(114b) Engineering of Assemblies of Inorganic Particles

Nano-particles, of particle size less than 1 µm, tend to be difficult for users to handle. Therefore, the particles are often assembled into larger granules for ease of handling and to allow controlled delivery in their desired functions. Engineers who manufacture granules want to be able to control and predict the structures formed and ultimately predict the functional performance of the granular materials.

This project studies the use of spray drying to engineer titanium dioxide (TiO₂) pigmentary granules of controlled morphology and mechanical properties. The first stage considers the characterisation of TiO₂ slurries (TiO₂ pigments dispersed in water) in order to develop an overall understanding of the feed materials for the subsequent spray drying experiments. Following, these materials were spray dried and the product morphologies assessed. Techniques for controlling product morphologies were explored via the manipulation of the particle-particle interaction strength in the TiO₂ slurry feed, as well as via the drying process conditions (e.g. drying temperature, slurry feeding rate etc.). Empirical Models have been used to better understand the drying transformation (structural evolution of the droplets) throughout the entire spray drying process. Finally, the resulting granule morphologies were correlated with the mechanical properties such as the granule compressibility. The relationship(s) between the slurry characteristics, processing aspects of the spray drying and the morphologies of the produced granules is being developed.

The processing of non-coated and alumina-coated TiO₂ slurries has been investigated as a function of the slurry pH. SEM images of the non-coated particles (in slurry) showed that the primary particles were elliptic with diameter of 300 ± 100nm, while the coated particles were found to be highly aggregated with diameter of 5 µm. Zeta potential measurements confirmed the iso-electric point (IEP) of the non-coated particles to be pH = 4.5 and pH = 8.5 for the alumina-coated particles. Sizing, sedimentation and rheology studies demonstrate the formation of weak aggregated structure at the IEP for non-coated slurry. However, the behaviour of the alumina-coated slurry was very similar with no dependence on the slurry pH.

Spray drying of the non-coated slurries produced spherical granules in the size range 20-60 µm. However, these spherical granules were found to be weak in strength. The slurry pH and processing conditions had only a minor effect on either morphology or size of the granules. The alumina-coated slurries formed spherical granules at non-IEP pHs, however buckled elliptical morphology was observed when drying the slurry at the IEP. The mechanical strength of the granules made from the alumina coated-slurries were found to be stronger than non-coated slurries.