(139a) Relationship Between Rheology and Morphology In Aqueous Dispersions of Hydrogenated Castor Oil Crystals

Hydrogenated castor oil (HCO) dispersions are widely used as the rheological modifier in paint, cosmetics and personal care products. Three HCO crystal morphologies have been observed, which we refer to by their geometric shape as fiber, rosette and irregular crystal. Due to the difficulty of obtaining samples with a single morphology, rheology studies of suspensions containing mixtures of the three morphologies in a non-Newtonian surfactant solution have been undertaken. The viscometry of dilute suspensions has shown that the magnitude of intrinsic solution viscosity follows the sequence of samples having a majority of a particular crystal morphology, i.e. fiber > rosette > irregular crystal. Huggins coefficients have been found to be the function of crystal morphology, indicating that a solution with a fractional distribution of 0.43:0.40:0.17 of fibers:rosettes:irregular crystals, shows the highest particle/particle interaction. The shear dependent viscometry of concentrated dispersions shows that Cross model fits well for the data at low volume fraction while a power law model fits well for the data at high volume fraction. The volume fraction dependent zero shear viscosity of concentrated dispersions has been found to be a function of crystal morphology with the highest zero shear viscosity found in fiber-rich systems and least zero shear viscosity for systems rich in irregular crystals. The linear viscoelasticity has been investigated, showing fibers and rosettes contribute more to elastic properties than irregular crystals. There is also an aging phenomenon, where irregular crystals transform to short-branched rosettes over a period of two months. Both the steady shear viscosity and dynamic moduli have been found to increase, which may be significant in certain applications.