Adsorption of polymers and the stability of pigment dispersions

Abstract

The stabilization of dispersions of titanium dioxide and iron oxide pigments in benzene by polyesters of adipic acid and neopentyl glycol terminated with different functional groups has been studied. Adsorption of the polyesters corresponded to approximately a close-packed monolayer, being somewhat lower for the hydroxyl-terminated polyester. Stability of the pigment dispersions was high in solutions of carboxyl- or hydroxyl/carboxyl terminated polyesters, but extremely low with hydroxyl-terminated polyesters. Interaction of the ester groups with the pigment surface was found to occur at low surface concentrations, but not at higher concentrations, suggesting that at low concentrations the polyester molecules lie flat on the surface, while at higher concentrations the polymer lies on the surface in random coils. The dispersed pigments were found to be uncharged, and stabilization by an adsorbed layer mechanism is postulated.

In experiments with practical alkyd (modified polyester) paints, the degree of flocculation of phthalocyanine pigments was related to the particle size distributions of the pigments. The pigments were found to adsorb monolayers of alkyd polymer sufficiently thick to stabilize smaller (<0.2 µ) but not larger particles.