Effects of latex and thickener polarities on rheology and phase stability of latex–HEUR mixtures

In this study, hydrophobically modified, ethoxylate urethanes (HEURs) and latexes of well-defined structure and composition were used to examine the rheology and phase behavior of HEUR–latex mixtures. The mixtures comprised one of four HEURs that were synthesized with varying molecular weight polyethylene glycols (PEG 35 K, 20 K, 12 K, and 8 K) and end-capped with octadecyl hydrophobes, as well as one of two latexes (butyl acrylate/styrene [BA-Sty] and butyl acrylate/methyl methacrylate [BA-MMA]) that each contained a small amount of methacrylic acid. The two experimental latexes represent compositions commonly used in commercial paint formulations. The polarity of the HEURs decreases as the PEG molecular weight decreases and the surface of the styrene containing latex is less polar than that containing methyl methacrylate. In all HEUR–latex mixtures, the latex polymer volume fraction was maintained constant at 0.25 which provides a dispersed component spatial crowdedness that represents a fully formulated paint. All latex–HEUR mixtures were stable at very low (i.e., below 0.10% by weight) HEUR levels. At critical concentrations dependent on the polarity of thickener and latex, flocculation was accompanied by syneresis for latex-thickener mixtures prepared with all HEURs except the one with the longest PEG spacer. Further increase of HEUR levels revealed another critical concentration above which the mixtures were stable. Effects of the latex polarity diminished for the mixtures prepared with more hydrophobic HEURs.