The Influence of Interparticle Interactions on Rheology and Stability in Stirred Media Mills

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Peukert, Wagner, Sommer, Mallembakam
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Size Reduction
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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 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 grinding limit in the range of 0.5 µm is reached despite high energy inputs and aggressive milling conditions. Furthermore, the product size can even increase with increased energy input, a seemingly counterintuitive result that is attributed to aggregation of fine particles during the milling process. By producing particles smaller than a median particle size of 1 µm a steady state between breakage and agglomeration exists in the milling process. This equilibrium is controlled by interparticle interactions as well as the milling conditions. In this report we demonstrate that this steady state particle size is independent of the feed particle size and can be reached by agglomeration of small particles as well as by real breakage of large particles. Furthermore, we extended our studies to non-aqueous systems, because of the high industrial demand on nano particles in organic solvents. Milling studies with steric and electrostatic stabilization showed that the size of the particles milled in organic solvents is conspicuously smaller than the size of the particles milled under the same conditions in the aqueous phase. However, ions dissolved from the media wear and impurities in the suspension can destabilize the suspensions if the electro- static stabilization mechanism is chosen. This is not the case for steric stabilized systems. An important question is whether the mechano-chemical activation which was observed in aqueous media is crucial to obtain nano-particles. Differential Scanning Calorimetry (DSC) and X-ray difraction (XRD) measurements showed no mechano-chemical changes during milling in ethanol and toluene. In the aqueous phase the stabilization mechanism has no influence on the amount of hydroxide phase. A protecting polymer cover around the particles does not prohibit the mechano-chemical changes.