Diffusion-Weighted PFGNMR Study of Molecular Level Interactions of Loops and Direct Bridges of HEURs on Latex Particles

Abstract

Viscosity building in latex coatings to provide desirable shear thinning rheological properties is a key property commercially achieved with hydrophobically modified ethylene oxide urethane (HEUR) rheology modifiers (RMs). Prior studies focused on the aqueous solution properties of HEURs, resulting in the well-known transient network model that describes solution rheology reasonably well. Relatively fewer studies have probed the molecular level interactions between the hydrophobe groups of HEUR and latex surfaces under conditions of realistic latex volume fractions and HEUR concentration. The presence of ubiquitous surfactant and oligomer molecules in the latex aqueous phase makes it difficult to detect these interactions directly for any off-the-shelf (industrial) materials. In this work, we outline the use of pulsed field gradient (PFG) NMR spectroscopy as diffusion-weighted filter to remove the signals of low molecular weight species in order to detect hydrophobe end groups and urethane linkers. This in situ approach does not have any perturbation issues that are inherent in prior methods involving centrifugation and avoids the questions raised by the use of custom pyrene hydrophobes in fluorescence spectroscopy. From this study we conclude that there are no HEUR transient network structures present in HEUR−latex composites with less than about 2% HEUR and 30% latex relevant for coatings applications. Our results explain the shear thinning rheology of latex−HEUR composites based on molecular level interactions between hydrophobe end groups and urethane linkers of HEURs and latex particles to produce HEUR PEO loops on latex and direct bridges between pairs of latex particles.

Development and Implementation of Dispersion Phase Diagrams (DPDs) for Four Different Hydrophobically Modified Ethoxylated Urethane (HEUR) Based Acrylic Paint Systems

“…These non-ionic associative thickeners are made from water-soluble poly(ethylene oxide) (PEO) backbone with internal, terminal, and even pendant hydrophobes. The PEO and other components are linked with urethane groups via isocyanate chemistry (Beshah, Izmitli, Van Dyk, Rabasco, & Bohling, 2013).…”

“…One traditionally accepted viscosifying mechanism for a fully formulated system is show in Figure 5 (Münzenberg, 2011). The adsorption of HEUR polymers onto latex surfaces through hydrophobic end groups, with the PEO portion forming a thin shell is understood and recognized; however, the effects of component contributions of core-shell hydrodynamic volume, particle aggregation, viscous drag on aggregated particles, and HEUR transient network on a given system's rheology is unclear (Beshah, Izmitli, Van Dyk, Rabasco, & Bohling, 2013). Early research proposed a transient network, shown in Figure 5, in which HEUR polymers form flower-like micelles which coexists with latex.…”

“…Early research proposed a transient network, shown in Figure 5, in which HEUR polymers form flower-like micelles which coexists with latex. In this model, some of the network nodes were believed to be occupied by latex particles and the HEUR bridges formed the transient network (Beshah, Izmitli, Van Dyk, Rabasco, & Bohling, 2013).…”

“…A drawback to this method is that there is other evidence that has shown centrifugation to disrupt the weakly bound hydrophobes resulting in inaccurate detection of free HEUR in the aqueous phase (Beshah, Izmitli, Van Dyk, Rabasco, & Bohling, 2013). A newer technique developed by Beshah et al utilized This test was conducted in a 1% HEUR and 30% latex composite which is a typical representation of commercial paints and coatings.…”

“…shows a different case in which well-dispersed pigment is surrounded by flocs of latex (Kostansek, 2007 PFGNMR results for 1% HEUR and 30% latex composite. Small black dots represent hydrophobic end groups of HEUR molecules (Beshah, Izmitli, Van Dyk, Rabasco, & Bohling, 2013 (Wicks, Jones, Pappas, & Wicks, 2007).…”