Nanomilling in Organic Solvents in Stirred Media Mills and Microstructural Analysis with Whole Powder Pattern Modelling (WPPM)

Fine and ultra-fine grinding is of great interest to many industries. Examples of ap- plications are fillers for paper and plastic coatings, pigments, ceramics for abrasive and structural applications, toners for photocopy and printing machines. Besides the direct synthesis of these materials by chemical methods, wet grinding in stirred media mills is a suitable method for the production of sub-micron particles. In the sub-micron size range the behaviour of the product suspension is more and more in uenced by increasing particle-particle interactions. Due to these interactions, often spontaneous agglomeration of product particles occurs and the viscosity of the product suspension increases [1] [2]. To overcome this problem the milling suspension has to be stabilized by means of electrostatic, steric or electrosteric stabilization. In this study, milling of electrostatically stabilized alumina particles in water, sterically stabilized alumina particles in water and ethanol has been accomplished. Preliminary to the milling experiments of sterically stabilized alumina particles in different media, adsorp- tion isotherms of the polymer DAPRAL GE 202 on alumina particles in water, ethanol, 2-butanol and toluene has been explored. These adsorption isotherm curves show that the amount of adsorbed polymer on the surface of the alumina particles in 2-butanol is higher than in ethanol, water and toluene. This indicates that the affinity and conformations of the hydrophobic and hydrophilic parts of the polymer chains are oriented differently according to the nature of the solvents. The median particle sizes of the milled product particles of sterically stabilized alumina in ethanol is less than the milled product particles of sterically stabilized and electrostatically stabilized alumina F-320 particles in water at the same milling conditions. This is supported by different measuring techniques like DLS, SEM and BET. SEM pictures show a particle size of 50 nm for the milled sterically stabi- lized particles in ethanol. Mechanochemical changes from alumina to alumina hydroxide have been observed during wet grinding of sterically and electrostatically stabilized parti- cles in water. The amount of the hydroxide phase is the same regardless of the stabilization method. This point is supported by characterizing sample with the DSC method. In con- trast to milling experiments with alumina particles in water no mechanochemical changes occur for sterically stabilized alumina milled in ethanol. In this system the obtained median particle sizes are the result of pure mechanical grinding, because the formation and dissolv- ing of an hydroxide layer is not observed. This fact is supported by different characterizing methods like XRD, DSC and FTIR analyses. And also in this study with the help of Whole Powder Pattern Modelling, the mi- crostructural study of the materials based on the analysis of the X-ray diffraction patterns of the milled samples has been carried out. The particles are breaking at the interface of the crystallites leading to smaller particles until a crtical domain size is reached, which we believe is the real grinding limit. In case of tin oxide the critical domain size or grinding limit is reached at 2 nm. Whereas, the critical domain size of tin oxide obtained from Rietveld and Scherrer methods are 5 and 15 nanometers.