Dynamic colloidal processes in waterborne two-component polyurethanes and their effects on solution and film morphology

While waterborne two-component polyurethanes (WB 2K-PURs) offer an attractive approach toward the reduction of volatile organic compounds (VOC), a number of fundamental processes related to the colloidal stability and solution morphology of WB 2K-PUR reactive dispersions are not understood. These studies focus on mechanisms of inter-particle reactant aggregation in heterogeneous aqueous dispersions resulting from solution morphological changes of poly(ethylene glycol) modified water-dispersible polyisocyanates (WDPI) and water-reducible polyester polyol coreactants, as revealed by particle size measurements and in situ NMR T2 studies. Based on the results of these experiments, a model of film formation is proposed which relates solution morphological features and chemical reactions. WDPI reactant droplets become hydrated over time, leading to formation of carbon dioxide, which dissolves in the aqueous continuum as carbonic acid and subsequently destabilizes anionically stabilized polyol droplets, resulting in their adsorption to WDPI droplet surfaces. The implications of polyol collapse, as they pertain to film formation, are discussed in the context of solution and film morphologies.