Fundamental Investigations of Nanoparticle Production by Stirred Media Milling

摘要

Nanotechnology applications in the pharmaceutical, materials, and chemical industries has renewed interest in the use of wet grinding in stirred media mills for the production of colloidal and nanoparticles. However, challenges arise in the production of sub-micron particles that are, in part, due to colloidal surface forces influencing slurry stability and rheology. As often observed in the literature, a lower bound of 1 μm is often reached despite high energy inputs and aggressive milling conditions. Furthermore, the product agglomerate size can even increase with increases in energy input, a seemingly counterintuitive result that may be attributed to aggregation of fine particles during the comminution process. In this work we postulate that colloidal stability and rheology must be considered in wet grinding to understand these results and to surmount limitations on the production of nano-sized particles. Experiments are performed on a well-characterized, model system of monodisperse primary nanoparticles that are destabilized and aggregated under various milling conditions. Conditions spanning Brownian to turbulent collision aggregation in model stirred media mills are explored to study the effects of colloidal stability on the aggregation process. The agglomeration kinetics are measured using dynamic light scattering (DLS) as a function of particle and electrolyte concentrations. Further information on the agglomeration process and the structure of the agglomerates are also obtained from small angle neutron scattering (SANS) experiments. Theoretical predictions based on independently measured particle and solution properties as well as mill characteristics are compared against the experimental results to demonstrate that particle aggregation kinetics in a stirred media mill can be controlled through the colloidal interactions and the milling conditions. This research provides a theoretical basis for understanding stirred media milling of nanoparticle slurries and as such, is a step towards a predictive model of sub-micron stirred media milling.