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Applied Rheology

SPE Library content related to rheology
Combining Boron Nitride with a Fluoroelastomer: An Enhanced Polymer Processing Additive
Manish Seth, Franky Yip, Savvas G. Hatzikiriakos, May 2001

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
Choon K. Chai, Gordon Adams, Jason Frame, May 2001

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
D. Chopra, M. Kontopoulou, D. Vlassopoulos, S.G. Hatzikiriakos, May 2001

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
Mark Grehlinger, Charles L. Rohn, Suneet K. Sikka, John Suwardie, May 2001

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
Alfonsius B. Ariawan, Sina Ebnesajjad, Savvas G. Hatzikiriakos, May 2001

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
D. Laroche, P. Collins, P. Martin, May 2001

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
David W. Riley, May 2001

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
Garth L. Wilkes, Matthew B. Johnson, Ashish M. Sukhadia, David C. Rohlfing, May 2001

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
Q. Xiang, M. Xanthos, S. Mitra, S.H. Patel, May 2001

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
C. Wan, M. Xanthos, S. Dey, Q. Zhang, May 2001

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
Donald De Laney, Jeffry Sweinhart, May 2001

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
Michael J. Bortner, David L. Godshall, Donald G. Baird, Priya Rangarajan, May 2001

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
Xingfu Shi, Paul A. O’Connell, Gregory B. McKenna, May 2001

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
L.M. Dehnke, Permadi, Jose M. Castro, May 2001

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
Sunny Jacob, Gary Lawrence, May 2001

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
Karen Xiao, Costas Tzoganakis, Hector Budman, May 2001

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
M.T. Martyn, T. Gough, R. Spares, P.D. Coates, May 2001

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.

Processing and Morphology of Hyperbranched Polyester Polyol/Polystyrene Blends
Christian D. Smialek, Nora Beck Tan, Donovan Harris, Tom Mulkern, May 2001

Recent work has shown that hyperbranched polymers have promise for use as processing aids for polyolefins and as toughening agents for thermosetting resins. This promise stems from the high reactivity and unique rheological properties of hyperbranched polymers, which are attributable for the unusual molecular structure. However, there are difficulties associated with the blending of these small, branched molecules into high molecular weight polymers, and processing can be challenging. In this study, we investigate the blending of hyperbranched polyester polyols (HBP) and high molecular weight polystyrene using batch and continuous processing techniques. The overall size, or generation, of the individual hyperbranched polyols is varied, as is the reactivity of the thermoplastic matrix toward to the polyol. Large reductions in system viscosity resulted from the addition of the hyperbranched polymers to the polystyrene. High energy processing and reactive compatibilization were effective in producing finely dispersed morphology in the blends. The processing characteristics, compatibility, and morphology of the blends are reported as a function of HBP generation, reactivity, type of processing, and shear rate.

Ethylene Styrene Propylene Terpolymers: Structure/Property Relationships
Martin J. Guest, Y. Wilson Cheung, Teresa P. Karjala, James M. Ruiz, Brian W.S. Kolthammer, May 2001

The terpolymerization of ethylene(E), styrene(S) and propylene(P) has been enabled by INSITE* Technology. ESP terpolymers differing in monomer composition ratio have been produced and characterized by solid-state dynamic mechanical spectroscopy and differential scanning calorimetry. Crystallinity and thermal transitions are correlated with the comonomer composition of the ESP terpolymers. Melt rheology and stress/strain behavior of selected ESP terpolymers are described and compared to ES and EP copolymers. Models developed to interpolate the characteristics of the terpolymers further help to develop structure/property relationships of these novel polymeric materials.

Process Monitoring at the National Institute of Standards and Technology: Celebrating 100 Years of Measurement Excellence
Anthony J. Bur, Kalman Migler, Steven C. Roth, May 2001

Two events that coincide in the year 2001 are the NIST centennial year anniversary and the inauguration of the SPE Special Interest Group on Process Monitoring and Control. To celebrate these two events, this paper will highlight polymer process monitoring activities at NIST describing the full range of measurement and sensor developments for real-time monitoring of resin temperature, rheology, morphology, molecular orientation, and dielectric properties of polymers, filled polymers and polymer blends.

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