The rheology of aqueous alumina suspensions in the presence of hydroxyethylcellulose as binder

The rheological behaviour of aqueous colloidal alumina dispersions stabilis ed by the dispersant Darvan C, an ammonium salt of polymethacrylic acid, wa s investigated in presence of hydroxyethyl cellulose (HEC) at room temperat ure. Measurements were taken under both oscillatory and steady shear. It wa s found that the polymer induces flocculation of the particles probably, du e to a depletion mechanism. The flocculation process results in the creatio n of a particle network, which restricts particle movements and gives rise to a wide distribution of relaxation rimes. At high HEC concentrations, two sets of relaxation times can be distinguished, one at low and the other at high frequencies. At high solids content and high HEC concentrations, the response of the system becomes predominantly elastic at low frequencies. Th e behaviour is similar to an entangled network of high molecular weight unc ross-linked polymzer solution. Thus, HEC causes the formation of 'topologic al restraints' comprised of chains of particles that gives rise to a wide d istribution of relaxation times with significantly lower terminal relaxatio n times. In addition, similarly to a polymer solution, complete superpositi on occurs of the apparent and dynamic viscosity in accordance to the Cox-Me rz rule. Taking into account the combined data, a Gross type of formulation was found to fit the behaviour of all dispersions that can be represented in terms of a single dimensionless parameters. The behaviour of the adjusta ble parameters of this empirical model suggests a diffusion controlled clus ter-cluster aggregation process that induces a more closed packed particle arrangement at high shear rates. (C) 1999 Elsevier Science Limited. All rig hts reserved.