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SPE Library content related to rheology
Rheological Behavior of Thermotropic Liquid Crystalline Copolyester Vectra A950
An investigation of the transient, steady and dynamic flow properties of the thermotropic liquid crystalline polymer Vectra A950 (Ticona) is presented. The steady viscosity curve shows the typical three-region flow curve of LCPs. In the transient shear experiments, the shear stress grows during the start-up of constant shear rate flow and goes through a maximum at approximately ?=2. Dynamic mechanical experiments show the existence of a linear viscoelastic region at small strains, and that the onset of nonlinearity varies with frequency. In the linear viscoelastic region, the storage and loss modulus, G'(?) and G(?) do not display the typical dependence on ?2 and ? respectively. The results obtained for a frequency sweep using constant strain and constant stress instruments are qualitatively different."
Study of Flow Marks during Thin-Wall Injection Molding
In this paper, the effect of polymer rheology, injection speed, mold geometry, melt temperature, mold temperature and lubricant on flow marks was studied. The results show that the most important factor affecting the flow marks is injection speed. It is found that the flow marks did not occur at high injection speeds. Mold geometry also has an effect on the flow marks. However, mold temperature and melt temperature were determined to have little effect on the flow marks. It is also found that the polymer with the highest dynamic viscosity, elastic modulus and first normal stress difference, and longest relaxation time exhibits flow marks over the widest range of processing conditions.
Strain Hardening Behavior in Elongational Viscosity for Blends of Linear Polymer and Crosslinked Polymer
Rheological properties for the binary blends of a linear polymer, such as isotactic polypropylene (PP) and polystyrene (PS), and gel fraction of a crosslinked terpolymer composed of ethylene, 1-hexene, and ethylidene norbornene (gEHDM) have been studied. Blending of the gEHDM, which is characterized as the gel just beyond the sol-gel transition point, much enhances the strain hardening behavior in the elongational viscosity of PP, even though the amount of the gEHDM is only 1 wt%. On the other hand, the PS/gEHDM (97/3) blend shows no strain hardening in the elongational viscosity. The entanglement couplings between the gEHDM and the PP will be responsible for the strain hardening behavior.
Large, Structural, Class A" Thermoplastic Automotive Part Production without Painting"
The Valyi surface finishing/compression molding process (SFC) has successfully been used to produce large structural panels with Class A finishing under low pressure. The material used in the SFC process must meet certain performance requirements in order to fully exploit the capability of the process. This paper compares the mechanical properties and rheological properties of short and long glass and carbon fiber reinforced materials. The Long fiber reinforced PP resins show enhanced stiffness and impact strength. Degradation of surface appearance due to long fiber read through is an issue to be addressed in future work.
The Role of Melt Dynamics in Shear-Enhanced Crystallization of Isotactic Polypropylene
Processing flows are known to accelerate polymer crystallization kinetics, strongly altering the orientation distribution of the crystallites and producing dramatic changes in material properties. Our research probes the molecular level processes that give rise to these effects. To clarify the role of macromolecular relaxation, we investigate the effects of shear history on the crystallization of isotactic polypropylenes. A unique apparatus enables us to subject a subcooled melt to precisely controlled intervals of shear at stress levels similar to those encountered in industrial processes.(1) Brief intervals of shear enhance the rate of subsequent crystallization by orders of magnitude. Previous rheo-optical experiments have indicated that the creation of long-lived, oriented structures during flow is controlled by the dynamics of the melt.(2) We present polarimetry and synchrotron wide-angle x-ray diffraction (WAXD) data obtained during and after shear of an iPP believed to contain chains with long branches. Results suggest that shearing near the nominal melting temperature induces the formation of a slow relaxing species that templates subsequent oriented crystal growth, emphasizing the importance of rheology to shear-enhanced crystallization.
A Closer Look at the Long-Chain Branch Formation in Ethylene Polymerization Using Metallocenes
Polymerization of ethylene using metallocene catalysts, particularly the constrained geometry catalysts (CGC), was studied. The main focus of the paper is on the control of chain microstructure in olefin polymerization using metallocene catalysts, particularly long chain branching in ethylene polymerization. Combined metallocene catalysts, consisting of CGC and a conventional metallocene catalyst, which only produces linear chains (linear catalyst), were used to manipulate long chain branching degree. The feasibility of this technique was verified using a mathematical model developed for the polymerization of ethylene in a semi-batch reactor using combined catalyst systems. Polymerization experiments were performed to verify the validity of the proposed technique and some of the modeling results. It was shown that by choosing a proper catalyst system and polymerization conditions chain microstructure could be tailor-made. Monte Carlo simulation was also used to study the structure and length of the branches in metallocene catalyzed ethylene polymerization. This information is essential for making any correlations between LCB degree and rheological properties.
Nonlinear Rheological Behavior of LDPE Melt during Capillary Extrusion under Vibration Force Field
In order to study the rheological behavior of polymer on vibration force field, a new capillary dynamic rheometer has been successfully developed by us for the vibration extrusion experiments of polymer melts. In this paper the measuring principles for the capillary rheological behavior of polymer melts under vibration force field will be introduced. By the experiment study of low-density polyethylene (LDPE), it has been discovered that melt viscosity and extrusion swelling ratio nonlinearly changed with the frequency and amplitude of vibration sources. The viscosity of the LDPE melt, the swelling ratio and unstable flow of LDPE decreased during capillary extrusion under vibration force field, and had a minimum with vibration frequency's change. It has great significance to the researches on dynamic extrusion and injection processing of polymer materials.
The Effect of High Shear Rate and Shear Duration on the Properties of Injection Molded Plastics
Shear rate is an important part design and processing consideration in injection molding. Excessive shear rates can cause polymer chains to break and degrade, but actual limits are currently unknown. Four materials (polycarbonates and polypropylenes) were processed through specially designed runner inserts that varied shear rate. Each material was processed through each insert three times by regrinding and reprocessing. The resulting material was evaluated for mechanical and rheological properties. The mechanical tests showed an insignificant change in properties. Rheological tests showed a progressive decrease in viscosity as shear rate and shear duration was increased.
Constitutive Analysis of the Nonlinear Shear Rheology of Flour Suspensions Obtained from Different Botanical Sources
The evaluation and development of validated models for the nonlinear viscoelastic (VE) behavior of materials is an important area of research which has impact on a number of industrial processes including those in the food industry. Various nonlinear VE models have been developed over the years and evaluated for petroleum-based polymers; however, our understanding of the nonlinear VE behavior of biopolymers of industrial import lags our understanding of synthetic polymers. In the work reported herein, the nonlinear VE behavior of suspensions (20 % by weight in deionized water) of defatted oat flour, oat bran, barley flour, and oat flour were investigated. The rheological properties were measured using a Rheometrics Series IV controlled-strain rheometer equipped with a cone and plate fixture. The measurements were conducted at 23 ± 0.1°C. The rheological data were interpreted using a strain separable K-BKZ type (Wagner) model. The K-BKZ model was found to provide an accurate description of the rheological behavior of the four flour suspensions.
Mixing Silica and Other Fillers into Elastomers and Their Agglomerate Breakdown during Mixing in an Internal Mixer and Their Rheological Behavior
The rate of silica particles agglomerate breakdown in an laboratory internal mixer were measured and compared with carbon black, calcite, talc and zinc oxide particles. Silica agglomerates exhibited the highest agglomerate size. The rheological behavior of each compound was investigated. Small silica particle filled system exhibited highest viscosity level.
Thermorheological Investigation of Entangled Branch Polybutadienes Having Different Architectures and Arm Lengths
Rheological behavior of entangled six-arm and eight-arm 1,4-polybutadiene melts of the types A3-A-A3 and A3-A- A2-A-A3 is investigated using low amplitude oscillatory shear and viscosity measurements. Experiments covered a time (frequency) and temperature range broad enough to characterize the complete relaxation spectrum. In oscillatory shear, three separate relaxation modes are identified. At high frequencies a maximum in the loss modulus is linked with segmental relaxation. At intermediate frequencies a new relaxation mode characteristic of the arms is observed. Finally, at low frequencies a terminal relaxation process is identified. This process is characterized by a lower plateau" modulus and is thought to reflect cross-bar reptation in an enlarged tube."
Mixing Characteristics and Mechanical Properties of Polypropylene-Clay Composites
Polypropylene(PP)-clay composites were prepared by melt mixing in an intensive mixer. Three grades of PP's having different melt viscosities were employed to investigate the mixing characteristics of the composites with various clays which belong to organically modified montmorillonite(org-MMT). Depending on the matrix viscosity and nature of the organic layer in MMT, significant variations in the phase structure of the composites were found. In addition to the simple combination of PP and clay, modified PP's having various content of maleic anhydride were also incorporated. Major interest was focused on the effect of varying thermodynamic affinity between the components on the phase evolution and mechanical properties of the composites. Requirements for the effective dispersion of clay in the PP matrix are discussed in terms of both rheological effect and thermodynamic interaction.
Dynamic Vulcanization of Elastomers in Polypropylene Blends
The use of innovative crosslinking agents for the preparation of thermoplastic vulcanizates (TPVs) is investigated. In this preliminary study, the most common TPVs systems, based on polypropylene (iPP) and rubber ethylene-propylene-diene terpolymer (EPDM) blends, are studied. Typical vulcanization agents, such as sulfur, phenolic resins and peroxides do not permit to crosslink saturated elastomers and, furthermore, give rise to dynamic vulcanization of the polyolefins. For this reason, the main goal of the present study is to investigate a new vulcanization agent for elastomeric matrices. This agent is based on azide derivative, 1,3-bis sulphonyl azide benzene that, for the specific behavior of the sulphonyl azide group, allows its interaction with the C-H bonds of the elastomeric phase and of the polyolefin. The study includes the dynamic vulcanization of PP-EPDM blends and their rheological, mechanical and thermal characterization. A comparison with traditional blends prepared with sulfur as vulcanization agent is also presented.
Dynamic Rheological Properties of HDPE/Ethylene-Alpha-Olefin Copolymer Blends
The blends of linear and branched polyethylenes have received much attention in recent years. Many studies have been carried out to evaluate the miscibility of these mixtures and its influence on the final properties. There still some degree of controversy on the issue of melt miscibility even though partial co-crystallization has been demonstrated for some of these systems.
Rheology of Randomly Branched Polymers
The molecular structure of randomly branched polymers is understood using percolation theory. Once the chain length between branch points and the extent of reaction relative to the gel point are specified by synthesis, both the molecular structure and the linear viscoelastic response can be determined using simple models. We demonstrate these ideas using randomly branched polymers with known chain lengths between branch points. Then we exploit this finding to characterize the chain length between branch points for polyethylenes from knowledge of their weight-average molecular weight and zero-shear-rate viscosity.
Clear and High Heat Resistant TPEs
There is a market need for soft (35 - 70 Shore A) clear TPEs with heat resistance high enough for repeated boilable applications. Flexible PVC meets most of the requirements, but it is out of the scope of this work. Styrenic Block Copolymer based compounds have been developed to fill this need. Important parameters, such as molecular weight of the base polymer, surface quality of the molded part and rheology of the material, have been correlated with clarity and heat resistance. Some of the myths about the clarity of SBC compounds will also be discussed.
Polymer Raw Material, Process and Production Fingerprints in Injection Moulding
In-process monitoring of polymer melts is found to provide a fingerprint of the: • polymer (process-relevant polymer rheometry); • the process (injection pressure-time curves which reflect the material-machine-mould combination, including polymer batch to batch variation and machine dynamic repeatability); and • production trends, with 'process indices' offering an efficient basis for 100% automatic inspection, Statistical Process Control, and even insight into factory housekeeping. Specific pressure indices in an indentified low noise region of the primary injection stage of injection moulding have been found to provide a sensitive indicator of changes in a polymer, including batch to batch changes and process-induced changes, as such measurements are closely related to the rheology of the polymer melt. The same information has also been found to provide sensitive indications of variation in the processing operation for a given polymer-mould combination, and also consequently to allow meaningful statistical analysis of trends in the injection moulding process. Laboratory and factory data for raw material, process and production trend analysis (the latter involving data from substantial production runs) are presented.
Standard Reference Materials: Non-Newtonian Fluids for Rheological Measurements
NIST develops Standard Reference Materials for calibration, quality assurance and research into improved measurements. Two fluid standards are being developed to exhibit shear thinning and normal stresses typical of polymeric fluids. SRM 2490 is a solution of polyisobutylene dissolved in 2,6,10,14-tetramethylpentadecane. At this time, SRM 2491 is expected to be a poly(dimethylsiloxane) melt, giving less temperature dependence than SRM 2490. NIST will certify linear viscoelastic behavior and the shear-rate dependence of the viscosity and first normal stress difference at 0 °C, 25 °C and 50 °C. A round robin with the fluids will investigate variability in rheological measurements. We report progress on the project.
New Boron Nitride Processing Aids for the Extrusion of Molten Polymer
It is already known that the use of Boron Nitride (BN) in the extrusion of molten polymers may eliminate surface melt fracture and postpone the critical shear rate for the onset of gross melt fracture to significantly higher values depending on resin type and additive content. In this work several new Boron Nitride powders (Carborandum Co.) are tested that exhibit superior behavior from the previous studied ones. Critical parameters for this unique behavior (elimination of gross melt fracture) are good dispersion, small average particle size and free of agglomeration. The equipment used for the testing of the new powders include both an Instron capillary rheometer with special annular dies (Nokia Maillefer wire coating crosshead) attached to the rheometer, and a parallel-plate rheometer. A metallocene polyethylene with all types of boron nitride is tested at various additive concentrations. The additive having the finest particle size and that is free of agglomeration was found to have the greatest influence on the gross melt fracture performance of the polymer tested in crosshead dies and tips. Moreover, one of the additives was found to enhance melt slippage and as a result relieved internal shear stresses. This action is believed to eliminate surface melt fracture and postpone the critical shear rate for the onset of gross melt fracture to significantly higher values depending on the additive concentration. The influence of the boron nitride type and its concentration on the polymer rheology is also discussed.
On-Line Rheological Measurements
Rheometers are being used increasingly as sensors for process control. This is because of the sensitivity of rheological properties to polymer characteristics such as molecular weight. The process rheometers now being used are mostly of the on-line, pressure flow type, in which a gear pump feeds a capillary or slit, and the pressure drop is measured. For process control applications, the signal delay associated with the flow of melt from the main flow to the capillary or slit slows the system response, and this problem has been addressed by the use of a larger sampling line together with a bypass around the rheometer. Other recent advances are aimed at providing a more extensive characterization of the melt.
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