The Influence of Slurry Formulation on the Multi-Scale Structure and Functional Properties of Spray Dried Detergent Powders

Globally, spray drying is the most commonly used process by which detergent slurries can be transformed into low-density and free-flowing powders. The structure and formulation of these granules have a profound effect on the functional properties, e.g. flow behaviour, storage stability and dissolution rate, of the final product. Nevertheless, little is known about the structure and functional properties of the spray-dried powders due to the lack of scientific works in the public domain. In this thesis, four simple model formulations based on linear alkyl benzene sulphonate (NaLAS) and sodium sulphate were used to probe the influence of initial slurry water content and binder, SiO2:Na2O molar ratio of sodium silicate, on the structure. Moreover, flow behaviour and water sorption characteristics were chosen to be investigated as the findings of these studied can provide a better understanding of the influence of slurry formulation on the granule structure. The complex multiscale structure of these granules was characterized using a complementary range of techniques including scanning electron microscopy (SEM), image analysis, x-ray micro-tomography, wide angle and small angle x-ray scattering (WAXS, SAXS). A complex composite structure is revealed due to the multiphase nature of the starting slurry and its evolution during the drying process. The structure revealed can be viewed as a continuous matrix consisting of NaLAS, sodium sulphate and binder in which large, dense, crystals of sodium sulphate are embedded. These were initially undissolved in the slurry and are consequently reduced in number in the product made from higher water content slurry. Air is also dispersed in this matrix at two length scales, large vacuoles, at sizes of the order of the particle diameter which are the result of ‘puffing’ i.e. water boiling, and micro-scale porosity which evolved during the crystallisation of the drying matrix. The matrix is a composite structure of binder, dried surfactant liquid crystalline phase and sub-micron scale crystallites of sodium sulphate. Changes in the initial composition, water content or binder, of this phase are seen to change both the sulphate polymorphs present and the d-spacing of the NaLAS phase. Morphology and surface characteristics are also seen to change with the high initial moisture content particles producing more irregular structure and a rougher surface. The SAXS data were further analysed by reconstructing electron density profiles (EDP) of the one-dimensional repeat of bilayer stacking by Fourier analysis, and the parameters of lamellar d-spacing, bilayer thickness (dHH ) and water layer thickness (dW ) were determined. The relative humidity was found to be a critical factor determining the number of coexisting lamellar polymorphs. The greater the RH value, the fewer is the number of lamellar polymorphs. The presence of binders is another factor, due to its hygroscopic properties, controlling the short-range order structures of self-assembled surfactants. To investigate the influence of relative humidity on the microstructural evolution and moisture sorption behaviour of spray-dried detergent powders, the samples were stored at various relative humidity. The moisture sorption isotherms of the powders were gravimetrically determined using a dynamic vapour sorption (DVS) technique. The extent of moisture uptake in these samples was found to be determined by the solid state of sodium silicate. The powder flowability was investigated using a Schulze ring shear tester. The initial water content of the slurries was found to have a profound effect on the flowability of the resulting spray-dried powders. The powders produced from high-water content showed a better flow behaviour. This was attributed to the influence of water content on the matrix composition. The chemical composition of the detergent powders was also probed using Raman confocal microscopy techniques. The powders containing lower surface concentration of surfactant had a better flowability. Sodium silicates significantly improved the flow behaviour of the particles. The silicate is thought to form a glassy continuum linking sulphate crystals and surfactant domains, which consequently increases the apparent granule strength, and hence improves the flow behaviour.