One-Step versus Two-Step Synthesis of Hydrophobically Modified Ethoxylated Urethanes: Benefits and Limitations

Associative thickeners, such as hydrophobicallymodifiedethoxylatedurethanes (HEURs), are an important class of rheological modifiersallowing precise control and optimization of the rheology of waterbornecoatings. In this work, we present a novel, comprehensive investigationof one-step and two-step HEUR synthesis processes, highlighting theirimpact on the final HEUR properties. In the conventional one-stepprocess (current industrial practice), there are inherent limitationsin producing high molecular weight polymers due to the complex competitionbetween end-capping and polymerization. We show that the two-stepmethod allows for much higher molecular weight polymers than the one-stepmethod while using less amounts of toxic diisocyanates. Additionally,using the two-step method, the polymerization can be simply and efficientlycontrolled by the addition timepoint of the end-capping agent, whichcan be tailored to provide HEURs with a wide range of molecular weightand polydispersity index. However, the efficient end-capping of highmolecular weight polymers remains a challenge when using conventionalmixing equipment in batch reactors due to mass transfer and mixinglimitations associated with the significant increase in the bulk viscosityof the reaction mixture. To overcome these limitations, alternativeand more efficient mixing technologies, such as reactive extruders,should be considered for the efficient end-capping of high molecularweight polymers.