Experimental and numerical investigations of the shear behavior of binary particle blends

In this study, a Schulze ring shear tester and the discrete element method (DEM) are employed to investigate the effect of polydispersity on the binary shear flows. Both experimental results and DEM simulations show that the preshear stresses are greater for binary blends than for monodispersed particles. The flowability of these mixtures is strongly affected by the solid fraction, with minimal flow function values correlating to maximum packing fraction. However, minimum flow function values are not observed at the same packing fractions where the maximum preshear stress occurs. Using DEM, it is demonstrated that the decrease of angular velocity of larger particles due to the addition of small adhesive particles reduces and the fraction of large-small particle contact both make contributions to shear stress difference. A mechanism is proposed to quantify the effects of these two factors.     

THE FLOW BEHAVIOR OF SUBMICRON α-Al2O3 AQUEOUS SUSPENSIONS

The viscosity of aqueous, charged a-Al₂O₃ suspensions with and without added polyelectrolytes (polyacrylic acid (PAA 1800)) is investigated over a wide range of volume fractions and shear rates. The Carreau-Yasuda model is used for both cases for examining the shear-rate dependence; for the electrostatically-stabilized suspensions the Krieger-Dougherty and Quemada models are used to determine the maximum packing fraction. The latter analysis shows that the suspension behavior changes from liquid-like to solid-like at a volume fraction of 0.71 in the low-shear limit and at 0.89 at the high-shear limit (because of the polydispersity of the suspension). The results with polyelectrolytes indicate that the dosage of the polymer plays an important role in the viscosity of suspensions and that there is an optimum dosage of polyelectrolyte that must be added to reduce the viscosity at high volume fractions of solids.     

Improved Equation of the Continuous Particle Size Distribution for Dense Packing

The Furnas model describes the discrete particle size distribution for densest packing. Using a model that considers a continuous particle size distribution for the densest packing to be a mixture of infinite Furnas discrete particle size groups, an equation for the cumulative particle size distribution providing the densest packing was derived. Monosize particles with different shapes have a different packing pore fraction. One parameter in the equation is the pore fraction of packed monosize particles; the particle size distribution for achieving densest packing is a function of this pore fraction. A reduced form of this equation is also presented as a working equation. The equation derived here is compared to the modified Andreasen equation for dense packing. An equation and the correlated graph for calculating theoretically the geometric mean particle size and an equation for calculating the specific surface area of the particle size distribution of the improved equation are also derived.     

Rheology of Glaze Suspensions

Rheological properties of suspensions and ceramic glaze slurries under steady flow conditions have been considered. Colloidal forces play an important role in the rheology of such ceramic slurries. Since the potential function characterizes the rheology of colloidal systems, a new dimensionless group, viz. potential number, is introduced within a dimensional analysis representing the relative significance of the potential to the Brownian energy. In order to relate the relative viscosity to other dimensionless groups, a new model is proposed by the inclusion of an extra term in addition to that of the hard‐sphere theory owing to the fact that the presence of colloidal forces always increases the fluid viscosity with respect to that predicted by the hard‐sphere. Steady viscosity measurements have been carried out on ceramic glaze suspensions at different volume fractions, particle diameters, and shear rates. Experimental results have been used to modify the model relating the relative viscosity to the Péclet number, potential number, volume fraction, and maximum packing fraction.     

Characterization of polyisoprene-clay nanocomposites prepared by solution blending

The effects of clay dispersion and the interactions between clays and polymer chains on the viscoelastic properties of polymer/clay nanocomposites are investigated using oscillatory shear rheology, X-ray diffraction (XRD), small-angle X-ray scattering (SAXS), and transmission electron microscopy (TEM). Four different montmorillonite silicates of natural clays, plasma-treated clays, and organically modified clays (OCs) have been used in this study. For the polyisoprene (PI)/clay nanocomposites, the exfoliation of the OC dispersed in the PI matrix is confirmed with XRD and SAXS although TEM images show both exfoliated and non-exfoliated nanoclay sheets. In contrast aggregation or intercalation is obtained for the other PI/clay composites studied here. Additionally, the effective maximum volume packing fraction of OC for the exfoliated nanocomposites is determined from the overlapping of dynamic viscosity at low frequency regime, in which the effective maximum volume packing fraction is larger than the percolation threshold determined from the storage modulus of the nanocomposites.     

Rheological behavior of highly filled polymer melts

The rheological behavior of highly filled polymer melts has been examined. At concentrations near the maximum packing fraction, strain-dependent behavior was observed at strains as low as 1 percent. Selected surface treatments were shown to reduce particle agglomeration. This produced composite melts with lower viscosities and higher maximum loadings. While η* – ω plots provide information on the shear strength of the interparticle network, G′ – ω plots show evidence of phase separation.     

Electrical conductivity of carbon black filled polymers—effects of morphology and processing

This paper discusses the relation of the morphology of conductive carbon black and its critical volume fraction, ?_c, required to achieve semiconductor property. We also discuss the influence of processing on the electrical conductivity of polymer composites. An equation based on the crowding factor of concentrated suspension rheology and Janzen's particle contacts percolation is proposed to describe the relationship between ?_c, and the maximum packing fraction of conductive fillers. The relationship could explain the influence of particle morphology on conductivity and the conductivity difference in the high shear and low shear region of a processed polymer composite part.     

Transverse compression failure of unidirectional composites

The mechanical response of unidirectional composites subject to uniaxial transverse compressive loads was measured and analyzed by finite element simulation. Consistency in failure plane orientation was observed when comparing simulated matrix shear band angle to measured crack angle. A model based on hexagonal packing of fibers was proposed and the shear band angle was shown to depend on the fiber volume fraction. The effects of strong and weak fiber–matrix interfaces were considered using models with randomly distributed fibers for a valid statistical analysis. The results of these models showed that the composite compressive strength increased with the fiber loading for the strong interface case, while the strength was independent of the fiber loading for the weak interface case because of interface debonding. POLYM. COMPOS., 36:756–766, 2015. © 2014 Society of Plastics Engineers     

Packing Density of Composite Powder Mixtures

A model of particle packing in binary composite systems is developed. The effects of both inclusion surfaces and touching inclusions on the packing density are taken into account. To implement the model, a statistical approach is used to determine the number of inclusion contacts as a function of inclusion content. The statistical approach indicates that the average number of inclusion contacts is a linear function of the inclusion volume fraction, a result which agrees very well with independent computer simulations. The model suggests that the packing efficiency, defined by the ratio of the packing density to the ideal packing density (as originally derived by Furnas), is governed by the inclusion volume fraction ( f_i ) and the particle to inclusion size ratio ( r ). Good agreement is obtained between the theory and experimental literature data.     

Low shear rheological behaviour of two-phase mesophase pitch

The low shear rate rheology of two phase mesophase pitches derived from coal tar pitch has been investigated. Particulate quinoline insolubles (QI) stabilised the mesophase spheres against coalescence. Viscosity measurements over the range 10–10⁶ Pa s were made at appropriate temperature ranges. Increasing shear thinning behaviour was evident with increasing mesophase content. At low mesophase contents the dominant effect on the near Newtonian viscosity was temperature but at higher contents it was the shear rate; temperature dependence declined to near zero. The data indicated that agglomeration could be occurring at intermediate mesophase volume fractions, 0.2–0.3. The Krieger–Dougherty function and its emulsion analogue indicated that in this region the mesophase pitch emulsions actually behaved like ‘hard’ sphere systems and the effective volume fraction was estimated as a function of shear rate illustrating the change in extent of agglomeration. At the higher volume fractions approaching the maximum packing fraction, which could only be measured at higher temperatures, the shear thinning behaviour changed in character and it is considered that this is possibly due to shear induced deformation and breakup of dispersed drops in the shear field.