Abstract
The flow properties of aqueous slurries containing salicylic acid, sodium salicylate and gum arabic, prepared as feed-liquids into spray drying chamber, were investigated by analysis of their flow curves obtained using a cone and plate viscometer. Their flow curves showed a thixotropy, having two Newtonian flow curves at low and high shear rates and a characteristic spur-like protrusion between two Newtonian flow curves. Spur in their curves is thought to result from a sharp breakdown point in the card-house structure in slurry at the movement of salicylic acid particles. Apparent viscosity of the first Newtonian flow (ηN1) at the low shear rate was larger than that (ηN2) of the second Newtonian flow and was related to sedimentation volume of salicylic acid only. The second Newtonian apparent viscosity was related to concentrations of gum arabic and sodium salicylate, and active energy for it was 3-12kcal/mol. When geometric mean diameters of salicylic acid particles in slurries were large, or geometric standard deviations of distributions or shape factors of them were small, thixotropy coefficients (ηR=ηN1/ηN2) were large and represented resistance for movement of solid particles in slurries to be large at the low rate of shear. When the distribution of solid particles were broad and shape factor of them were large, small and narrow particles act as glidants or rollers to facilitate the rotation of larger particles resulting in the reduction of the thixotropy of slurry.
