Dispersion engineering and modeling of silica filled rubber compounds

Mechanical properties of filled plastic/rubber systems are strongly affected by the mixing process and the interfacial interactions between filler and polymer. Mixing involves the breakage of powder agglomerates into fragments by the hydrodynamic action of a flow field (dispersive mixing) and the distribution of the resulting fragments in the matrix (distributive mixing). Dispersion is of paramount importance since the efficiency of this process, the filler fragment size distribution, and the uniformity of the mixing outcome are crucial parameters for the properties of the final compound.; Several factors affect the dispersion process, namely powder properties (agglomerate/aggregate structure, morphology and surface properties), flow field characteristics (fluid viscosity, shear rate, geometry and hydrodynamic stresses), and interfacial interactions between polymer and filler. Investigating, characterizing and understanding how these factors affect the dispersion process is important for better engineering of the processing operations.; Dispersion studies on single spherical powder agglomerates were undertaken in two different devices able to induce a simple shear flow under constant and oscillatory shear conditions. We used transparent polymers, enabling direct observation of the dispersion process and measurement of the erosion kinetics.; The strength of powder agglomerates and the hydrodynamic forces transmitted to the solid are also influenced by the presence of liquid inside the agglomerate pores. Infiltration studies allowed us to characterize and model infiltration kinetics of liquids into powder beds based on a hierarchical structure for the agglomerates.; Powder characteristics can be tailored and/or modified by adding coupling agents, used also to aid the silica-rubber bonding. Optical microscopy and light scattering were employed to investigate the effect of surface treatment on silica morphology and packing properties. The aggregate size distribution of the treated powder was found to be smaller than for the untreated powder. Packing properties and permeability were also influenced by powder surface treatment. Dispersibility was greatly enhanced by powder surface treatment and the effect was improved with increasing coupling agent loading.; The dispersion of silica agglomerates was investigated under different flow conditions, flow strength and geometries. The information obtained was translated into an erosion model, which can predict the dispersion process under a variety of conditions. The model assumes the erosion rate to be proportional to the difference between hydrodynamic and cohesive forces and with the rotation velocity of the dispersing agglomerate. For dispersion under identical hydrodynamic conditions, the model predicts faster erosion for larger agglomerates. Moreover, at comparable stresses, erosion is enhanced at higher shear rates. The model can also predict erosion for fractal agglomerates and can potentially include the effect of fluid infiltration on the kinetics of the process. The influence of flow geometry was investigated experimentally and theoretically by analyzing the erosion history of any given point on the agglomerate surface. Flow geometry was found to affect the uniformity of the erosion history, and from the analysis of experimental results, it seems that a more uniform distribution leads to enhanced erosion. The model can be used to make predictions in different flow fields and to compare dispersing devices in terms of their efficiency. Under certain condition, an oscillatory shear flow may be more efficient than a steady flow. The window of applicability of the model was also discussed.