The low shear rate rheology of two phase mesophase pitches derived from coal tar pitch
has been investigated. Particulate quinoline insolubles (QI) stabilised the mesophase
spheres against coalescence. Viscosity measurements over the range 10–106 Pa s were
made at appropriate temperature ranges. Increasing shear thinning behaviour was evident
with increasing mesophase content. At low mesophase contents the dominant effect on
the near Newtonian viscosity was temperature but at higher contents it was the shear rate;
temperature dependence declined to near zero. The data indicated that agglomeration
could be occurring at intermediate mesophase volume fractions, 0.2–0.3. The Krieger–
Dougherty function and its emulsion analogue indicated that in this region the mesophase
pitch emulsions actually behaved like ‘hard’ sphere systems and the effective volume fraction
was estimated as a function of shear rate illustrating the change in extent of agglomeration.
At the higher volume fractions approaching the maximum packing fraction, which
could only be measured at higher temperatures, the shear thinning behaviour changed in
character and it is considered that this is possibly due to shear induced deformation and
breakup of dispersed drops in the shear field.