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SPE Library content related to rheology
The bending curvature of an elasto-viscoplastic beam under three-point flexion solicitation
The thermoplastic polymers have a very narrow range in which their behavior can be assumed to be elastic linear. When a thermoplastic is loaded, it rapidly exhibits a clear nonlinear behavior due to its viscous character, or to its plastic character or to both. However, the contribution of those two characters may be linear or nonlinear. The problem becomes more challenging when the material is subjected to a non-uniform stress field such us in the case of the flexion solicitation. In this case, the same material may have different behavior from one area to another according to the local stress state. The objective of this paper is to develop a rheological model to represent the behavior of an elastoviscoplastic and homogeneous beam, with straight and constant cross section, subjected to a three-point flexion solicitation. It is assumes the Bernoulli and Saint-Venant conditions are met in order for the strain field to be considered uni-dimensional. The work starts with the presentation of the analogical model followed by the mathematical formulation of the model established according to the elasticity, the viscoelasticity and the viscoplasticity laws. The aim of the work is to deal mainly with one specific geometrical parameter of the flexion which is the beam bending curvature. This parameter allows the determination of the deformation field and the stress field in the beam. It is found that in the elastoviscoplastic area of the beam the differential equation of the curvature requires numerical integration performed using the implicit Euler method. This theoretical modeling approach is supported by creep tests results carried out on samples of polyamide 66. The actual test results are qualitatively consistent with the predicted behavior of the rheological model.
Verfication of Rheological Mixing Rules to the Application of Masterbatches
Polymer flow behavior is influenced by the rheology of additives such as colors, fibers or fillers. To prevent expensive and time consuming material testing after compounding, mixing models have been developed to estimate the resulting viscosity. In this investigation, three different mixing models were applied to the mixture of masterbatches including different colors and carbon black. The prediction of the three mixing models were compared to the measured viscosity of the compounded mixture. The results suggest significant predictive capability of the mixture rheology, but not sufficient to identify the ratio of the constituent materials given rheological data of a compounded material.
Melt Flow Simulation of an Injection Molding Cavity For Validation of a Multivariate Sensor
Utilizing mold filling simulation to validate a multivariate molding sensor is described. The multivariate sensor uses an infrared thermopile for temperature measurement and a piezoelectric ring for measuring pressure. A zinc selenide window is used in order to transmit melt radiation, pressure, and prevent leakage. Velocity is estimated based on the rise time of the melt temperature signal. Viscosity is estimated using the pressure and velocity based on constitutive rheological models. Results indicate that the frozen layer thickness is significant in estimating the shear rates and viscosity, but an accuracy of ±5% can be obtained across a wide range of processes.
Improved Exfoliation and Dispersion of Nano-clays in Nylon 6 Using Supercritical Carbon Dioxide
The use of supercritical carbon dioxide (scCO2) as a processing aid to help exfoliate nanoclays and improve their dispersion after melt blending in polymer matrices has been reported in the literature. Previous work has focused on nonpolar polymers such as polypropylene. In this work, the supercritical aided melt blending method was applied to a Nylon 6/ organoclay composite system with favorable organoclay/polymer interactions. Transmission electron microscopy (TEM), rheological results, and tensile tests are provided to investigate the effect of processing with scCO2 on the final composite properties and morphology. It was found that the scCO2 aided method improved the composite properties compared to reported twin screw results in literature. At 7.6 wt% the modulus is observed to reach about 4.75 GPa which is one of the highest increases (~1.7 GPa) reported for these materials at intermediate concentrations. We note that beyond 7.6 wt% the improvement due to scCO2 processing only matches that of direct blending. It is possible that with the use of a twin screw extruder, or reduction in processing steps, the modulus would continue to increase.
Determination of Extensional Viscosity of Polypropylene Melt by the Rehotens Test
In this work, the extensional viscosity of polypropylene (PP) melt was determined by the rheotens test. Three different extrusion velocities were employed and the extensional viscosities were calculated following a “Newtonian local approach”. A new test mode, the “steady state rheotens test”, was tentatively used to obtain a reliable extensional viscosity. The results indicate that the extensional viscosity increased with the increase of extrusion velocity in the standard rheotens test. In the steady state rheotens test, the influence of extrusion velocity on the extensional viscosity was eliminated and a superposition of the extensional viscosity curves appeared at relatively low extrusion velocities.
Investigation of Crosslinking and Crystallization Behaviors of Polyethylene Blends
Crosslinking kinetics and its influence on the subsequent crystallization of polyethylene blends were investigated by rheological measurements and thermal analysis. Results indicate that addition of small amount of polyolefin elastomer (POE) in the HDPE matrix does not accelerate the crosslinking process and the final crosslinking degree of the blends is independent of the molecular weight of each component. Crosslinking can significantly retard the crystallization process and decrease the crystallinity due to the hindrance on chain folding caused by the formation of the network structure.
Effects of Molecular Weight Changes on the Foaming Behavior of Thermoplastic Polyurethane (TPU) and its Acoustic Properties
This paper investigates the effects of molecular weight changes on the foaming behavior of thermoplastic polyurethane (TPU) and its acoustic properties. In order to vary the molecular weight of TPU, the additional melt extrusion processes are introduced and the foam samples are manufactured via injection foam molding technology. The effects of each additional extrusion process on the molecular weight changes are examined by analyzing heatcycle and rheological behaviors. In addition, the cellular morphologies and acoustic properties of injection foam molded samples are evaluated and their relationships with molecular weight changes are discussed. The foaming behaviors are varied significantly due to reduced molecular weights and different foam structures result in different acoustic performances. In general, the foamed samples from the processed TPU resin are able to achieve higher acoustic absorption coefficients.
A Device for Simulataneous In Situ Structuring and Measurement of Rheological Properties of Polymer Blends and Composites
A prototype In Situ Structuring Rheometer (ISSR) was developed to study the shear behavior of polymer blends and composites in tandem with forming material components into a variety of fine-scale structural types. The ISSR utilizes a regime of fluid mechanics known as chaotic advection which enables progressive structure development, whereby specific blend morphologies are derived in sequence from in situ structural transitions. The ISSR resembles a conventional parallel plate rheometer and as such can be incorporated into commercial rheometer instruments. The ISSR provides new means to explore interactions between material components at the micro- and nano-scales, validate theoretical rheology models, and characterize melt properties to support manufacturing processes. The ISSR design and results of initial trials are presented.
A Study on the Morphological Properties and Foaming Behavior of PS/Polyolefin Blends
In this study, the blends of polystyrene (PS) with three different polyolefins of polypropylene (PP), low density polyethylene (LDPE) and linear low density polyethylene (LLDPE) that consists of 20 wt% polyolefins with/without 4phr UPES interpolymer, were prepared using a twin screw extruder. Scanning electron microscopy (SEM) was used to describe the phase morphology of the blends. The rheological characteristics of neat polymers and their blends were analyzed to evaluate the viscosity ratio of blends and the interaction between dispersed and matrix phases. Finally, the foaming behavior of PS/polyolefin blends in the presence of supercritical Co2 were studied by using a batch foaming technique at 140°C and 145°C under 1500psi pressure. The results show that the higher interfacial area of LLDPE dispersed phase in the PS matrix plays a key role in controlling the cell size, cell density and expansion ratio and improve the foaming behavior of the PS.
The Effect of Hydroxyl-functionalization on the Structure and Properties of Polypropylene
Hydroxyl-modified polypropylenes (PPOH) with side chains containing OH groups were synthesized by the copolymerization of propylene with undecenyl-oxytrimethylsilane monomer. Copolymers with OH concentration ranging from 1.3 to 3.9 mol% were produced and their properties compared with unmodified polypropylene. The presence of intermolecular H-bonding between the OH groups affected the base polypropylene structure as well as its thermal and rheological properties. As predicted by Flory’s theory of melting point depression in copolymers, the melting point reduced with increasing [OH]. While the crystal structure remained unaffected, the crystal size reduced by approximately 15%. Crystallinity for PPOH polymers was also reduced by as much as 40% because of the hindrance to the ordered packing of backbone chains into the lamellar structure. On the other hand, the rheological properties such as the melt strength and the elasticity increased for the PPOH polymers. PPOH with only 3.9 mol% OH groups displayed a gel-like rheological behavior suggesting the formation of a weak elastic network in the melt.
Constant Temperature Embossing (CTE): Process Analysis
The goal of this paper is to analyze the Constant Temperature Embossing (CTE) process by constructing a process model to determine the change in material parameters during the process. The process model is developed by combining the non-isothermal crystallization kinetics and suspension based rheological models. A shift factor is determined from the non-isothermal crystallization kinetics to predict the crystallization behavior at temperatures where the properties are difficult to measure. A suspension based rheology model is chosen to represent the change in viscosity of the polymer during the process as the increase in particle concentration. The thermal and rheology models are then merged by considering certain key assumptions, and a process model to determine the change in material properties during the CTE process is constructed.
The Application of Rheological Techniques in Selecting Plasticizers for PVC Processing
This paper focuses on three different rheometric techniques to analyze how dibenzoate and other plasticizers affect flexible polyvinyl chloride (PVC) processability. Both plastisol and melt compounds will be considered. This analysis includes the use of a research rheometer in an oscillatory mode to evaluate plastisols. A torque rheometer was used to evaluate melt compound formulations. New dibenzoate plasticizers and a new monobenzoate have been introduced and the nature of the solvator class of these new benzoates will be evaluated.
Manufacturing of Polypropylene/Ground Tire Rubber Thermoplastic Elastomers by Ultrasonically Aided Extrusion
Compounding ground tire rubber (GTR) with thermoplastic polyolefins, such as polypropylene (PP), is a possible way to manufacture thermoplastic elastomers and also to recycle waste tires, thus solving a major environmental problem. The effect of ultrasonic treatment on the mechanical, rheological and morphological properties of PP and PP/GTR (ground tire rubber) blends in an ultrasonic single screw extruder (SSE) and an ultrasonic twin screw extruder (TSE) were investigated. PP and GTR was fixed at a ratio of 50:50. The treatment was carried out under amplitude of 5, 7.5 and 10 ?m, and at a flow rate of 2 lbs/hr. Pressure and ultrasonic power consumption were measured. Mechanical and rheological properties of untreated and ultrasonically treated PP indicated that TSE provided more degradation than in SSE. For 40 mesh blends from SSE, the mechanical properties improved with increasing ultrasonic amplitude. The viscosity indicated very little dependence on ultrasonic amplitude, which is evidence a formation of covalent bonds between PP and GTR. Viscosity of 140 mesh blends was lower than that of 40 mesh blends from both SSE and TSE, indicating a larger degree of degradation of blends with smaller rubber particle size. In addition, with smaller rubber particle size, much better elongation at break is obtained which indicates better adhesion between PP and GTR.
Preparation and Properties of Polyethermide (PEI)/ Graphite Composites with Ultrasound Assisted Extrusion
Polymeric composites containing carbon materials, such as carbon nanofibers (CNF), carbon nanotubes (CNT), graphite, expanded graphite, graphene, were extensively studied. However, the agglomeration of carbon fillers makes their dispersion challenging and limits their potential use. In this paper, an ultrasound assisted twin screw extruder was developed to prepare polyetherimide (PEI)/graphite composites with untreated nature graphite (UG), modified graphite (MG) and expanded graphite (EG). The effect of ultrasound on rheological, mechanical and electrical properties of PEI filled with up to 10 wt% graphite was investigated. For PEI/UG composite, ultrasound showed little effect on these properties. However, for PEI/MG and PEI/EG composites, the storage moduli (G’), loss moduli (G”) and complex viscosity were all increased and damping characteristics were decreased with application of ultrasound. The PEI/EG showed an electrical percolation between 2.5 to 5 wt%, but PEI/UG and PEI/MG did not show any percolation even at 10 wt% graphite concentration due to their large particle size and agglomeration. Ultrasonic treatment improved the electrical conductivity of PEI/EG and reduced percolation threshold, but did not show any effect on electrical properties of PEI/UG and PEI/MG. Mechanical properties of all these composites showed slight differences with ultrasonic treatment.
Thermal and Rheological Characterization of Polylactic Acid
The thermal and rheological properties of polylactic acid are investigated using differential scanning calorimetry and parallel plate rheometer. Polylactic acid is found to be able to crystalize both on cooling and on heating, and the extent of crystallization depends on the scan rates used. If the material is cooled faster than 30 K/min, the crystallization disappears on cooling. If the material is cooled slower than 2 K/min, no cold-crystallization is observed on heating regardless of the heating rate used. Additionally, rheological measurements are performed at different temperatures. Master curves are successfully constructed using time-temperature superposition.
Continuous High Power Ultrasonic Extrusion of PEEK - CNT Nanocomposites
A micro-compounder using ultrasonic assisted technology has been developed and used for the dispersion of multi walled carbon nanotubes (CNTs) to produce polymer nanocomposites. This process is continuous and can be easily adapted to equipment that is currently being used commercially. The effects of compounding with and without ultrasound on torque, power consumption, rheological, morphological, mechanical and electrical properties in polyetheretherketone (PEEK) filled with 0-5 wt. % CNT were evaluated. Ultrasonically treated nanocomposites show increased viscosity with a slight improvement in mechanical properties. An electrical percolation of less than 1 wt. % was observed. Samples ultrasonically treated at 10 ?m showed a significant increase in viscosity due to improved dispersion at the molecular level. The incorporation of CNTs into PEEK showed up to a 15% increase in modulus with loadings up to 5 wt. % CNT and a significant decrease in electrical resistivity at CNT loading of 1 wt. %. The rheological percolation is observed to be between 2 and 5% and the electrical percolation is observed to be below 1 wt. % loading. Further investigation of the interaction between the CNTs and the matrix are in progress.
The Role of Surface Interactions in Renewable Poly(Butylene Succinate)-Silica Nanocomposites
Polymers from renewable resources are beginning to compete with conventional fossil fuel derived materials as fossil resources become increasingly expensive and difficult to extract. The same lightweight, high-strength properties of petroleum-based polymers and composites are required for renewable materials, and a better understanding of processing properties will improve their prospects in the market. One route to widening the thermophysical property window of biobased polyester poly(butylene succinate) (PBS) is the incorporation of reinforcing fillers. In this work, PBS is melt-mixed with high-surface-area fumed silica to create nanocomposites. The surface of the silica nanofiller is chemically modified to explore the effects of surface functionality on filler dispersion and required mixing energy. Rheological and thermal measurements show that structural properties of the filler have a larger influence than surface modification. Comparison of blending techniques provides guidance for improved nanocomposite preparation. The demonstrated mechanical property improvements over neat polymer enable a broader range of applications for these novel renewable materials.
Poly(butylene succinate)/fumed silica nanocomposite: functionality and rheology
Plastics Engineering Department, University of Massachusetts Lowell Nanocomposites based on biodegradable poly(butylene succinate) (PBS) and silica fillers were prepared by a melt-blending process. Two types of unmodified fumed silica and octadecyltrichlorosilane (OTS) functionalized silica were used as fillers. Rheology was used to study relaxation dynamics and viscoelastic properties of these nanocomposites in the melt state. The effects of polymer-particle and particle-particle interactions on viscoelastic properties of nanocomposite materials were investigated. Linear viscoelastic data indicate a transition to a solid-like response at low oscillation frequencies for particle weight fractions as low as 5%. The long-time response upon a step shear strain demonstrates that liquid-like behavior persists in the nanocomposites below 5 wt% loading, which is related to the relaxation of the temporal polymer-particle network. Dynamic viscoelastic and dynamic mechanical thermal analysis (DMTA) measurements of the PBS/silica nanocomposite reveal that fumed silica with the smallest primary particle size has the largest dynamic moduli over the testing temperature range. The hydrophobic functionalization of silica filler does not appreciably change the thermal transition temperatures in the nanocomposites.
Blow head design and optimization
Major sectors with high demands and specifications for polymer products are packaging and automotive. Due to the complexity of polymeric materials and the high specifications regarding the product quality and e. g. homogeneity of wall thicknesses, a key issue is the rheological design of the extrusion die that is used for the primary forming of the polymer melt. Usually, numerical die design approaches (e. g. based on computational fluid dynamics) are time consuming, costly, tie down manpower and highly depends on the experience and training of the responsible engineers. Applying a holistic approach based on the analogy between electrical engineering (voltage, current, resistance) and hydrodynamics (pressure drop, volume flow, flow resistance) offers a promising way to achieve good die design results very time efficient. In order to describe flow phenomena the control volume approach (also referred to as network theory) is used and a simulation model for complex multi-level extrusion dies is implemented. Interdependencies between different levels of the extrusion die are taken into account. The approach aims for a fast and automatic flow calculation. The results of the flow simulation are compared against user specifications and a quality value is computed that describes the quality of the design. This value is used for optimization techniques tin order to develop a smart and time-efficient way to find optimal solution for complex multi-level extrusion blow heads.
Effect of mixing time and temperature on the rheology and morphology of immiscible polymer blends prepared from polytrimethylene terephthalate [PTT] and polyamide 6,10 [PA6,10]. Kevin Lucero, Marissa
Thermoplastic immiscible polymer blends were prepared from polytrimethylene terephthalate [PTT] and polyamide 6,10 [PA6,10] by melt processing in a Brabender mixer to assess morphology developed between the immiscible domains. PTT and PA6,10 were selected as a pair of engineering polymers with complementary properties and as a blend prepared to significant extent from bio-based precursors. Overall, a 50/50 blend of these polymers has a renewable content of nearly 50%. The overall objective is to develop an engineering blend with good stiffness, strength, and dimensional stability while simultaneously being easy to process. In the present phase of the work, blend homogeneity was studied as a function of mixing time and temperature in the range of t=0-25 minutes and T=240-260 C. Results are presented in terms of torque versus time and temperature curves that are interpreted in terms of domain formation and SEM micrographs are used to define domain size and overall morphology.
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