Volume 4, Issue 1

A dispersive mixing model focusing on the rupture of agglomerates as the step that primarily determines the dynamics of the mixing process was developed and analyzed. Rupture is predicted to occur when hydrodynamic forces exerted on the outer surface of the agglomerate exceed cohesive forces binding the agglomerate together. Agglomerates are modeled as clusters of aggregates bound by van der Waals forces. The magnitude and orientation of the rupturing hydrondynamic force depend on the local stress field in the fluid. Cleavage of the agglomerate is predicted to occur at the mid-plane of the agglomerate where the effects of hydrodynamic tension is the largest. Under the assumption that parent agglomerates and their fragments have the same shape, the kinetics of the rupture process is found to be independent of the absolute size of the agglomerate. Following from this is the result that the dispersion process is governed by flow dynamics, i.e., a minimum value of flow strength, which depends on the geometry of the bulk flow field, is required for rupture of the agglomerate. Agglomerate rupture was investigated in four flow geometries: simple shear; pure elongation; uniaxial extension, and biaxial extension. The efficiency of each flow geometry is compared on the basis of power and time requirements to achieve a given degree of dispersion.

A mathematical model is developed for simulation of flow in an internal mixer with two counter rotating non-intermeshing rotors is presented. The model presumes rotors with a screw flight which is separated into two sections. The mixing chamber is taken to be fully filled and rotor curvature neglected in analogy to screw extruder analyses. Lubrication theory is used and the fluid presumed Newtonian. The flow patterns within the mixing chamber are computed. The conditions under which rotor designs lead to global circulation patterns within the mixing chamber and good distributive mixing are considered. We also investigated the extent of fluid motion over rotor tips which should lead to dispersive mixing.

During gelation and fusion process PVC plastisols changes from a suspension of resin particles in a liquid plasticizer to a uniform mass. The manner of change was observed with a scanning electron microscope, SEM. Four different plasticizers varying solvent power were used: they were dioctyl phthalate, dioctyl sebacate, dibutyl sebacate and tricresyl phosphate. Significant differences were observed among different plasticizer systems in a manner of dissolution of PVC into a plasticizer, swelling of PVC particles with a plasticizer and fusing to form a uniform mass. The SEM micrographs are presented with reference to the viscoelastic properties at the corresponding temperature.

A flow visualization study of the blending of elastomers and the subsequent mixing of carbon black and oil in an internal mixer is described. The elastomers studied were natural rubber, cis-polybutadiene and a solution styrene-butadiene copolymer. Studies were made of addition to the mixer of mini bales of earch rubber pigmented with different colors and their subsequent homogenization. Subsequently we investigated the blending of bales of the different rubbers and determined the homogenication times. Similar investigations were made of the addition of carbon black and oil black homogenization times and oil absorption times were determine. These are correlated with the rheological properties of the elastomers.

The dissipation factor and dielectric constant of thermally aged recycled high impact polystyrene (HIPS) were measured at different aging temperatures. The results indicates that the degradation of HIPS is mainly due to surface action with minor chemical change.

A method is developed for the simulation of the transient structure and rheology of concentrated suspensions of generally-shaped particles. This procedure is a modification of the Stokesian dynamics method using boundary element to evaluate the multiple interactions of an arbitrary number of particles. The effect of commonly-used pairwise additivity assumptions on the particle velocities and suspension stress is shown for several three-spheroid structures. In general, superposition of forces gives better results than additivity of disturbance velocities. Methods for improving the simulation and an experimental program to measure the transient particle orientations and viscosity of concentrated suspensions are discussed.

The wire-coating coextrusion process has been simulated numerically using the Lubrication Approximation Theory (LAT) and the Finite Element Method (FEM). The analysis includes both Newtonian and shear-thinning fluids under isothermal or nonisothermal conditions. Special Streamline-Upwind/Petrov-Galerkin schemes have been used for an accurate solution of the energy equation in highly convective flows. The determination of the unknown a priori interface and free coating surface was carried out through an iterative procedure that allows for no cross-flow along these surfaces. The analysis shows how an initial die design for high-speed wire-coating coextrusion can be modified to meet several criteria deemed necessary for better operation. These involve: (a) lack of recirculation everywhere inside the die, (b) the development of smooth stresses along the die wall, especially near the contact region of the two melts, and (c) the location of an interface that runs almost parallel to the wire. The improved die design that meets these criteria consists of moving the torpedo to a position of optimum “gum space” and also reducing the taper in the die region. Subsequent non-isothermal studies of two typical polymer resins, namely HDPE and PS, show that the initial melt and boundary temperatures do not influence so much the results and especially the interface location as do the individual flowrates (hence flowrate ratios) for a given total coating thickness. The analysis also provides a wealth of information regarding pressure and shear stress distributions, stream-line patterns and temperature fields, along with such overall quantities as pressure drops, wire tension, maximum temperature rise and maximum stresses.