A “Sequential Design of Simulations” approach for exploiting and calibrating discrete element simulations of cohesive powders

The flow behaviours of cohesive particles in the ring shear test were simulated and examined using discrete element method guided by a design of experiments methodology. A full factorial design was used as a screening design to reveal the effects of material properties of partcles. An augmented design extending the screening design to a response surface design was constructed to establish the relations between macroscopic shear stresses and particle properties. It is found that the powder flow in the shear cell can be classified into four regimes. Shear stress is found to be sensitive to particle friction coefficient, surface energy and Young’s modulus. A considerable fluctuation of shear stress is observed in high friction and low cohesion regime. In high cohesion regime, Young’s modulus appears to have a more significant effect on the shear stress at the point of incipient flow than the shear stress during the pre-shear process. The predictions from response surface designs were validated and compared with shear stresses measured from the Schulze ring shear test. It is found that simulations and experiments showed excellent agreement under a variety of consolidation conditions, which verifies the advantages and feasibility of using the proposed “Sequential Design of Simulations” approach.     

Studies on the melt flow behavior of thermoplastic elastomers from polypropylene—Natural rubber blends

The melt flow behavior of thermoplastic polypropylenenatural rubber blends has been evaluated with specific reference to the effects of blend ratio, extent of dynamic crosslinking of the rubber phase and temperature, on viscosity, flow behavior index, and deformation of the extrudate. The proportion of rubber in the blend and the extent of dynamic crosslinking of the rubber phase were found to have profound influence on the viscosity of the blends at lower shear stresses. But at higher shear stresses, the effect of blend ratio on viscosity was comparatively less for the uncrosslinked blends than that for the crosslinked blends. At lower shear stress, the viscosity of the blend increased with increase in degree of crosslinking but at higher shear stress, the effect of crosslinking on viscosity was found to vary depending on the ratio of the plastic and rubber components in the blend. The deformation of the extrudates was also very much dependent on both blend ratio and degree of crosslinking.     

Effects of shear and electrical properties on flow characteristics of pharmaceutical blends

This article examines the effects and interactions of shear rate, shear strain on electrical and flow properties of pharmaceutical blends. An unexpectedly strong relation between the flow and passive electrical properties of powders is observed to depend on the shear history of the powder bed. Charge density, impedance, dielectrophoresis, flow index, and dilation were measured for several pharmaceutical blends after they were subjected to a controlled shear environment. It was found that the increase in the shear strain intensified the electrical properties for blends that did not contain MgSt. The opposite effect was found in blends lubricated with MgSt. Different shear conditions resulted in different correlations between flow index and dilation. Flow properties of powders were found to improve with continuous exposure to shear strain. It was also found that flow properties correlated to charge acquisition and impedance for different shear treatments. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

An observed correlation between flow and electrical properties of pharmaceutical blends

We study the relation between flow and electrical conductivity of multiple formulations of pharmaceutical powder blends. Ten formulations were tested, consisting of two excipient sets, two active preparations, and a variety of food-grade additives including magnesium stearate (MgSt), and ionic and conductive materials such as ascorbic acid, talc, sodium carbonate, colloidal silica and TiO₂. Electrical impedance, flow index and dilation were independently measured for all of the blends, and a strong correlation was found between every pair of these three properties. The relation between flow and dilation has been observed before; we find for the first time that there is an exponential relationship between flow index or dilation and impedance. This indicates that cohesive powder behavior depends on powder electrical properties, raising the questions of whether additives such as MgSt affect friction and conductivity per se and what mechanism and phenomenon links cohesion and conductivity.     

Melt Rheology and Morphology of Nitrile Rubber and Ethylene-Vinyl Acetate Copolymer Blends

The melt Theological behavior of nitrile rubber (NBR)/ethylene-vinyl acetate (EVA) copolymer blends was studied with special reference to the effect of the blend ratio, cross-linking systems, and shear rate using a capillary rheometer. At a given shear stress at 90°C, the viscosities of the blends vary slightly with composition. The effect of cross-linking systems [viz., sulfur (S), peroxide (DCP) and mixed (S+DCP) systems] on the viscosity of NBR/EVA blends is negligible. The melt viscosity of the blends decreases with increasing shear rate, showing pseudoplastic behavior. The flow behavior index values also support the pseudoplastic nature of these blends. Various theoretical models were used to predict the melt viscosity of the blends. Parameters such as die swell, principal normal stress difference, recoverable shear strain, and shear modulus were calculated to characterize the melt elasticity of these blends. The melt elasticity of the system was increased by the addition of NBR to EVA. The extrudate deformation at different shear rates was also studied. It was observed that as the shear rate increases, the extrudate surface exhibits a higher degree of deformation. The morphology of the extrudates of the blends at different shear rates has been examined by a scanning electron microscope. The morphology was found to be dependent on the blend ratio and shear rate.     

Mixture Rules for Critical Shear Values of Extruded Linear PE/Branched PE Blends

Oscillatory flow and elastic turbulence belong to the types of flow instabilities frequently encountered during extrusion of polymer melts. The onset of these defects corresponds to the flow conditions when the critical shear stresses or the critical shear rates are attained. The critical values of shear stresses and shear rates were experimentally determined for linear polyethylene/branched polyethylene blends (IPE/bPE) that were prepared with various weight ratios. Consequently, mixture rules of the logarithmic type are proposed. These rules relate the critical value of shear stress (shear rate) of blend to the critical values of shear stresses (shear rates) of the individual pure components, weight fractions, and interaction parameters. There is a good agreement between the proposed mixture rules and experimentally determined critical values.     

Shear flow of assemblies of cohesive granular materials under constant applied normal stress

We have studied plane shear flow of nearly homogeneous assemblies of uniformly sized, spherical, cohesive particles in periodic domains under constant applied normal stress. Our focus has been on (a) exploration of the effect of inter-particle attractive forces on the flow behavior manifested by dense assemblies under constant applied normal stress, and (b) comparison of the rheological characteristics observed under constant-applied normal stress and constant-volume conditions. As a model problem, the cohesion resulting from van der Waals force acting between particles is considered. Simulations were performed for different strengths of cohesion, shear rates, and applied stresses. From each simulation, the volume fraction, shear stress and the average coordination number have been extracted. We find that cohesive assemblies sheared under constant applied normal stress shear differently from those sheared at constant volume only in the dynamic sense, while the time-averaged rheological characteristics are essentially indistinguishable. At constant volume, the fluctuations in shear stress are larger than, but have the same dependence on cohesion as under constant applied normal stress. This study has also exposed a pronounced dependence of the apparent coefficient of friction on particle volume fraction in the quasi-static flow regime.     

Melt rheology and morphology of physically compatibilized natural rubber–polystyrene blends by the addition of natural rubber-g-polystyrene

Blends of natural rubber (NR) and polystyrene (PS) were prepared by melt mixing in a Brabender plasticorder and by solution casting using chloroform as the casting solvent. Earlier studies have indicated that these blends are incompatible and immiscible, and their compatibility can be improved by the addition of a graft copolymer of NR and PS (NR-g-PS). The rheological behavior of these blends has been carried out in the presence and absence of the compatibilizer using a capillary rheometer and a melt flow indexer. The effects of blend ratio, processing techniques (melt mixing versus solution casting), shear stress, and temperature on the rheological behavior have been studied in detail. Both in the presence and absence of the copolymer, the blends showed a decrease in viscosity with an increase of shear stress, indicating pseudoplastic nature. Solution-cast blends showed a higher viscosity as compared to melt-mixed blends. The viscosity versus composition curve of both melt-mixed and solution-cast blends showed negative deviation from the additivity at a higher shear rate region. This is associated with the interlayer slip between the highly incompatible NR and PS phases. The effects of graft copolymer loading and temperature on solution-cast blends were studied, and it was found that as the copolymer loading increases, the shear viscosity increases. This is due to the high interfacial interaction between the two components in the presence of the copolymer. The copolymer, in fact, locates at the interface and makes the interface more broad. However, at higher loading of the copolymer, the viscosity of the blends decreases. This may be associated with the formation of micelles, which have a plasticizing action on the viscosity of the blends. Melt elasticity parameters like principal normal stress difference, recoverable elastic shear strain, and die swell were evaluated. Master curves have been generated using modified viscosity and shear rate functions that contain the melt flow index as a parameter. The extrudate morphology of the blends was studied using a scanning electron microscope. Addition of the copolymer reduces the domain size of the dispersed phase, followed by a leveling off at a higher concentration. The leveling off is an indication of interfacial saturation. The interparticle distance also decreased followed by a leveling off at a higher loading of the copolymer. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 69: 2673–2690, 1998     

A study of the synergism of poly(vinyl chloride)/polyether-ester blends

The synergism between blends of poly(vinyl chloride), PVC, and a polyether-ester elastomer, COPE, was investigated. Previous research indicated that blends of these two polymers produced synergistic results in the melt index and Izod impact properties of the system. This research confirmed the increased flow properties of the blends using melt index, capillary rheometer, and dynamic mechanical analysis. The synergism was found to be shear rate dependent and an additive relationship was approached at high shear rates. Differential scanning calorimetry showed that the blends were immiscible. This suggested that phase separation was the cause of the synergism in the flow properties and that elongation of the COPE domains most likely produced the shear rate dependency of the blend synergism. The phase separation would also have produced the improvement in the Izod impact properties of the blends.     

Constitutive model to predict flow of cohesive powders in bench scale hoppers

This communication empirically correlates flow in two systems; an instrumented rotating drum (GDR) and a set of bench scale hoppers. A flow index obtained from measurements in the GDR is directly correlated to the flow through hoppers, providing a predictive method for hopper design and a convenient experimental test for screening materials and determining their suitability for specific hopper systems. Simulations were performed to understand the dynamics of flow in hoppers by using the same flow parameters in hoppers and rotating cylinders. Simulations showed that as cohesion increased it becomes harder for the particles to flow through the hoppers, in good agreement with the experiments. The effect of hopper angle also yields similar findings to experiments for Avicel, K=60, where the powder does not flow through the 45° hopper but flows well in a 75° hopper. Simulations were also used to calculate the normal forces on the walls of the hopper and the wall pressure distributions in both hoppers. As depth increases, the wall pressure increases for all cases. Finally, the simulations also helped understand the different flow behaviors (funnel and mass flow) that take place in a hopper. The simulated dynamics of flow in the rotating drum and in the hopper correlate very closely to experiments, indicating that the model cohesion parameters are, as desirable, material-specific but independent of geometry.     

Rheological behavior of reactive blending of epoxidized natural rubber with cassava starch and epoxidized natural rubber with natural rubber and cassava starch

Epoxidized natural rubbers (ENR‐25 and ENR‐45) were prepared using the performic epoxidation method. Two‐component (ENR–cassava starch) and three‐component (ENR–NR–cassava starch) blends were prepared. ENR‐25 and ENR‐45 were blended with various quantities of gelatinized cassava starch in the latex state. The pure ENR exhibited lower shear stress and shear viscosity than those of the blends with cassava starch. Furthermore, the shear stress and shear viscosity were increased with an increase in the cassava starch concentration. The chemical interaction between the epoxide groups in the ENR and the hydroxyl groups in the cassava starch molecules might be the reason for the increasing trends of the shear stress and shear viscosity. The blends are classified as compatible blends because of the strong chemical bonding between different phases. SEM micrographs were used to clarify the compatibility. Power law behavior with pluglike flow profiles was observed for all sets of ENR–NR–cassava starch blends. Very low power law index values (<0.34) and highly pseudoplastic fluid behavior were also observed. The log additive rule was applied to plots of zero shear viscosity (consistency index) and the shear viscosity versus the concentration of ENR‐25. Positive deviation blending was observed, which indicates compatible blends. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1752–1762, 2004     

Melt rheology and elasticity of natural rubber—ethylene–vinyl acetate copolymer blends

The melt rheology of blends of natural rubber (NR) and ethylene–vinyl acetate copolymer (EVA) has been studied with reference to the effects of blend ratio, cross-linking systems, shear stress, and temperature. When EVA formed the dispersed phase, the viscosity of the blends was found to be a nonadditive function of the viscosities of the component polymers at lower shear region, i.e., a positive deviation was observed. This behavior has been explained based on structural buildup of dispersed EVA domains in the continuous NR matrix. The effect of the addition of silica filler on the flow characteristics of the blends has been investigated. The melt elasticity parameters such as die swell, principal normal stress difference, recoverable shear strain, and elastic shear modulus of NR–EVA blends were also evaluated. © 1993 John Wiley & Sons, Inc.     

Flow Behavior of Polypropylene/Ethylene-Propylene Diene Terpolymer/Natural Rubber (PP/EPDM/NR) Blends by Torque Rheometer: The Effect of N,N-m-Phenylene Bismaleimide (HVA-2) Addition

Blends of polypropylene (PP)/ethylene-propylene diene terpolymer (EPDM)/natural rubber (NR) with different ratio were investigated using a Haake torque rheometer. The effect of N,N-m-phenylene bismaleimide (HVA-2) addition on the flow behavior of PP/EPDM/NR blends also was studied. The torque data was collected at different rotor speeds in the range of 30–60 rpm and different processing temperatures in the range of 170–190°C. The recorded data were interpreted in terms of apparent shear rate, apparent shear stress, and apparent viscosity. The shear stress–shear strain curve shows that all blends follow the power law where the pseudoplasticity behavior of melt viscosity increases with increasing NR content as well as addition of HVA-2. The apparent viscosity of the blends was found to increase with increasing NR content in the blend. The addition of HVA-2 increases the apparent viscosity due to the formation of cross-linking in rubber phase. However, blends with HVA-2 show lower flow activation energies than do similar blends without HVA-2. Scanning electron microscopy (SEM) shows good correlation with the flow properties of the blends.     

Study of the rheological properties of melts of polypropylene and poly(butylene terephthalate) blends

The viscosity properties of melts of fibre-forming polypropylene (PP) and poly(butylene terephthalate) blends were investigated in the entire range of ratios at different stresses and shear rate gradients at 230–250°C. It was shown that melts of fibre-forming PP and PBT blends are weakly crosslinked systems. The effect of the mass ratio of PP and PBT on the apparent activation energy of viscous flow of melts of the blends was investigated at different shear stresses and shear rate gradients. It was hypothesized that this blend can be assigned to the group of limitedly compatible systems. The probability of compatibility of the polymers in the melt appears when up to 20% PBT is incorporated in the PP. The blends are not compatible for the remaining ratios of polymers in the investigated system. __________ Translated from Khimicheskie Volokna, No. 5, pp. 17–21, September–October, 2006.     

Flow properties of vacuum gas oil–low density polyethylene blends

In this paper, the viscous flow behaviour of Vacuum Gas Oil (VGO) with different fractions (0–10% wt.) of Low Density Polyethylene (LDPE) under dynamic shear has been investigated. Viscosimetry measurements of the blends at temperatures between 333 and 433 K using a BOHLIN Controlled Stress Rheometer, as well as compatibility studies using Differential Scanning Calorimetry (DSC) were carried out. The effects of the variation of the blends polymer content on the activation energy of flow has also been investigated. The results obtained reveal that the blends show Newtonian flow behaviour at higher temperatures for all polymer concentrations studied, while at lower temperatures and at higher polymer concentrations, they show non-Newtonian shear-thinning behaviour. Furthermore, at lower temperatures, these behaviours are more pronounced at lower shear rates than at higher shear rates. As the polymer content in the blend is increased, the shear viscosity increases, the flow behaviour index decreases, and the application of an Arrhenius type equation shows an increase in the activation energy of flow at the lower shear rates.     

Study on ester–amide exchange reactions between Nylon 1010 and Ethylene‐vinyl acetate rubber

Reactive processing is a useful method to improve the compatibility of immiscible polymer blends. Nylon 1010/Ethylene‐vinyl acetate rubber (EVM) blends were prepared via melt blending at 240°C and tetrabutyl titanate (Ti(OBu)₄) was used as a catalyst. Ester–amide exchange reactions were proven to take place between Nylon and EVM during the shear processing. Melt flow index, Fourier transform infrared spectroscopy, and proton nuclear magnetic resonance spectra were used to study the reactions. It was demonstrated that tuning the shear rate could control the properties and reaction extent of Nylon 1010/EVM/Ti(OBu)₄ blends. The results revealed that the reactions were promoted by high shear rate. Tensile strength of the blends increased from 4.5 to 11.4 MPa when the shear rate increased from 20 to 80 rpm. Meanwhile, scanning electron microscopy was adopted to study the morphology of the reactive blends. It was found that the morphology of the blends was changed from sea‐island structures to co‐continuous structures while increasing the shear rate from 20 to 100 rpm. Dynamic mechanical analysis confirmed that high‐shear processing was found to promote the compatibility of the blends. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40064.     

Studies on blends of polychloroprene and polybutadiene rubber containing phosphorylated cardanol prepolymer: Melt rheology,cure characteristics,and mechanical properties

Melt rheology, cure characteristics, and mechanical properties of blends of polychloroprene (CR) and polybutadiene rubber (BR) in the presence and absence of phosphorylated cardanol prepolymer (PCP) were studied. The melt rheology parameters of the blends over a wide range of shear rates and temperatures were studied using a capillary rheometer (Rheoflixer SWO). The plasticizing effect of PCP in the blends was indicated by reduction in apparent melt viscosity and activation energy for melt flow. Good compatibility between the blend components (CR and BR) in the presence of PCP was evidenced by the lower values of principal normal stress difference. The self crosslinking behavior of the blends in the presence and absence of PCP was studied at different temperatures, using a Brabender Plasicorder and the kinetic parameters of crosslinking were evaluated. The cure characteristics of blends of CR and BR containing different dosages of PCP (0–10 phr) in a semi efficient vulcanization system were also studied at temperatures ranging from 150°C to 180°C, using an oscillating disk rheometer. The increase in tensile modulus, tensile strength, and tear strength of the vulcanizates in the presence of 5 phr of PCP is presumed to be an indication of reinforcement resulting from accelerated cross linking reaction as evidenced by higher chemical crosslink density index. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3195–3200, 2006     

Quantitative experimental study of shear stresses and mixing in progressive flow regimes within annular-flow bioreactors

Annular-flow bioreactors are normally operated under laminar Couette flow conditions in order to minimise shear-induced damage to cells. In this study, we computed the fluid shear stresses in model annular vessels over a range of laminar flow regimes, from Couette flow to Taylor-vortex flow, and at two geometric scales, using a shear rate model for freely suspended particles, together with experimental Laser Doppler Anemometry data for a 2-D velocity field. The shear stresses were greatest in the boundary layers adjacent to each wall in each case, with values typically 6 times higher than the mean stresses in the annular space; their respective magnitudes were significantly lower in the larger of the two vessels studied, however. Cell viability studies were also performed in which mammalian cells were cultured under dynamic conditions in a functional bioreactor having the same dimensions as the smaller vessel. The results of these studies demonstrated that a significantly greater number of cells remained in suspension in Taylor-vortex flows than in Couette flow, but at the expense of cell viability at higher Taylor numbers. Taken together, these findings suggest that the benefits of enhanced convective mass transport afforded by Taylor-Couette flows could be realised without risk of appreciable shear induced damage of cells and tissues in larger vessels operating under dynamically similar flow conditions.     

Microstructural evolutions of LDPE/PA6 blends by rheological and rheo-optical analyses: Influence of flow and compatibilizer on break-up and coalescence processes

In this paper we investigate the relationships between flow and morphology in immiscible diluted non-Newtonian (LDPE and PA6) blends. Both rheological and rheo-optical techniques have been used. The effect of a steady shear flow on the blend microstructure has been studied at various shear rates. Moreover, the effects of a compatibilizing agent on rheology and morphology have been analysed. The compatibilizer causes decrease of average drop sizes and stabilization of the blend morphology during a steady shear flow. An attempt to relate the evolution of the volume-average radius with flow has been carried out by following the approach generally pursued for Newtonian polymer blends. The rescaling of both the capillary number and the viscosity ratio by using the shear-rate-dependent viscosity of blend constituents allows to roughly estimate the shear rate wherein severe drop break-up phenomena occur for the uncompatibilized system. Conversely, quantitative discrepancies between experimental data and theoretical expectations have been found for compatibilized blend. The coarsening behaviour in the early stages of an annealing process at rest has been studied, and simple model has been proposed to describe the volume average drop radius growth rate. The coalescence suppression detected for the compatibilized blend could be due to the shielding effect related to the presence of the copolymer layers at the interface between the phases.     

Rheological Properties of Thermoplastic Polyvinyl Alcohol and Polypropylene Blend Melts in Capillary Extrusions

A single screw extruder with a static mixer was used to prepare molten blends of thermoplastic polyvinyl alcohol (TPVA) and polypropylene (PP). The effects of shear rate, blending ratio and temperature on rheological properties for the blends in capillary extrusions were investigated, and ends correction was also carried out. Rheological parameters such as non-Newtonian index and activation energy were also calculated and evaluated. It was found that the viscosities of the blends were lower than those of TPVA and PP; moreover, the non-Newtonian indices and the activation energies of the blend melts were higher than those of the homopolymers. In particular, the blend with 60 wt% TPVA had the highest non-Newtonian indices and activation energies among blend melts. These results indicate that TPVA and PP blends are negative deviation blends. Furthermore, at a blending ratio of 60 wt% of TPVA, the shear-sensitivity of the viscosity was the lowest and the temperature dependence of the viscosity was the highest. In addition, an increase in temperature led to an increase in non-Newtonian index, therefore the shear-rate dependence of the blend viscosities decreased with a rise in temperature. As the shear rate was increased, the variation of the viscosity over blending ratios decreased while the activation energy of the blends decreased. Thus the effects of temperature and blending proportion on flow behavior were diminished by increasing shear rate.