(496g) Characterization of Elastomer Particle Aggregation and Implications for Industrial Processing
Coagulation is a typical process step to facilitate recovery of solid phase from the carrier liquid in the colloidal domain. Industrial coagulation takes advantage of the fact that particle aggregation is controlled not only by diffusive motion but as well by shear flow which limits particle size by breakage. For a given particle size the effectiveness of breaking aggregates greatly depends on their structure. This has implications on the processability of a coagulated colloid which is relevant for many applications e.g. in coagulation of elastomer particles. Aggregation studies in quiescent fluid can greatly contribute to understanding of particle size evolution and cluster structure. This work addresses the issue of transferability of information gained in static fluid to flow conditions.
Focus of our study is to characterize the aggregation of elastomer particles in static conditions. Simulation of the aggregation kinetics with comprehensive, physical models (population balances) provides useful information about the physics and chemistry governing the aggregation process. Experiments were carried out on partially as well as fully destabilized elastomer colloids at temperatures ranging from 25°C to 70°C. Static and dynamic light scattering were employed to measure particle size. Scanning electron micrographs of frozen colloids provided further information about cluster structure of selected samples.
Comparisons of latexes which differ in material as well as surface properties allow us to draw following general picture. Given a sufficiently low polymer viscosity or high polymer chain mobility clusters of surfactant stabilized elastomer particles always coalesce to form sphere-like clusters. Elastomer particles of the same bulk material but carrying fixed charges can coalesce at high temperatures e.g. 70°C but not at 25°C. This rather surprising finding is discussed in another contribution in terms of a temperature dependent structural barrier within the surface-to-surface contact region. High polymer chain mobility as pre-condition for coalescence seems greatly affected by elastomer bulk properties. Depending on the material composition cluster coalescence was found to be reduced or absent without significant influence of temperature. The results obtained from static aggregation have important consequences for aggregation in shear flow such that cluster breakage is impaired by coalescence resulting in increased steady state particle size. Even though latter quantity is the most important one for design of (industrial) coagulation process, static aggregation experiments are useful to characterize latexes in terms of cluster coalescence. Information about influence of material and surface properties might be therefore already considered in design of a preceding polymerization process.