Evolving empirical rheological limits to predict flow-levelling and sag resistance of waterborne architectural paints

Good flow-levelling and sag resistance are the two key properties that are expected soon after applying a waterborne latex paint. It is well established in literature that both these properties are predominantly governed by rheology. Numerous papers have been published to evolve fundamental understanding of rheology of waterborne latex paints and their use in predicting flow-levelling and sag resistance properties but most of them deal with the problem in relative scale. The current study focusses on developing an empirical rheology limit to predict the flow-levelling and sag resistance of waterborne paints. Thirteen commercially available waterborne latex paints were taken up for the study that ranged from primer-to-top coat and interior-to-exterior. Rheology studies of these products were investigated in detail at different dilutions (by volume) using hysteresis test and three interval thixotropy test. Rheology flow profiles of latex paints based on cellulosic thickener were distinctly different than that of the paint based on modified urethane thickener; therefore, rheology data of paints based on these two class of thickeners were treated separately. The viscosity after 100 s of recovery and the difference in viscosity after 100 and 10 s of recovery in a three interval thixotropy test were used to derive the rheology limits. Leneta flow-levelling and sag resistance chart tests were carried out on all the paints at different dilutions to define the criteria for acceptable flow-levelling and sag resistance. Viscosity ranges were then defined for cellulosic and modified urethane thickener based latex paints to predict the flow-levelling and sag resistance on non-absorptive surfaces. A good empirical correlation exists between rheological parameters and practical flow-levelling and sag resistance.