The manner in which a slurry flows, coats, and drains is referred to as slurry rheology. This behavior is an important characteristic of the slurry and affects the properties of shells made from these slurries. Slurry rheology can be measured with a Brookfield viscometer. Zhan cups are excellent for everyday control of viscosity, but for measuring the complex characteristics of non Newtonian fluids, they are inadequate.

One factor that effects slurry rheology is the particle size distribution of the flour. Three backup fused silica slurries were made using 120, 200, and 325 mesh flour. Slurry properties were measured, shell samples made and tested for strength, porosity and density. The slurries were then combined to form a fourth slurry that was again measured and shell samples produced and tested. Significant differences were observed. The slurry rheological properties are shown to result in different shell properties.

Experimental Slurries

Binder used was Nyacol 830 small particle sol adjusted to 25% SiO2. Flours were CE Minerals 120, 200, and 325 mesh fused silica.

There are vast differences between the flours as you will see in Figure 1. that shows exactly 200 grams of flour in identical jars.

These flours were mixed into the binder at a total solids content of 73% including binder solids. After two days, wetting agent was added. The slurry was allowed to mix in a closed jar and the Zahn #5 viscosity was measured for 3 weeks. The results are shown in Table 1. All slurries were adjusted to a Zahn #5 of 14 seconds, which was deemed to be a reasonable dipping viscosity.