Principal Normal Stress Difference Research Articles

The electrohydrodynamic plumes evolution in viscoelastic fluids with energy transfer process

Two-dimensional direct numerical simulation was carried out to investigate the various dynamic characteristics of electrohydrodynamic plumes in viscoelastic fluids in a complex blade-plate configuration. Here, the effects of different elasticities (Weissenberg number, Wi) and the maximum possible extension lengths (L) of polymer are studied. The results show that a new stem expansion process is found for viscoelastic plumes, which is explained by the transport mechanism of free charges. The rise and spread of plumes are promoted with an increase in elasticity and L, which we interpret from both an energy perspective and through the concept of the first principal normal stress difference (Np) for the first time. Our results suggest that when elasticity is weak, the total currents in the system are slightly enhanced due to the denser charge density, and above a certain value of Wi they are considerably inhibited maximum by 11.8% with the increasing of Wi or L mainly because of the variation in Uy.

Synthesis and Characterization of Chemically Crosslinked Styrene-Butadiene Rubber Nanogels and their Effect on Various Properties of the Rubber

Abstract Nano-sized styrene-butadiene rubber (SBR) latex gels were prepared by pre-vulcanizing SBR latex with different sulfur to accelerator ratios. These gels were characterized by swelling studies, dynamic light scattering, atomic force microscopy, mechanical and dynamic mechanical properties. With the increase in sulfur to accelerator ratio, the gels had higher amount of crosslink density and gel content. Particle size distribution did not alter much in the crosslinked gels. Incorporation of these nanogels into raw SBR led to the considerable drop in viscosity of the gel filled systems under capillary melt flow conditions. However, the reduction in viscosity was found to be dependent on the loading and crosslink density of the nanogels. Energy dispersive X-ray mapping of sulfur was used to check the dispersion of these gels into raw SBR matrix. The die swell values of gel filled SBR were much lower than that of the raw SBR. The reduction in principal normal stress difference values combined with the reinforcement effect of the gels was found to be responsible for the lowering of die swell values. Scanning electron photomicrographs of extrudates of gel filled systems showed much-improved surface smoothness compared to the unfilled SBR. The mechanical and dynamic mechanical properties also showed excellent improvement in modulus with the addition of gels in the raw rubber. A new empirical relationship was proposed to explain the reinforcement properties of nanogels as viscoelastic fillers.

Effects of quasi‐nanogel particles on the rheological and mechanical properties of natural rubber: A new insight

AbstractThe influence of sulfur‐crosslinked, quasi‐nanosized gels on the rheological and mechanical properties of raw natural rubber (NR) was investigated. Latex gels with different crosslink densities were prepared through the variation of the sulfur‐to‐accelerator ratio. These gels were characterized by dynamic light scattering, solvent swelling, and mechanical properties. The gels were mixed with raw NR latex at concentrations of 2, 4, 8, and 16 phr, and their effect on the rheological properties of NR was studied by Monsanto processability tester. The presence of gel in raw NR reduced the apparent shear viscosity and die swell considerably. Initially, the viscosity decreased up to a 8 phr gel loading and then increased with an increase in the gel loading. However, the change in the viscosity was related to the crosslink density of the gels. A new empirical equation relating the viscosity, volume fraction of the gels, and crosslink density was proposed. The die swell of gel‐filled raw NR was at least 10% lower than that of unfilled raw NR and decreased with an increase in the gel loading. The effect of the gels on the die swell properties was explained through the calculation of the principal normal stress difference of gel‐filled NR systems. Scanning electron photomicrographs of the extrudates revealed much better surface smoothness for the gel‐filled virgin rubber systems than for the unfilled rubber. The addition of the gels to raw NR increased the modulus and tensile strength, whereas the elongation at break decreased. The effect of the gels on the dynamic mechanical properties of NR was also investigated. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Effect of electron beam-cross-linked gels on the rheological properties of raw natural rubber

Electron beam (EB)-cross-linked natural rubber (NR) gels were prepared from latex and characterized by various techniques. The addition of a small amount of these gels to raw NR was found to reduce the apparent shear viscosity and die swell remarkably. This effect was further enhanced with the addition of butyl acrylate as a sensitizer. The apparent shear viscosity first decreased up to 8 phr of gel loading and then increased. However, the percent die swell value decreased steadily upon gel loading. These were explained by calculating principal normal stress difference, the activation energy of melt flow and characteristics of EB-cross-linked gels. These effects were also reflected in the changes of mechanical and dynamic mechanical properties of gel-filled raw NR. Tailoring of the above properties could be done with the help of these gels.

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

AbstractMelt 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

Melt elasticity and flow activation energy of nylon 6/polystyrene blends

Nylon 6 (PA 6) is blended with polystyrene (PS), so as to reduce the moisture absorption and hence to impart dimensional stability to the polymer. The melt elastic properties of the blends including die swell, principal normal stress difference and recoverable shear strain have been analysed. The temperature sensitivity of the flow behaviour of the blends is studied using Arrhenius plots.

Melt elasticity behaviour and extrudate characteristics of LLDPE/EVA blends: effect of blend ratio, compatibilisation and dynamic cross-linking

A study on the melt elasticity behaviour and extrudate characteristics of linear low density polyethylene (LLDPE)/ethylene vinyl acetate (EVA) blends was done. Extrusion studies were carried out in a capillary rheometer. Examinations of the surface characteristics of extrudates were made by taking photographs. With a view to characterize melt elasticity of these blends, parameters such as die swell, principal normal stress difference, recoverable shear strain, and shear modulus were calculated. Melt elasticity of the blends increases with EVA concentration and shear rate. The morphology of the blends was modified by physical compatibilisation process and dynamic vulcanisation. Compatibilisation reduces extrudate deformation, whereas dynamic cross-linking increases extrudate deformation and fracture.

Rheological behavior of highly filled ethylene propylene diene rubber compounds

The rheological behavior of highly filled ethylene propylene diene rubber (EPDM) compounds was studied with respect to the effect of curative system, grafted rubber, shear rate, temperature and die swell using a Monsanto Processability Tester (MPT) to gain an understanding of the molecular parameters that control the surface finish. All systems show pseudoplastic behavior. At a particular shear rate, shear viscosity increases with blend ratio. The dependence of flow behavior on extrusion velocity indicates a surface effect. The extrudate die swell and maximum recoverable deformation are related by a linear relationship, which is independent of sulfur/accelerator ratio, extrusion temperature and shear rates and blend ratio. The principal normal stress difference increases nonlinearly with shear stress. Activation energy decreases with shear rate in most cases. The faster relaxing system produces extrudate of better surface quality.

Melt rheology and morphology of thermoplastic elastomers from polyethylene/nitrile-rubber blends: The effect of blend ratio, reactive compatibilization, and dynamic vulcanization

A study of the melt-rheological behavior of thermoplastic elastomers from high-density polyethylene and acrylonitrile butadiene rubber (NBR) blends was carried out in a capillary rheometer. The effect of the blend ratio and shear rate on the melt viscosity reveals that the viscosity decreases with the shear rate but increases with NBR content. Compatibilization by maleic anhydride modified polyethylene has no significant effect on the blend viscosity, but a finer dispersion of the rubber is obtained, as is evident from scanning electron micrographs. The melt-elasticity parameters, such as the die swell, principal normal stress difference, recoverable shear strain, and elastic shear modulus of the blends, were also evaluated. The effect of annealing on the morphology of the extrudate reveals that annealing in the extruder barrel results in the coalescence of rubber particles in the case of the incompatible blends, whereas the tendency toward agglomeration is somewhat suppressed in the compatibilized blends. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1104–1122, 2000

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.

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

Rheological Study of Filled SBR Compounds with Trans-Polyoctenylene (TOR) and their Interaction Mechanism

Abstract The purpose of this study is to find the suitability of a crosslinkable processing aid, trans-polyoctenylene (TOR) on general purpose rubber based filled compounds for application in thick rubber products. Objective is to have better processability as well as improvement in vulcanizate properties using TOR. Thus to investigate its effect as processing aid, rheological behaviour of TOR filled Styrene butadiene rubber (SBR) compounds has been studied in a capillary rheometer. Two types of carbon black were used. All compounds show pseudoplastic behaviour and obey Power-law equation in the shear rate range studied. True viscosity decreases marginally at 60°C and increases little at 100°C with addition of TOR. Die swell (index) decreases with incorporation of TOR, a probable mechanism has been suggested. Maximum recoverable deformation (vm ) has been correlated with die swell (α). Principal normal stress difference (N 1) decreases with increase in TOR which is explained through mechanistic theory of particle-particle interaction. Incorporation of TOR in the system lowers activation energy of flow. Study concludes with selection of TOR dose.

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

AbstractThe 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.

Rheological behavior and processability of neoprene and acrylic rubbers

AbstractRheological behavior of neoprene and acrylic rubbers and their carbon blackfilled compounds has been studied using a Monsanto Processability Tester. These systems show pseudoplastic flow behavior. Die swell increases with increase in shear rate up to a limit beyond which it decreases. It also decreases with increase in filler loading and temperature. The maximum recoverable deformation has been calculated by assuming that the viscous response of the rubbers obeys the power law model and elastic behavior is described by Hooke's law and has been correlated with die swell. A linear generalized relationship has been obtained between the die swell and the recoverable deformation. The principal normal stress difference has been found to increase nonlinearly with shear stress. Activation energy of melt flow process increases with increase in shear rate up to a limit, after which it decreases for acrylic rubber and 10 phr filled neoprene rubber.

Melt rheology of plasticized poly(vinyl chloride)/epoxidized natural rubber miscible blends

AbstractA study of the melt rheological behavior of miscible blends of plasticized poly(vinyl chloride) (PVC) and epoxidized natural rubber (ENR) was carried out with a capillary rheometer. Melt viscosity and melt elasticity behavior was determined and surface characteristics of the extrudate was examined by using a scanning electron microscope. Melt viscosity of plasticized PVC increased with ENR content at higher processing temperatures. The melt elasticity parameters such as die swell ratio, principal normal stress difference, and recoverable shear strain at a particular shear stress increased with temperature up to a 50 wt % composition of ENR in blends. For high ENR blends the die swell ratio and recoverable shear strain decreased with temperature, and the principal normal stress difference and elastic shear modulus remained independent of temperature for all values of shear stress. It was noted that the nature of deformation in the surface layer was linear for a smooth extrudate and nonlinear for a distorted extrudate. Moreover, recoverable shear strain was found to have a correlation with the extrudate distortion of the melts.

Melt elasticity behavior and extrudate characteristics of rigid poly(vinyl chloride)/epoxidized natural rubber miscible blends

AbstractA study on the melt elasticity behavior and extrudate characteristics of melts of rigid poly(vinyl chloride), PVC, and rigid poly(vinyl chloride)/epoxidized natural rubber (ENR) miscible blends were conducted. Extrusion studies were carried out in a capillary rheometer and examinations of the surface characteristics of the extrudate were made by taking photomicrographs in a scanning electron microscope. The anomalous behavior in the die swell ratio of rigid PVC arising from the particle agglomerates continued in its blends up to 50 wt% composition of ENR. Temperature independence for high ENR blends was noted for the principal normal stress difference and elastic shear modulus, when shear stress was held constant. Recoverable shear strain and die swell ratio behaved identically in terms of blend composition and processing temperature. Factors which control the extrudate distortion and melt fracture of the melts of rigid PVC/ENR systems were fusion of particle agglomerates and strength of melts. Diamond cavitations were typical of the extrudate surface of PVC melts as those of the fracture surface of the tensile failure of PVC. Conditions to obtain a smooth extrudate surface of rigid PVC melts in blends with ENR have been found to be the low ENR content, low shear rate, or stress and high processing temperature.

Rheological and extrusion behavior of dispersed multiphase polymeric systems

An experimental study of temperature effect and composition effect on the rheological and extrusion properties of several dispersed multiphase polymer melts were investigated, using a cone-and-plate rheometer and a capillary rheometer. The polymeric systems studied included three homopolymers (two polystyrenes and one poly(methyl methacrylate) (PMMA)), a mechanically blended copolymer of polystyrene and PMMA, two graft copolymers (rubber-modified polystyrene and PMMA), and three particulate-filled polystyrene (CaCO3, milled glass fiber, and glass flake). It was found that the principal normal stress difference plotted against shear stress gives rise to a temperature independence for all dispersed multiphase polymeric systems. Composition independent correlations, however, do not exist for the principal normal stress difference. The extrudate swell plotted against shear stress becomes independent of temperature only for the homopolymers and graft copolymers. For the mechanically blended polymers and particulate-filled polymers, the temperature independent correlation does not exist. The reduction in viscosity of the glass fiber- and glass flake-filled polystyrenes is found due to the degradation of the base polymer during mixing.

A comparative experimental study of the isothermal shear and uniaxial elongational rheological properties of low density, high density and linear low density polyethylenes

The rheological properties of a series of sixteen polyethylene melts with different molecular weights, molecular weight distributions and types of long chain branching have been investigated. Measurement of shear viscosity, principal normal stress difference and elongational flow response are included. The behavior of high density polyethylenes and linear low density polyethylenes are described in an equivalent way in terms of their molecular weights and molecular weight distribution. The LDPE response is different, being strongly influenced by the extent of long chain branching. This appears to elevate normal stresses, and introduce substantial “strain rate hardening” in uniaxial extension. The experimental data are interpreted in terms of the theory of second-order fluids and the convected Maxwell model. The influence of long chain branching as well as molecular weight distribution on constitutive equation parameters is discussed.

High density polyethylene/polystyrene blends: Phase distribution morphology, rheological measurements, extrusion, and melt spinning behavior

AbstractAn experimental study of the development of phase morphology, rheological properties, and processing behavior of mechanical blends of a polystyrene (PS) and a high density polyethylene (PE) is presented. Phase morphologies were determined by scanning electron microscopy for (i) products prepared in a screw extruder/static mixer system, (ii) samples removed from a cone‐plate viscometer, (iii) extrudates, and (iv) melt spun fibers. Disperse phase dimensions were measured. The values varied from 1–5 μm in the products from static mixers. The dimensions of the dispersed phase in the blend products from the cone plate and capillary die were of the same order. The melt‐spun fibers exhibited disperse phase dimensions as low as 0.35 μm. Polystyrene was extracted from the blend fibers producing small diameter, PE fibrils, or minifibers. Both the initial melts and the blends were rheologically characterized. The shear viscosity and principal normal stress difference N1 exhibit maxima and minima when plotted as a function of composition. The characteristics of extrudates and melt spinning behavior of the blends were investigated. The shrinkage of extrudates of PE is much greater than PS. Additional small amounts of PE to PS greatly increase its shrinkage. Addition of PE to PS initially increases extrudate swell, though the swell shows maxima and minima when considered as a function of composition. The positions of the maxima and minima correspond to those of N1. The onset of draw resonance has been investigated in isothermal melt spinning. Wide angle X‐ray diffraction studies have been carried out on blend fibers and the orientation of the crystalline polyethylene regions has been determined as a function of process conditions. This orientation decreases rapidly with the addition of polystyrene when the melt‐spun filaments are compared at the same spinline stress or drawdown ratio.

Polypropylene/nylon 6 blends: Phase distribution morphology, rheological measurements, and structure development in melt spinning

AbstractA series of polypropylene (PP)/nylon 6 (N6) blends of composition 75/25, 50/50, and 25/75 have been prepared in a screw extruder combined with a Koch static mixer. The phase morphology was observed with a scanning electron microscope. The influence of heating in the reservoir of a rheometer followed by subsequent extrusion through a capillary on the phase morphology was investigated. Phase size growth as a function of time was observed under quiescent and mild deformation rate conditions. The discrete phase size was observed to decrease with increasing extrusion rate through dies. The shear viscosity and principal normal stress difference of the blends were measured as a function of composition. The crystalline orientation of both polypropylene and nylon 6 in blend melt spun fibers was characterized by wide angle X‐ray diffraction and interpreted in terms of Hermans–Stein orientation factors. The orientation increases with drawdown ratio. The orientation factors for the polypropylene phase vary with spinline stress in a manner independent of composition and identical to that for pure polypropylene. Extracting melt spun blend fibers with formic acid has produced small‐diameter polypropylene minifilaments with diameters of the order of microns.