Coagulation is frequently applied to polymer particles as a separation or material formation step. The resulting cluster structure greatly influences the properties of the resulting material. Universal cluster structure has been found for aggregates of rigid particles [1]. In contrast, rather soft elastomer clusters may densify after aggregation due to polymer chain diffusion. The extent of this sintering or coalescence process is governed by surface properties of the primary particles.

In order to investigate this phenomenon the aggregation kinetics of industrial elastomer latices has been studied in the diffusion limited cluster aggregation (DLCA) regime in quiescent fluid. The aggregation extent was followed by particle sizing via dynamic light scattering (DLS). Simulation of the aggregation using the kinetic approach (population balances) allows an interpretation of the light scattering result in terms of cluster structure, which is described by the mass fractal dimension – the key parameter for fitting the average hydrodynamic radius resulting from simulation to experimental DLS data. Additionally the cluster structure has been probed by cryogenic electron microscopy. Depending on the surface properties of the particles with respect to latex stability two limiting cluster structures can be found. Bare elastomer particles, stabilized with surfactants only (mobile charges), form dense, sphere-like clusters due to very strong sintering of primary particles. On the other hand sintering is practically absent if the particles carry a sufficiently high number of fixed charges. In the case of clusters made of latex particles that are stabilized by both fixed charges and surfactants the sintering extent is in between the two limiting cases.

References: [1] M.Y. Lin, H.M. Lindsay, D.A. Weitz, R.C. Ball, R. Klein, P. Meakin, Nature 339 (1989) 360-362.