Surface area and dispersant chemistry changes of milled titania pigment probed by relaxation NMR

Synthesised particle products are often formed as agglomerates of finer single crystallites. Understanding the internal clustering and structure, as well as overall size and chemical nature is extremely important, as the performance of these systems (e.g. optical properties) may be dominated by these primary forms. For instance, inorganic agglomerates can be formed in high temperature flame reactors, as with the production of titanium dioxide. The structure of these species is a complex mix of source crystals and final aggregated or sintered particles. High quality manufacturing often demands tight control of the surface area, size, distribution and other properties (e.g. chemical homogeneity) for end user performance. Standard measurement techniques to analyse physical and chemical properties do not always discern the raw crystal properties present in final products and are often very time consuming (e.g. BET surface area measurements). Due to these limitations, manufacturing knowledge is often limited to ex situ characterisation of extracted samples, where access to fast parameter analysis would greatly enhance process monitoring and control capabilities.

The focus of this thesis was to investigate the limitations of using relaxation NMR to probe changes in TiO2 particle size during milling. Furthermore, dispersants such as sodium hexametaphosphate (SHMP) are adsorbed onto the TiO2 particles during milling, to reduce particle-particle interactions allowing the suspensions to be handled more easily. The influence that changes in surface chemistry may have on relaxation NMR results during milling has not previously been explored. Therefore, a further focus of this thesis was to understand changes in titania surface chemistry with milling and relate this to any changes observed in the NMR relaxation data.