INTRODUCTION Previously, the rheological properties of different sized monodisperse polymer latices of polystyrene and polymethyl methacrylate
Many industrial concentrated dispersions, e.g., inks and (PMMA) were determined. This paper describes procedures in paints, are not monodisperse, but have a wide particle size which carefully prepared blends of the same monodisperse latices distribution. It is commonly known that increasing the volwere mixed together to examine the effect of the differences in ume fraction of a particulate suspension increases the viscossize between the latices (diameter ratio) on the rheology of the ity and that the viscosity increases markedly as the maximum blend. The relative viscosities of these blends were then measured packing fraction of the particles is reached. Furthermore, it as a function of the total volume fraction and compared with those is widely recognized that the maximum packing fraction of of the constituent parts. In a bimodal suspension, theory predicts a monodisperse system can be increased by broadening the that a maximum packing fraction and hence a minimum viscosity particle size distribution. Thus, polydispersity can give a is achieved with 27% small particles by volume. The experiments lower viscosity at the same volume fraction or permit a revealed that a minimum viscosity was obtained with 25% small higher volume loading of particles at the equivalent viscosparticles by volume and with a diameter ratio of 7.83. At this ity. Despite this practical significance, relatively few experidiameter ratio, the small particles are able to pass through the mental and theoretical studies have been made.
triangular pore between the large particles and this essentially
A systematic study of the effect of polydispersity can be leads to an increased maximum packing fraction of the suspension.
Hence the suspension will have a lower viscosity. Other diameter achieved by investigating systems in which it is well defined, ratios at 25% small particles by volume that led to suspensions such as a bimodal suspension. By introducing smaller partiwith lower viscosities were 4.03, 6.37, and 11.15. On the other cles, such that they fit in between the larger particles, it is hand the suspensions with diameter ratios of 2.81 and 5.67 led to conceptually possible to achieve higher volume fractions. increases in viscosity. This is a result of the small particles being Optimization of the size difference (or diameter ratio) and too big to fit in the gaps between the large particles. All the other the volume ratio (composition) could lead to bimodal sysdiameter ratios at all the other compositions (i.e., 50 and 75% tems having greater volume fractions for the same viscosity small particles by volume) resulted in viscosities higher than that compared to monomodal systems. Conversely, the viscosity obtained for the monodisperse polystyrene latex. These effects may may be greatly reduced by switching from a monomodal to be explained by changes in the value of the maximum packing a bimodal system at the same volume fraction. This could fraction. An increased maximum packing fraction of the system produce a more environmentally sensitive product due to leads to lower viscosities and vice versa. The results challenge the less polluting solvent. In other systems a lower viscosity conventional view that simply increasing the diameter ratio at a would minimize pumping costs. This is exemplified by refixed composition leads to reduced viscosities and increased maxisearch into high solids loading coal/water mixtures that can mum packing fractions. This simple picture does not appear to be easily pumped over long distances (1-3). In addition to hold for binary suspensions of colloidal particles at every particle the reductions in viscosity, other rheological improvements size ratios.