EFFECT OF ADDITION OF HYDROXYETHYL CELLULOSE ON THE VISCOELASTIC PROPERTIES OF CONCENTRATED STERICALLY STABILIZED LATEX DISPERSIONS

Concentrated sterically stabilised polystyrene latex suspensions were prepared by physical adsorption of a graft copolymer with poly (ethylene oxide) (PEO) side chains. The relative viscosity (ηr) -volume fraction (Φ) curves showed that these dispersions behave as hard spheres. By fitting the ηr - Φ to the Dougherty-Krieger equation, an adsorbed layer thickness of 4.0 nm was obtained and this showed a tendency to decrease (to 2.5 nm) with increase in Φ indicating some interpenetration and/or compression of the PEO chains. These concentrated sterically stabilised dispersions were used to study the effect of addition of hydroxyethyl cellulose (HEC) with three different molecular weights (70 000, 124 000 and 223 000) and a hydrophobically modified HEC. Rheological studies showed flocculation to occur above a range of HEC concentration (volume fraction Φ+ p) which tended to decrease with increase in molecular weight of the polymer. From the extrapolated yield value (τβ) the energy of separation particles in the flocculated structure (ESep) was calculated assuming two values (4 and 8) for the average number of contacts between the particles. These values were compared with the theoretical calculations of the free energy of attraction due to depletion, (Gdep) based on Asakura and Oosawa as well as Fleer, Scheutjens and Vincent (FSV) models. The results showed considerable deviation between ESep and Gdep based on Asakura and Oosawa's theory. However, some agreement was obtained using the FSV model, particularly at low Φp values. © 1990.