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

Associative thickeners, such as hydrophobically modified ethoxylated urethanes (HEURs), are an important class of rheological modifiers allowing precise control and optimization of the rheology of waterborne coatings. In this work, we present a novel, comprehensive investigation of one-step and two-step HEUR synthesis processes, highlighting their impact on the final HEUR properties. In the conventional one-step process (current industrial practice), there are inherent limitations in producing high molecular weight polymers due to the complex competition between end-capping and polymerization. We show that the two-step method allows for much higher molecular weight polymers than the one-step method while using less amounts of toxic diisocyanates. Additionally, using the two-step method, the polymerization can be simply and efficiently controlled by the addition timepoint of the end-capping agent, which can be tailored to provide HEURs with a wide range of molecular weight and polydispersity index. However, the efficient end-capping of high molecular weight polymers remains a challenge when using conventional mixing equipment in batch reactors due to mass transfer and mixing limitations associated with the significant increase in the bulk viscosity of the reaction mixture. To overcome these limitations, alternative and more efficient mixing technologies, such as reactive extruders, should be considered for the efficient end-capping of high molecular weight polymers.