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Applied Rheology

SPE Library content related to rheology
Producing Long Chain Branched Polymers from Linear Polyolefins
Edward M. Phillips, May 2015

Polyolefins, (polyethylene and polypropylene) are arguably the most commonly used thermoplastics used today in a broad range of processes and applications. There are hundreds of types of polyolefins that can be produced directly from the polymerization reactor considering
co-monomer type and concentration, molecular weight and molecular weight distribution etc. However, even with today?s state of the art process technology, not all of the polymer attributes required by many applications are achievable at the reactor level. Of the polyolefin family, only low density polyethylene inherently exhibits excellent melt strength resulting from it?s highly long chain branched structure. While the physical properties of linear low density polyethylene (higher tear and puncture resistance), high density polyethylene (higher stiffness, toughness) and polypropylene (higher stiffness, clarity, high heat resistance) may be more desirable for a particular application, they all exhibit poor melt strength due to their linear structure.

This paper reviews the history, techniques and practical application of modifying readily available, ?reactor grade? linear polyolefins to produce long chain branching. Some of these materials exhibit rheological properties comparable to or exceed the melt strength of low density polyethylene while maintaining 100% of their original properties. Examples of their commercial practicality will be discussed in terms of value in application.

Viscoelastic Models with Rotational Recovery
Donggang Yao, May 2015

Classical models for viscoelastic fluids typically assume that relaxation only occurs along the axial direction of the conformation tensor. A unsatisfactory consequence is that such models over-predict the strain softening effect in rotational flow such as simple shear. In this work, a new paradigm is proposed to formulate relaxation models with consideration of rotational recovery. Particularly, the relaxation models in the form of F=LF-1/?*(lnV+V^n*lnR*V^(-n)), where F is the deformation gradient, V is the left stretch tensor, R is the orthogonal tensor from polar decomposition, ? is a relaxation time and n is a material parameter, are particularly useful, with a flexibility embedded to adjust the straining and softening effect in 3-D flows. With this model, realistic shear thinning and elongational thickening can be simultaneously modeled and general agreements with experimental results are demonstrated.

Effect of Ultrasonic Treatment on Electrical and Rheological Percolation Threshold of Polycarbonate-Carbon Nanotubes Composites
Xiang Gao, Avraam Isayev, May 2015

A twin-screw extruder having an ultrasonic treatment zone was used to prepare polycarbonate (PC)/multi-walled carbon nanotubes (CNT) composites. The effect of ultrasonic amplitude and CNT concentration on processing characteristics, rheological properties, electrical conductivity and mechanical properties of both high (HPC) and low (LPC) PC filled with 0.2-1.5 wt% CNT was studied. Ultrasound showed significant effect on improving the dispersion of CNT in both HPC and LPC composites, as indicated by the increase of storage modulus at low frequencies, decrease of the rheological and electrical percolation threshold. Specifically, the rheological percolation of LPC composites decreased from 0.10 vol% for untreated samples to 0.055 vol% for treated samples at an ultrasonic amplitude of 13 æm. Meanwhile, the electrical percolation threshold of LPC composites decreased from 0.176 vol% for untreated samples to 0.088 vol% for treated samples at an amplitude of 13 æm. Additionally, obvious improvement in mechanical properties (including elongation, tensile strength, yield strength and Young?s modulus) of HPC composites after ultrasound treatment at 13 æm was observed. Finally, a possible mechanism of the decrease of both rheological and electrical percolation threshold by ultrasonic treatment was proposed.

The Influence of Rheological Properties of the Material Formulation on the Cell Size and Cell Density of Physically Foamed Polyethylene
Matthias Walluch, Anna Uray, Bernd Geissler, Stephan Laske, Clemens Holzer, Guenter Langecker, May 2015

In this study physical foaming of low density polyethylene (LDPE), high density polyethylene (HDPE) and different blends of the two polymers was examined. The rheological properties of formulations were characterized using a cone plate rheometer to compare the influence of the used polymers. The foaming behavior was investigated by using a grooved single screw extruder with a screw diameter of 45 mm. For the experiments three different die geometries were used to show the influence of the interaction of the deformations and the rheological properties of the polymer melts. For the study azodicarbonamide, normally used as a chemical blowing agent, was used as nucleating agent and supercritical CO2 as blowing agent. The aim of this study was to determine, how the rheological properties of the material formulations influence the cell size and the cell density of the polymer foams and to find out if there are correlations between the relaxation times of the material formulations and the foam morphology. The different dies were used to show the influence of the dwell time and the flow conditions in the dies. It was shown that there is a correlation between rheological properties and the foam morphology and that it is important to use proper die designs for polymer foaming, especially for formulations with high relaxation times.

Stress Relaxation Study of the Development of Microstructures in Blends of Isotactic Polypropylene, Sorbitol Nucleating Angent and Silsesquioxane
Jairo E. Perilla, Sadhan C. Jana, May 2015

Stress relaxation experiments were carried on the blends of isotactic polypropylene (iPP), dibenzylidene sorbitol (DBS) and a polyhedral oligomeric silsesquioxane (POSS) - tetra-silanol-phenyl-POSS (Tetra-POSS) in order to study the development of physical gels. Relaxation plots were discretized as a series of Maxwell elements in parallel. In order to find the minimum number of relaxation modes, the Pad‚-Laplace technique was applied to the data. It was found that the decaying part of the relaxation plot could be described by four relaxation modes. However, the Pad‚-Laplace method was unable to identify the relaxation modulus of the gel. Stress relaxation demonstrated to be a more powerful technique to understand the structure development in iPP/DBS and iPP/DBS/Tetra-POSS blends than using oscillatory shear rheometry.

Synthesis and Crystal Transition of HBA/HNA Copolymer
Rui Jiang, Zhenhao XI, Ling Zhao, May 2015

Thermotropic liquid crystal polymers (TLCPs) with high melting point are of scientific and technological interest because of their typical high strength and modulus, good heat resistance, unique rheological properties and excellent electric properties. With acetylation-melt polycondensation two-step, the HBA/HNA copolymer (PBN) is synthesized from 4-hydroxybenzoic acid (HBA) and 2,6-hydroxynaphthoic acid (HNA). The study of melt polycondensation with the acetylation product 4-acetoxybenzoic acid (ABA) and 6-acetoxy-2-napthoic acid(ANA) show that, with zinc acetate dehydrate as the catalyst, PBN can be prepared with the catalyst dosage of 300 ppm and polycondensation temperature of 320 oC. XRD and POM indicate that the polymer prepared is nematic liquid crystal polymer. By in situ XRD and in situ POM, the connection of microstructure and crystal transition is studied. TGA results that the average decomposition temperature is more than 500 oC with a fine thermal stability.

On the Entanglement Density and Chain Stiffness of Copolycarbonates Containing Rigid and Flexible Linkages
Manojkumar Chellamuthu, Shankar K. Subramanian, Sarah Grieshaber, Himanshu Asthana, May 2015

Thermal and rheological properties of LexanTM XHT (rigid) and LexanTM HFD (flexible) copolymers were investigated using oscillatory rheology and thermal characterization techniques. The incorporation of 20% rigid co-monomer on a neat Lexan TM has increased the glass transition temperature (Tg) by 30øC while increasing the entanglement molecular weight by 50%. Also, for the copolymer with a flexible co-monomer, the glass transition temperature (Tg) has decreased by 20 øC while hardly affecting the entanglement molecular weight. These results suggest properties related to chain stiffness can vary in a non-linear fashion with the nature of co-polymer content.

Mechanical Property Enhancement in Recycled High-Density Polyethylene (rHDPE) via Solid-State Pulverization Methods
Katsuyuki Wakabayashi, Evan Miu, Samuel Jubb, Andrew Fox, May 2015

Low-temperature, solid-state pulverization processes are explored for transformation of postconsumer, recycled HDPE (rHDPE) into value-added applications. A process called solid-state/ melt extrusion (SSME), comprising sequential solid-state pulverization and melt extrusion in a single twin screw extruder, was found to impart significant morphological and rheological changes in rHDPE, which in turn lead to improvements in tensile ductility and toughness to the level of those found in typical neat, virgin HDPE.

Melt Rheology of in-situ Polymerized Polyamide 6/Cellulose Whisker Nano-Composites
Shahab Kashani Rahimi, Joshu U. Otaigbe, May 2015

Polyamide 6 composites reinforced with various cellulose nanocrystal (CNC) contents were prepared by in-situ polymerization technique. Melt rheological properties of these composites were studied in both linear and non-linear viscoelastic regimes. The results showed that incorporation of cellulose nanocrystals even at low loadings results increased storage and loss modulus, complex viscosity and development of elastic properties in the melt. The composites showed shear thinning behavior especially at higher CNC loadings. The low frequency linear viscoelastic region showed the onset of the formation of percolated network structures of CNCs within the polymer matrix. The structure break-up and recovery tests further confirmed the evolution of CNC agglomerates and development of elastic properties, as well as, high shear-rate dependency of the filled samples.

A Method for Determining the Seven Coefficients of the Cross-WLF Equation
Wei Zheng, Justin Claus, Jesse Pischlar, Bryce Holm, Adam Kramschuster, May 2015

Commercial simulation software employs mathematical models to simulate the plastics flow and to provide useful guidelines on the process set-up. One such model is the well applied Cross-WLF viscosity model. With seven material coefficients, the model is able to predict the viscosity as a function of temperature, shear rate, and pressure. However, to the best knowledge of the authors, there is no readily accessible methodology for determining these seven coefficients, which presents a challenge for users pursuing such information. In this work, we present a methodology on determining such coefficients using thermal and rheological data generated for a polylactic acid. The methodology presented is expected to be robust at least for amorphous plastics.

Inline Detection of Material Storage Effects on Processing Behavior of Rubber Compounds
Michael A. Fasching, Gerald R. Berger, Walter Friesenbichler, May 2015

Due to the chemical activity of rubber compounds, storage temperature and -time of the material prior to processing do have a significant influence on the processability of rubber compounds in injection molding.
The aim of this work is to quantify storage-induced material changes in the laboratory as well as to evaluate how these changes affect the injection molding process. Furthermore, two key process indicators (KPIs) are presented which allow the evaluation of the material condition inline.
Therefore, a batch of a NBR model compound filled with carbon black was produced to be used for injection molding experiments in its fresh state as well as for repetitive experiments after a storage period of three months at cooled ambience. This is generally the maximum storage period for commercially processed rubbers.
Several process settings were varied systematically to identify the most important parameters regarding the processing stability of the compound. Simultaneously to the injection molding experiments, the material was characterized in the laboratory for its changes in rheological, thermodynamic and curing behavior in dependence on the storage time.
While a significant viscosity increase with storage time was observed, the incubation time of the rubber decreased which diminishes the possible processing window. The results of the injection molding experiments clearly show that the material changes observed in the laboratory can also be monitored in the injection molding process. With two KPIs based on machine signals, the inline evaluation of the material storage condition is possible and allows timely triggering of counteractions to ensure process stability as well as the estimation of expectable part quality.

Extrudate Swell of High-Density Polyethylene Using Integral and Differential Constitutive Equations
Vinod Kumar Konaganti, Mahmoud Ansari, Marzieh Ebrahimi, Anna Bardetti, Zohir Bernabah, Robert DiRaddo, Haile Atsbha, Christoph Hilgert, Savvas G. Hatzikiriakos, May 2015

The polymer extrudate swell phenomenon is studied to understand the effects of parameters such as past deformation history, die design characteristics, viscoelastic characteristics and process conditions. Accurate prediction of extrudate swell is very crucial in applications such as blow molding, extrusion and fiber spinning. The integral K-BKZ and differential PTT constitutive models are used to simulate the extrudate swell phenomena of a high molecular weight HDPE in capillary extrusion, under various geometric and operating conditions. The simulated swell results were further compared with experimental observations. It was found that the K-BKZ model overpredicts and the PTT model slightly under-predicts the experimental measurements. An attempt has been made to explain the discrepancy between swell predications using K-BKZ and PTT models by studying various material rheological functions using these rheological models.

Polystyrene-c-Poly (Ethylene-co-Butylene)-c-Polystyrene/Zinc Oxide Block Copolymer Nanocomposites: Rheological and Dielectric Properties
Emna Helal, Nicole R. Demarquette, Eric David, Michel Fr‚chette, May 2015

In this paper, nanocomposites of polystyrene-b-poly (ethylene-co-butylene)-b-polystyrene (SEBS) filled with different Zinc Oxide (ZnO) nanoparticles loadings are studied. An homogeneous distribution of the nanoparticles within the polymer matrix was achieved and confirmed by Scanning Electron Microscopy (SEM). Although individually dispersed nanoparticles were not observed, the average size of agglomerates was limited to the nanoscale and only few micrometric agglomerates were formed. The rheological and dielectric properties were evaluated as well and correlated to the morphology. In particular, an increase of the storage modulus G? was achieved at low frequencies while a progressive decrease of the breakdown strength was observed with increasing nanofillers concentration.

Relationship between Moisture Absorption, Crystallization and Rheological Property of Recycled PET Filled Pellets with Talc and Glass Bead
Takanori Negoro, Supaphorn Thumsorn, Wiranphat Thodsaratpreeyakul, Jian Jun Liu, Hiroyuki Inoya, Hiroyuki Hamada, May 2015

Talc, glass bead and poly(ethylene glycol) (PEG) were used for developing moisture absorption and improving crystallization of recycled poly(ethylene terephthalate) (RPET). RPET and fillers were compounded in twin screw extruder with using air drying along the conveyer to yielding high crystallinity of the compounds. The effect of additional fillers and PEG on intrinsic viscosity, moisture absorption, crystallization and rheological behavior was investigated. The incorporation of fillers and PEG slightly decreased intrinsic viscosity and yielded lower moisture absorption rate than neat RPET. Crystallization kinetic parameters indicated that RPET compounds were faster crystallization, which resulting in higher crystallinity than RPET. Rheological properties of RPET compounds decreased at higher talc contents and the addition of PEG. It can be noted that RPET compounds were lower moisture contents, higher crystallinity and better rheological properties at lower shear rate, which would improve mechanical properties of talc and glass bead filled RPET compounds.

Effect of Particle Dispersion on the Rheological Behavior of LLDPE/CaCO3 Composites
Tong Wu, Baodi Zhao, Guo Li, Ping Xia, Haitao Liu, Yulu Ma, Linsheng Xie, Lih-Sheng Turng, May 2015

The dispersion effect has a direct influence upon the rheological behavior of filled blends. This paper aims to study the steady and dynamic rheological properties of different CaCO3 dispersions in LLDPE. A rheological model was chosen to fit the steady viscosity curve. Then the relationship between the dispersed forms and the rheological properties was investigated in order to describe the different filler dispersions via changing the parameters of the equations and consolidating the theoretical support for evaluating the effect of different dispersions based on rheological tests.

Study on Worm Melt Fracture of Blow Molding Process Using Capillary Rheometer
Yongwoo Inn, Ashish M. Sukhadia, William M. Whitte, May 2015

During the blow molding process of large size drums, string-like defects, which are called worm melt fracture, can be observed on the extrudate surface. Such string-like defects are observed in the capillary extrusion at high shear rate range after the slip-stick transition. It is proposed that the cohesive slip layer which is a failure within the polymer melts inside the die could emerge out as the sting-like defects on the extrudates. The resin having more small chains and lower plateau modulus can be easier to have such an internal failure and consequently exhibit more ?worm? defects.

Modelling the Rheolgocial Behavior of Blowing Agent Laden Melts That Simulates the Foam Injection Molding Process
Christian Hopmann, Daniel Sander, May 2015

Currently a new approach for simulating the foam injection molding process is being developed at the Institute of Plastics Processing (IKV) Aachen, Germany. As existing simulation software for foam injection molding works with restrictions and assumptions, especially for the nucleation phase of the foam injection molding. Simulation results often do not correspond with practical tests at the injection molding machine.
In the new approach the influence of the blowing agent on the PVT- and rheological behavior is measured with two injection molds directly on the injection molding machine. The characterization of the material behavior is than prepared as input data to the injection molding simulation software.
In this paper the approach for modeling the rheological behavior of blowing agent loaded melts for simulating the foam injection molding process is described. Step one is to measure the effect of a blowing agent on the viscosity of a polymer melt at different melt temperatures and blowing agent concentrations through an online rheometer. Afterwards the measured data is adjusted, optimized and inputted into the injection molding simulation software.

Rheo-Kinetic Study of a Model TPU System for Reactive Extrusion
Jesse L. Gadley, Jo?o Maia, May 2015

Bulk polymerization of thermoplastic polyurethanes (TPU) in reactive extrusion is a very complicated topic. Understanding how to efficiently produce TPUs is dependent upon the relationship between the extrusion process and the polymerization. The goal of this study was to determine the response of the polymerization kinetics to both composition and shear rates. Rheological measurements were taken while concurrently collecting in situ Fourier transform infrared (FT-IR) spectra. The results indicated that polymerization kinetics were sensitive to both the hard segment (HS) composition of the system and the shear rate imposed on the system.

Pitfalls of MFI Targeting in the Specification of Golf Ball Thermoplastic Polyurethane Cover Layers
Shane R. Parnell, May 2015

Injection moldable thermoplastic polyurethanes (TPUs) have been used in golf ball cover layers since the 1980s and can provide an attractive combination of formulation flexibility, performance, ease of processing, and overall lower cost when compared to cast thermoset polyurethane and polyurethane-urea systems. In developing raw material specifications for TPU and in their polymerization at the material supplier, melt flow index (MFI) is often used as a means to target certain processing and property requirements. However, using MFI as the only metric for quantifying a given TPU formulation can lead to problems as it does not capture polymerization and thermal history dependent structural variations inherent to these materials. In this study, final golf ball performance attributes, mechanical properties, thermal properties, dynamic mechanical properties, and dynamic rheological properties of two batches of TPU with identical composition and MFI were compared and contrasted. The results will be used to illustrate why TPU copolymer segmental structure and both temperature and time dependent morphology are important in the processing characteristics and properties of these versatile materials.

Properties of Crosslinked Polyurethane-Clay Nanocomposites
Shirley Peng, Jude Iroh, May 2015

Polyurethanes (PU) are versatile materials displaying desirable properties that can be modified through the addition of fillers. The introduction of organoclay into the matrix can change the mechanical and overall properties of the polymer. In this paper, the effect of clay concentration on the behavior of crosslinked polyurethane was investigated. The prepared systems were evaluated through analytical techniques in which their rheological and dynamic mechanical properties were studied. Results demonstrated that the introduction of clay improved the storage modulus by more than 30%. Furthermore, clay decreased the molecular weight and viscosity of nanocomposite solution by 40% and 90%, respectively for up to 10 wt% clay, which significantly improves processability.

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