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Applied Rheology

SPE Library content related to rheology
Viscoelastic Models with Logarithmic Strains
Donggang Yao, May 2014

Analysis of 3-D viscoelastic flows is of a particular interest in polymer processing. Classical approaches rely on the Finger tensor and its inverse for strain evaluation. In this paper, we present a general Maxwell model with a logarithmic tensor for strain determination. Since the logarithmic strain provides a better representation of the state of straining of the material, the resulting constitutive model is expected to have a number of advantages including more uniform straining in different directions and extended linear behavior towards moderate deformations. Model testing on simple shear with only two material constants demonstrated simultaneous prediction of shear thinning and first and second normal stress differences.

Modeling of Moisture Effects on the Rheological Behavior of Polymers and Influences on the Injection Molding Process
Felix A. Heinzler, Christoph Mielicki, Jens Siepmann, Johannes Wortberg, May 2014

By processing technical polymers like polyamide or polybuthylene terephthalate, possible causes for process variations are residual moisture of the material or different conditions with respect to time and temperature. To show influences on the quality of an injection molding process, the changes have to be measured and described by a rheological model. The material behavior is measured by a rotational rheometer and a high-pressure capillary rheometer at different conditions. Effects like thermal and hydrolytic degradation or viscosity reduction by residual moisture are examined during this investigation. Combining measurements to a rheological model allows the simulation of influences on the processing and compensations. Therefore a new extension of the Carreau-model is presented. Parameters describing the material behavior are completed by a thermodynamic and calorimetric analysis of the material.

Quality Improvement by Enhanced Pressure Controlled Injection Molding
Felix A. Heinzler, Maurice Mistler, Johannes Wortberg, May 2014

Today’s machine capability in injection molding is at a high standard and the variation of material properties are within a small range. Nevertheless variation in material properties and conditions influence the process- and product-quality. Examples are residual moisture or drying conditions by varying material handling. With respect to surface properties especially the injection-phase has a large influence on the part quality. By a recently developed process adapted pressure control during the injection phase a compensation of variations in the rheological behavior of the material is possible within the processing. Combined with a control of the switch-over point and packing-pressure, the quality of the process can be improved. Process variations by, for example, varying residual moisture content of the material are compensated by this new control strategy.

Influence of Moisture of the Basic Raw Materials on the Mechanical and Rheological Properties of Polylactide Cellulose Fiber Composites Made by Twin Screw Extrusion
Tobias Koplin, Hans-Josef Endres, Gerhard Ziegmann, Andrea Siebert-Raths, May 2014

In this experimental series the influence of the initial moisture content of the materials in the extrusion-technical production of Polylactide Cellulose Fiber Compounds are examined. The target values in the tests are the mechanical properties (such as impact strength and tensile strength), the rheological behavior, the color, and the molecular weight of the produced compounds. The compounding was done by using a co-rotating twin screw extruder. The results show a significant effect of the variation of initial moisture of the materials on the tensile and flexural strength and the discoloration of the compounds as well as the melt temperature during compounding. Furthermore, it was found that the rheological properties of PLA-cellulose fiber compounds are linearly dependent on the material moisture before experiment. The other target values show no dependence on the moisture of the raw materials.

Influence of Cellulose Fiber on the Foaming Behavior of a Polypropylene Copolymer
Bernd Geissler, Anna Uray, Stephan Laske, Clemens Holzer, Günter R. Langecker, May 2014

This paper investigates the effect of different cellulose fiber grades on the rheological properties and the foaming behavior of a linear polypropylene copolymer. Three cellulose fiber grades with different fiber lengths were used in this study. The average length of the fiber grades were 300 ?m, 500 ?m and 700 ?m. The preparation of the compounds were carried out on a co-rotating twin screw extruder and the foaming experiments were executed with a grooved feeded 45 mm single screw extruder. For the foaming experiments supercritical CO2 was used as blowing agent. All compounds were characterized with a cone plate rheometer to compare the influence of the fiber length and the amount on the rheological properties. The different formulations were compared, due to their effect on the maximum cell density, in terms of cell size and the foam density. It was shown that the effect on the materials viscosity increases with increasing fiber length. The foam morphology was not affected by the fiber length. However the volume expansion ratio depends on the fiber length.

Styrene-Butadiene Rubber- Surface Modified Carbon Nanotube Nanocomposites: Morphology, Rheology and Dynamic Mechanical Properties
Sateesh K. Peddini, Clive P. Bosnyak, Nancy M. Henderson, Christopher J. Ellison, Donald R. Paul, May 2014

Because of the exceptionally high modulus of multiwall carbon nanotubes (MWCNT), they can be used as reinforcing fillers in polymer and rubber nanocomposites. However, the commercial implementation of such nanocomposites has generally been met with very limited success owing to poor dispersion of the MWCNT in the polymer matrix. A strategy that overcomes many of these difficulties is described here with a view towards replacing a portion of the carbon black or silica with MWCNT for improved elastomer performance. Tire treads are very prone to experience micro-cracking at the edges, which eventually leads to overall failure. MWCNT can serve as good bridging elements to avoid the growth of micro-cracks if they are well dispersed and discreet in the rubber matrix. A concentrated, easy to process MWCNT-rubber masterbatch, with the freedom of diluting to various lower loadings, and feasibility of blending with different rubbers, would be of commercial benefit to the tire industry. Discreet oxidized MWCNTs were dispersed in an SBR matrix and the rheology, tube dispersion, dynamical mechanical, and electrical properties of these composites were examined. Morphology and mechanical properties of the cured nanocomposites were investigated and related to the effective aspect ratio of MWCNTs.

Morphology and its Influence on Rheological Parameters of Nanocomposites Filled with Layered Silicates Based on Polylactic Acid
Bernd E. Haar, Stephan Laske, Petr Stloukal, Sabine Bodner, Clemens Holzer, May 2014

In this research work the morphology of polylactic acid (PLA) filled with various types of layered silicates, as well as its influence on the rheological properties regarding melt strength and complex viscosity, were investigated. Thereby the morphology of the produced nanocomposites were examined with small angle X-ray scattering (SAXS) measurements. For the determination of melt strength a Rheotens apparatus, fed by a bypass system during extrusion, was used. Complex viscosities were measured with a rotational rheometer with cone-plate setup. Results show that the different modifications of the layered silicates have a significant impact on the final morphology as well as on the rheological properties of the produced nanocomposites. Beside different achieved interlayer distances and grades of intercalation of the layered silicates inside the polymer matrix, also increased melt strength as well as increased complex viscosities were observed. Thereby a homogenous dispersion and high grade of exfoliation is also preferable for the production of thin products like films.

A Preliminary Comparison between Long Glass and Carbon Fiber Composites in Shear Flow
Mark J. Cieslinski, Donald G. Baird, May 2014

Injection molding of fiber composites causes a complex microstructure formed by the intricate flow field created during the molding process. This work aims to understand the transient orientation evolution of long fiber suspensions in a well-defined flow and to ultimately apply the findings to complex flow fields. A sliding plate rheometer is used to measure the shear stress growth created during the startup of shear flow. Preliminary results show that a carbon fiber suspension produces a stress growth response similar to that of higher aspect ratio glass fibers, indicating similar orientation kinetics. Rheological observations are further investigated by experimentally measuring fiber orientation taken at different times during the startup of shear flow experiments.

Modeling of the Droplet Morphology Evolution in Polymer Blends under Shear by the Pseudo-Potential Lattice Boltzmann Method
Lin Deng, Zhigao Huang, Yun Zhang, Shubiao Cui, Huamin Zhou, Dequn Li, May 2014

In this work, a novel modeling strategy is proposed to simulate the morphology evolution of polymer blends by the inter-particle-potential lattice Boltzmann method. In this model, the binary components of the polymer blends are tracked by two sets of density distribution functions governed by lattice Boltzmann equation. The model is verified by computing the droplet behavior of deformation, breakup and coalescence under shear, and the results are found in good agreement with the theoretical and experimental observations.

A Mesoscale Simulation Method for Underfill Encapsulation
Hui Wang, Huamin Zhou, Lin Hua, Yun Zhang, Dequn Li, May 2014

Underfill process is applied in flip chip encapsulation to prevent interconnection failures caused by the mismatch of CTE between die and substrate. In order to better characterize the capillary flow in underfill process, and otherwise to eliminate the troubles in calculating the capillary force in current underfill simulation methods, i.e., result-based and interface reconstruction, we present a mesoscale underfill simulation method based on three dimensional lattice Boltzmann method. In our method, the improved interparticle-potential model is used to model the fluid-fluid interaction, and Benzi's model is used to model the solid-fluid interaction. A geometric model for underfill simulation is then developed, and allows each surface to have a different wettability by assigning a different mesoscale interaction parameter. For verification purpose, two underfill cases are examined. It has been demonstrated that the proposed method has a good performance in the underfill simulation.

Morphological Analysis of Natural Fibers and Fiber Orientation Measurements for the Evaluation of Simulation Tools for Injection Molding Materials – NFC-Simulation
Katharina Albrecht, Jörg Müssig, May 2014

Due to environmental and sustainability issues, the request for renewable resources increases. Natural fiber-reinforced injection molded materials are therefore an interesting prospect for the automotive industry. To achieve a broader market launch of this new material in the automotive industry, numerical simulation of this new material is essential. Besides rheological and mechanical properties, the fiber morphology and the fiber orientation are the most important properties for the simulation. To evaluate the simulation results experiments are necessary. The morphology of natural fibers (sisal, hemp and regenerated cellulose fibers) was determined by image analysis of the original fibers and the fibers after the procedures of compounding and injection molding. Therefore the fibers were extracted from the granules and the injection molded components. The size of the fibers was significantly reduced during the compounding process, whereas no further reduction could be observed during the injection molding process. Quantitatively, the same results could be found in simulation based on a mechanistic model. Fiber orientation measurements were done via TeraHertz Spectroscopy to evaluate the simulation of the injection molding process and to be able to predict the mechanical properties of the components.

Compatibilizing Immiscible Blends from Polyethylenes and Polyamide Using Reactive Extrusion
Christoph Burgstaller, Carlo Augusto Puppo Bigarella, Bernhard M. Riedl, Wolfgang Stadlbauer, May 2014

The aim of this work was to investigate the possibilities of compatibilizing immiscible blends of HDPE – PA6 via reactive extrusion. We investigated the influence of the compatibilization on the mechanical and rheological properties, as well as the morphology of the samples was investigated. We found, that it is possible to compatibilize immiscible blends via the in situ production of a compatibilizer from a pre-cursor and a radical generator in the blends. The effectiveness of this method is comparable with the compatibilization via the addition of pre-fabricated, industrially available additives.

Effect of Molecular Structure on the Heat Seal Performance of Polypropylene Films
Redouane Boutrouka, Sayed H. Tabatabaei, Abdellah Ajji, May 2014

Three polypropylene resins (homopolymer, ethylene copolymer and elastomer based ethylene copolymer) were selected to investigate the effect of molecular structure on the heat seal performance of polypropylene films. The molecular structure of the resins was analyzed using dynamic rheological measurements and gel permeation chromatography (GPC). Thermal analysis was also performed to determine crystallinity and melting points. Heat seal test was conducted on multilayer cast films and it was found that the seal initiation temperature (SIT) and seal strength depend on the ethylene comonomer content, crystallinity, and molecular weight. The metallocene based resin having low branching content and narrow molecular weight distribution showed the lowest SIT.

Reactive Modification of High Density Polyethylene in a UV-Initiated Process
Pouyan Sardashti, Costas Tzoganakis, Alexander Penlidis, May 2014

Rheological properties of a high density polyethylene resin (HDPE) were modified by promoting long chain branching (LCB) through a novel photoinitiated reactive extrusion process (REX). Surface response methodology based on a central composite experimental design was employed with three processing variables, namely, photoinitiator concentration, polymer throughput, and extruder screw speed. The linear viscoelastic properties measured through oscillatory shear experiments indicated addition of LCB up to 0.055 branches per 1000 monomer units. The zero shear viscosity (?o) increased to a maximum of 11,600 Pa.s from a starting value of 1,900 Pa.s. Similarly, the average polymer relaxation time (?) increased from 0.05 s to 4 s. Both molecular weight (MW) and molecular weight distribution (MWD) slightly shifted toward higher values. However, the breadth of the distribution was not affected significantly.

Influence of Ultrasonic Treatment in PP/CNT Composites Using Masterbatch Dilution Method
Jing Zhong, Avraam Isayev, Keyuan Huang, May 2014

Multiwalled carbon nanotube (CNT) filled polypropylene (PP) composites of various concentrations were prepared by a twin screw extruder using direct compounding (DC) method without and with ultrasonic treatment. In addition, a masterbatch of 20 wt% PP/CNT composites were prepared without and with ultrasonic treatment and diluted to the same concentration as in the DC method without ultrasonic treatment. This is called the masterbatch dilution (MD) method. The rheological and electrical properties were investigated. The increased storage modulus and viscosity as well as the electrical conductivity indicates a better dispersion of CNT in PP matrix prepared by the MD method than by the DC method. The fractal dimension of CNTs, D, and the backbone fractal dimension, x, of the CNT network was determined by fitting the rheological data to the scaling model. The lower fractal dimension of CNT and higher backbone fractal dimension of CNT network in composites prepared by MD method compared with composites obtained by DC method indicates a better dispersion of CNT. Additionally, a lower value of D and a higher value of x as well as a higher storage modulus, viscosity and lower electrical percolation were achieved when the ultrasonic treatment at an amplitude of 13 ?m was applied in the MD method, indicating an advantage in use of the ultrasonic treatment in preparing the PP/CNT masterbatch. The macrodispersion was determined using optical microscopy to correlate the processing, properties and structure. It was shown that the MD method provided better dispersion of CNT in PP matrix than the DC method.

Dynamics and Hysteresis in Capillary Rheometry
David O. Kazmer, Amir Moshe, Stephen Johnston, Robert Malloy, Samuel Kenig, May 2014

Several sets of experiments were conducted to investigate the dynamics and hysteresis in capillary rheometry. Experimental data indicate that the imposed flow rate history can vary the apparent viscosity by a factor of 10, a magnitude as significant as a change between the minimum and maximum recommended processing temperatures. Contributing factors that were investigated and found insignificant included the capillary length:diameter ratio, viscous heating of the polymer melt, rheometer transmission, and instrumentation. Other contributing factors to be analyzed further include polymer compressibility and viscoelasticity.

Device for Measuring Microscale Viscosity in a Self-Aligned Microgap
Danyang Zhao, Huiqing Tian, Minjie Wang, Donggang Yao, May 2014

A novel rheometer for measuring shear viscosity in a self-aligned microgap was developed. Drag flow of simple linear motion was created inside two self-aligned parallel plates. A ball-bearing contact between the loading and moving parts was employed to facilitate self-alignment so that gap thickness smaller than those in standard rheometers can be obtained. Analysis based on the lubrication approximation indicated that a self-balancing liquid film can be generated between the two plates during rheological measurement. The new device was successfully used to measure the viscosity of selected fluids.

Binary Blends of Cycloolefin Copolymers
Emre Unsal, Alex Xue, Yatin Patil, Hoang Pham, Andrew Poslinski, May 2014

Binary blends of different grades of cycloolefin copolymers (COC) were prepared at different compositions via compounding in a twin-screw extruder. Thermal, mechanical and rheological properties of the COC blends were studied by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and rotational rheometry. Results revealed the differences in the miscibility of different grades of COCs. This could be attributed to differences in the microstructures of the COCs determined by their synthesis method (catalyst type) and norbornene content of each component. Blends of Topas 8007 and Topas 5013 were immiscible but they were compatible to a certain degree. Two distinct Tg’s were observed at DSC and DMA, with a slight shift upon changing the composition of the blends. DSC data was used to model this behavior using two parameter Kwei model for the prediction of the Tg for the immiscible COC blends (8007/5013). Rheology data of the blends revealed that the phase inversion occurs at 45%-55% for the blends of two different Tg COCs. On the other hand Topas 8007 and Topas 6013 blends were miscible and single Tg was observed at DSC. Fox equation was used to model the Tg behavior of the miscible blends and the experimental results were in close agreement with the model.

Curing Study of a Green" Thermosetting Resin"
Michael Davis, John P. Droske, Wei Zheng, May 2014

A series of green" thermosetting resins oligomers of bis(hydroxyalkylene)-2-mercaptosuccinate has been reported recently [12]. Curing in these thermosetting resins results from crosslinking via pendant thiol groups. As part of an effort to realize and assess the potential of these resins as sustainable materialsthe curing process was investigated using differential scanning calorimetry (DSC) and rheology. The progression of physical and mechanical properties such as the glass transition temperature (Tg) and the shear moduluswas monitored as a function of time and temperature. Tg of the resin was found to increase with curing and the averaged Tg of the fully cured resin was found to be 72.6 K ± 1.2 K higher than uncured resin. The increase in Tg corresponded with the change in rheological properties. The shear modulus obtained for fully cured samples reached a high modulus of 6.5 × 106 Pa at 200 °C. Additionallythe gel point was measured from the crossover of the storage and loss moduli. Based on the gel points the apparent activation energy of curing also was determined."

Thermal and Rheological Behavior of Plasticized Polyacrylonitrile and Melt Spinning of Precursor Fibers
Jianhua Huang, Donald G. Baird, Felix L. Paulauskas, May 2014

While there is tremendous interest in the melt spinning of polyacrylonitrile (PAN) with presence of plasticizers, the behavior of plasticized PAN melt and the spinning process have not been adequately investigated so far. In this paper the melting and rheological behavior was studied for PAN plasticized with water or water and a second plasticizer. A lab scale pressurized fiber spinning system was developed and used to generate melt-spun fibers as carbon fiber precursors. The morphological and mechanical properties of the fibers were evaluated and compared with commercial wet-spun fibers. The meltspun PAN fibers have surface and mechanical properties similar to those of commercial fibers.

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