Aggregation (coagulation or floculation) is a commonly used process to separate colloidal particles from liquid, where the particles after addition of appropriate amount of coagulant are aggregated to form large clusters. Besides Brownian motion, which is the controlling mechanism for sub-micron particles or clusters, effect of shear, induced by stirring of the suspension, contributes additionally to the cluster growth as well as it controls final size of formed aggregates. The final size of produced aggregates is strongly connected to the material and surface properties of the primary particles, internal structure of aggregates and inter-particles forces between primary particles composing an aggregate. When primary particles undergo coalescence the situation becomes even more complicated.

In this work we investigate aggregation behavior of elastomer particles with sub-micron size and different surface charge groups under both, static and flow conditions. In all experiments the amount of added coagulant (acid and various types of salts) was well above the critical coagulation concentration. Depending on the size of produced aggregates either light scattering (dynamic as well as static light scattering) or image analysis were used for characterization. Moreover, for selected conditions cryogenic scanning electron microscopy and confocal scanning microscopy were used to analyze the cluster structure. Simulation of the aggregation kinetics with comprehensive, physical models (population balances) provides useful information about the physics and chemistry governing the aggregation process.

It was found that depending on the nature of the particle surface charge groups as well as elastomer bulk properties, coalescence between primary particles can occur, resulting in significantly different steady state aggregate sizes. Moreover, depending on the experimental conditions one may obtain fractal aggregates under static conditions while under flow conditions particles exhibit coalescence. This will be demonstrated by the series of experiments carried out at different pH as well as at different temperatures.