Stability assessment of iron oxide yellow pigment dispersions and temperature dependent implications of rheological measurements

Abstract

Iron oxide yellow pigment dispersions were prepared using different surfactants. Dispersions were subjected to rheology analysis and accelerated stability test to understand and assess the shelf life/storage stability of the dispersions. The cohesive energy density (Ec) determined by analyzing Oscillatory amplitude sweep test indicated that, dispersion prepared using the surfactant, Polyoxyethylene Oleic Amide (S-1) should result in good storage stability compared to the other 3 dispersions prepared with surfactants Ethoxylated Cocamide (S-2), Cocomonoethanolamide Ethoxylate (S-3), and Polyoxy Ethylene Amide (S-4). However, the physical observations of accelerated stability test of dispersion prepared with S-1 showed separation and settling. The initial rheology data did not help for the assessment of shelf life of the dispersion probably due to the difference in test conditions e.g. the accelerated stability test was performed at 55 °C and the rheology profiles were recorded at 25 °C. The ‘Ec’ was calculated again for all dispersions for the Oscillatory sweep profiles recorded at 55 °C. Interestingly, the ‘Ec’ of dispersion prepared with S1 was reduced drastically. On the other hand, there was no significant difference in ‘Ec’ observed for other 3 dispersions. The ‘Ec’ calculated for profiles recorded at 55 °C reflected well with physical observation of accelerated stability samples. Dispersion prepared with S1 was found unstable over a period of time or at accelerated test conditions might be due to low HLB of S1. It was demonstrated that rheology data are of great help in assessing the stability of the dispersions provided the test conditions were considered appropriately.