement, Formation Mechanism and Routes to Reduction
Die deposit (sometimes known as die drool, die lip build-up, etc) occurs in melt extrusion of polyolefins. It is an undesirable build-up of material, normally on the lip or open faces of extrusion dies. In commercial scale polyethylene extrusion processes (e.g. blown or cast film, fibre spinning, etc), die deposit can have a significant influence on productivity, through the need to shut down processing line periodically to clean the die, and on the end-product quality. There are a wide range of factors or sources that lead to the formation of die deposit as its mechanism is not understood, these include low molecular weight species and volatiles, die swell, die design and polymer structure. This paper investigates, through laboratory-scale extrusion and rheology evaluations, some of the key relationships in the die deposit formation process. A deposition mechanism is thus proposed from these studies, with some suggestions for its reduction.
Effect of Maleic Anhydride Content on the Rheology and Phase Behavior of Poly(Styrene-Co-Maleic Anhydride)/Poly(Methyl Methacrylate) Blends
The thermo-rheological properties of lower critical solution temperature (LCST) poly(styrene-co-maleic anhydride) (SMA)/polymethyl methacrlylate (PMMA) blends, with varying amounts of maleic anhydride (MA) content (8%, 14% and 32% by weight) in the SMA component have been investigated, using differential scanning calorimetry and small amplitude dynamic oscillatory rheological methods. The effect of MA content on the phase behavior of SMA/PMMA blends has been determined. The resulting phase diagrams have been modeled using Flory-Huggins theory.
Rheological Characterization of the Molecular Weight and Molecular Weight Distribution of Linear Polyethylenes
It has been known for many years that the rheology of linear single phase polymer melts depends strictly upon their molecular weight and molecular weight distribution. Recently, theoretical relationships have been developed that permit transforming rheological data into molecular weight information. Rheology has several advantages over GPC or LC in determining the molecular weight distribution of linear polymers. For one, rheology is highly sensitive to the high molecular weight tail, which is usually excluded in chromatographic separations. These fractions dominate the elasticity of the polymer melt, which greatly affects processing behavior. Generally, in order to determine the entire molecular weight distribution of a polymer, the rheological data must characterize the complete range of relaxation times between the plateau and terminal regions. In practice, such measurements are very tedious and time consuming, and can involve multiple tests run at several different temperatures, with different sets of conditions. A solution to this problem has been developed by Mead et. al. that allows incomplete rheological data to be combined with appropriate mathematical models to produce the molecular weight distribution curves. This paper reports on the transformation of the frequency dependent viscoelastic material functions to molecular weight and molecular weight distribution curves of LLDPE samples using this method. These results are compared with the molecular weight distribution curves obtained from GPC.
Rheology of Polytetrafluoroethylene as Related to Paste Extrusion
The rheology of polytetrafluoroethylene (PTFE) pastes has been studied using an Instron capillary rheometer. Four different grades of PTFE have been tested. Three of them had a homopolymer structure with different molecular weights. The fourth polymer had a slight degree of branching due to the incorporation of less than 0.5% by weight of another perfluorinated monomer. The investigated parameters included those related to the die design (contraction angle, capillary diameter and length to diameter ratio), extrusion conditions (temperature, and extrusion speed) and the molecular weight and structure of PTFE. It was found that these parameters significantly affect the capillary extrusion pressure, which is important in the creation of fibrils during paste extrusion. Quantification of fibrils in the extrudate has been attempted by the use of Raman microscopy. An attempt has also been made to correlate the degree of fibrillation with the tensile strength of the extrudate.
Modelling of the Effect of Slip in Plug-Assisted Thermoforming
In plug-assisted thermoforming, the interaction between the sheet and the plug strongly affects the final part thickness distribution due to sheet cooling and slippage on the plug surface. The type of plug material and surface finish has to be carefully selected. The amount of slip on the plug surface depends on the rheology of the polymer sheet and on the friction coefficient. Both properties are temperature dependent. In this work a non-isothermal friction coefficient model is evaluated for its potential in predicting the amount of slip in plug-assisted thermoforming. The model has been implemented in a finite element analysis software for predicting the consecutive steps of the thermoforming process. The model has been applied to simulate industrial scale plug-assisted thermoforming and the predictions are compared to experimental measurements.
Rheology beyond One Million Reciprocal Seconds
In the wire and cable industry the speed of the insulated copper wire through the die is very fast. In 1963 a 24 gage wire was coated with plastic with a clearance in the die of only 5 mils (1 mm = 40 mils, i.e., 5 mils = 0.125 mm) at a speed that exceeded 2500 feet/min (almost 1 kilometer/minute). The shear rate calculates to be in excess of one million reciprocal seconds (actually this is 4 x 106 sec-1). Drastic changes were discovered in the molecular structure of the plastic. Shearing the polymer chain causes changes in the molecular structure that can be advantageous or severely detrimental. These running conditions and the effect on the PVC is the concern of this paper.
Optical Haze Properties of Polyethylene Blown Films: Part 1-Surface Versus Bulk Structural Considerations
In this paper we report on some recent findings regarding the factors affecting the optical (haze) properties of polyethylene (PE) blown films. The large majority of the contribution to the total haze in these blown films was a result of the surface roughness of the films, with the bulk (internal) contribution being relatively minor. Using several characterization techniques, we found, rather unexpectedly, that the surface roughness in some of these films was a result of the development of distinct spherulitic-like" superstructures formed during the blown film processing. Analysis of the rheological and molecular characteristics led us to conclude that in blown films of LLDPE-type resins the optical haze properties are adversely affected due to enhanced surface roughness caused by the formation of "spherulitic-like" superstructures in polymer melts that possess fast relaxing and low melt elasticity rheological characteristics."
VOCs Emissions and Structural Changes of Polypropylene during Multiple Melt Processing
Polypropylene, as a commodity recyclable thermoplastic, is studied in this research to evaluate the potential environmental impact resulting from volatile organic compounds (VOCs) emitted during multiple reprocessing. Unstabilized commercial polypropylene (PP) grade was processed several times by injection molding. Samples were examined after each cycle for total VOCs emissions with a flame ionization detector (FID) and cumulative VOCs emissions were obtained after each processing step. Corresponding structural changes were investigated with Fourier Transform Infrared (FTIR) Spectroscopy and results were correlated with rheological data that showed decreasing viscosity particularly after the 7th processing cycle.
Extrusion Foaming of PET/PP Blends
In order to develop new applications for recyclable commingled resin streams, blends containing PET and PP resins with different rheological characteristics were dry blended or compounded at different ratios and subsequently foamed by using PBAs and CBAs. Properties of the foamed blends were compared with those of similar products obtained by foaming the individual PET and PP components in the absence of compatibilizers/rheology modifiers. Foamed polymer blends with fine cell size and low density could be produced in the presence of suitable compatibilizer systems consisting of functionalized polyolefins or their combinations with reactive coagents
A New Method of Rheological Analysis for Polymer QA and Product Development
For many years the capillary rheometer has served as an important tool in R&D and product development laboratories. It has even gained in popularity in the QA laboratory as a tool for assuring the processability of a resin. More recently two and even three barrels designs of the capillary rheometer have become more generally available. The main applications of these instruments have been for providing directly corrected viscosities and for doing measurements of extensional properties. However, very little has been said about the opportunity these rheometers provide for applications in QA and product development. This paper discusses the capabilities of the dual barrel rheometer for direct comparisons of materials in QA and for the fast and easy observation of changes in the flow properties of a new product, as it is modified in the development process.
Plasticization with Carbon Dioxide to Facilitate Melt Spinning of High Acrylonitrile Content Materials
The effects of plasticizing Barex, a commercial polyacrylonitrile/methyl acrylate (PAN/MA) copolymer, with carbon dioxide (CO2) are studied. Differential scanning calorimetry (DSC) is used to evaluate the resulting shift in the glass transition temperature (Tg) of Barex following plasticization. Pressurized capillary rheometry is used to evaluate the differences in melt rheology prior to and after plasticization. Dynamic rheology data is used to perform an Arrhenius type analysis that allows an estimation of the shift in processing temperature. An estimated 20°C decrease in processing temperature can be obtained upon CO2 plasticization, which slows the kinetics of the copolymer degradation (crosslinking) at processing temperatures.
Rheological Response of Polystyrene/Ortho-Terphenyl Solutions
The linear and nonlinear viscoelastic responses of concentrated solutions of polystyrene in ortho-terphenyl are being investigated. Three aspects of behaviour are being addressed: 1) the time-temperature and 2) the time-concentration dependence of this material system and 3) the time-strain separability of the response in the non-linear regime. Linear viscoelastic responses are being measured using an ARES rotary rheometer and the non-linear response will be investigated using an RMS7200 where both the torque and normal force response in torsional deformations will be measured. The polymer is a commercial specimen having a weight average MW of 192,000 g/mol. Concentration is being varied between 0.2 and 0.7 mass fraction of polystyrene. Since time-strain separability is fundamental to the tube theory of reptation, the non-linear response will be discussed in the reptation framework.
Application of Chemo-Rheology to Establish a Process Window for a New Solventless System to Manufacture Pre-Pregs and Laminates for Electronic Applications
The most common commercial processes for manufacturing pre-pregs for electronic applications use solvent-based epoxy systems. Solvents are environmentally unfriendly and contribute to voids in the pre-preg and laminate. Voids cause product variability, which is a major source of scrap in board shops. In this paper, we use chemo-rheological and kinetic measurements to identify a potential epoxy-based resin system for a solventless process, based on injection pultrusion. DSC and rheological data show that the candidate system does not react appreciably without catalyst to temperatures of 170°C or with catalyst at temperature below 110°C. The system solidifies below 105°C. It was found that the overall viscosity of the resin system is dependent upon the temperature, degree of cure, and filler content. Kinetic rate and viscosity rise expressions to be used in process modeling and optimization have been developed. A preliminary process window for the process has been established.
Novel Thermoplastic Elastomers (TPEs) for Automotive Interior Applications: TPEs with Improved Molded Surface Appearance and Reduced Fogging
Injection molded parts from today's commercially available thermoplastic elastomers (TPEs) including thermoplastic vulcanizates (TPVs) can show surface defects such as halos, gloss variations, gate blemishes, flow lines, etc. Novel TPEs that overcome the above mentioned defects were developed for injection molded applications. These new materials, produced by dynamic vulcanization, showed significant improvements in reducing surface defects in molded parts and also showed greatly reduced fogging (i.e., low volatile release) as compared to existing commercial TPEs. The fogging, measured at 100°C for 16 hrs, for these new TPEs were remarkably low (0.7 - 1.3 mg) compared to standard commercial SEBS-based TPEs which were measured at 2.0 - 2.3 mg. Rheology studies indicated that these materials have a unique flow behavior which may be responsible for the improved surface appearance. The new TPEs are known as Santoprene® thermoplastic rubber M300. These are suitable for injection molding of automotive interior parts such as coin trays, mats, cup holders, etc., where aesthetics of the parts are important.
Modification of Rheological Properties of LDPE for Coating Applications
The effect of shear modification and blending on the coating performance of a commercial LDPE resin was studied. Shear modification of the virgin resin as well as of its blends with another LDPE was performed in a single and in a twin screw extruder. The rheological properties of the modified materials were measured and no significant differences in their behavior under shear deformation were detected. On the other hand, changes in the entrance pressure drop and extensional viscosity were observed. These changes in extensional behavior were found to affect coating properties. Overall, neck-in improved through shear modification while it was not sensitive to blending with a higher molecular weight resin. On the other hand, blending had an effect on the draw-down speed and it may be used to achieve a balanced coating performance.
Visualisation of Melt Interface in a Co-Extrusion Geometry
Past studies, principally confined to numerical simulations, have led to several theories being proposed to explain the source of interfacial instabilities that often occur during coextrusion of polymer melts. However the mechanisms, kinematics and rheological correlations leading to the onset of instability are not yet fully understood. Instability is thought to arise from one of two locations within a die. One location is that coincident with the point where an interface is created when the melt streams combine; the other region is a point just prior to the die exit region where the interface usually experiences a maximum shear stress. It is further suggested that interracial instability can develop when the layer thickness ratio of the melt streams exceeds a critical limit. This paper presents the results of flow visualization studies performed in order to gain a better understanding of the interracial instability phenomena of polyolefin melts within a flat film coextrusion die. Visualisation studies have been initially confined to melts of the same viscoelastic properties, using a new modular flow cell. The objective of the experiments is to determine the effects of geometry and processing on the source and nature of flow instabilities in coextrusion.
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