Tailoring hydrophobic segment lengths in reactive HEURs: A Novel approach to optimizing rheology for UV-curable waterborne coatings

Abstract

The transition from solvent-based to water-based coating technologies is driven by environmental concerns over volatile organic compounds (VOCs). However, waterborne systems, such as UV-curable coatings, face challenges like suboptimal rheological properties, leading to defects in flowability and leveling. Hydrophobically ethoxylated urethanes (HEURs) are among the most effective rheology modifiers for water-based resins due to their associative micellar behavior. In this study, reactive HEURs (RHs) with varying hydrophobic segment lengths (10, 14, and 18 carbons) and methacrylate reactive groups were synthesized and characterized using FTIR, H¹ NMR, and GPC. Rheological and viscoelastic analyses were conducted in both aqueous solutions and resin systems at pre- and post-curing stages.

The physicochemical characterization techniques confirmed synthesis of RHs with optimal molecular weights of approximately 14000 g mol⁻¹ and narrow distributions. The RHs self-assembled into micellar networks above the critical percolation concentration, with RH-18 forming the densest networks due to its longer hydrophobic chains and enhanced hydrophobic interactions. UV curing polymerized the methacrylate groups, transforming transient micellar networks into robust, crosslinked structures, leading to significant increases in viscosity and elastic modulus. RH-18 consistently exhibited the strongest rheological performance, demonstrating the highest viscosity, stability under a wide range of shear stress, and gelation properties. RH-14 also performed well, particularly in resin systems, where it enhanced gelation, elasticity, and flow stability, making it a suitable choice for applications requiring intermediate performance. These results establish RH-18 as an optimal rheology modifier for waterborne UV-curable coatings and highlight RH-14 as a viable alternative for systems requiring balanced rheological and viscoelastic properties.