Role of surfactants on stability of iron oxide yellow pigment dispersions

Iron oxide yellow pigment (PY42) dispersion was prepared using the mixture of dispersing agents (surfactants) alkyl aryl sulphonates (S1), lecithin (mixture of phospholipids) (S2), alkoxylated fatty alcohols (S3), and polyoxyethylene tri-decyl ether phosphate (S4). Evaluation of this pigment dispersion in an aqueous paint resulted in acceptable coloristic properties. However, stability of the dispersion with respect to a significant increase in viscosity and settling behaviors when subjected to accelerated stability test (at 55 degrees C for 30 days) was observed. Rheology profiles have been used to characterize the pigment dispersions. Role of various combinations of surfactants on rheology of pigment dispersion was studied. It was found that, certain surfactants played a crucial role on stability of pigment dispersions during accelerated stability. Elimination of surfactant S3 or S4 resulted in an acceptable viscosity at ambient conditions. However, in the accelerated stability test pigment dispersion without S3 showed significantly high viscosity whereas, dispersion without S4 showed no increase in viscosity. From the physical examination, viscosity measurements and analysis of rheology profiles it can be concluded that polyoxyethylene tri-decyl ether phosphate (S4) is the key contributor for the viscosity pickup and settling behavior during the accelerated stability test. Moreover, the rheology study is of great help in predicting the relative stability of the samples.