Minimising the viscosity of concentrated dispersions by using bimodal particle size distributions

The effect of composition (i.e. the volume of small particles compared to t he volume of large particles) on the rheological properties of a bimodal di spersion has been studied in detail using two monodisperse polystyrene latt ices. The diameter ratio of these lattices was 4.76. The relative high shea r rate viscosity and dynamic viscosity of these bimodal dispersions ware me asured using the Bohlin VOR. Firstly, the rheology of the two monomodal dis persions uas measured as a function of volume fraction, then 10 composition s ranging from 10% to 35% small particles by, volume wire prepared and agai n the rheology followed as a function, of volume fraction. A minimum in the relative high shear rate Limiting viscosity was found in t he range 15-20% by volume of small particles and another minimum was also f ound at 20% by volume of small particles for the dynamic viscosity measurem ents. Thus, it can be concluded that reductions in viscosity can be achieve d with a bimodal dispersion with the small particles occupying 20% of the t otal volume fraction. This: is in agreement with other studies. However, if the adsorbed layer thickness is taken into account then the effective comp osition works out to br 27-36%: the lower limit of which is in excellent ag reement with theory. The relative high shear rate viscosities of the bimodal dispersions were th en compared to the theoretical viscosities calculated from an effective med ium model. The results are in reasonable agreement with the measured data, but a rigorous analysis could not be performed due to the inability to calc ulate accurately the effective volume fi action, due to the presence of an adsorbed polymer layer on the particles.. (C) 1998 Elsevier Science B.V. Al l rights reserved.