Characterization of an Aqueous Dispersion of a Hydrophilic Polyisocyanate for Waterborne Two-Pack Polyurethane Coatings

Waterborne two-component polyurethanes (WB 2K-PUs) represent an environmentally friendly alternative to solvent-borne coatings, with reduced use of volatile organic compounds (VOCs). These coatings still cannot achieve the outstanding performance of solvent-borne 2K-PU formulations. In many WB 2K-PU formulations, a key step involves coalescence between polyol latex particles and nanoparticles formed from a water-dispersible polyisocyanate (PIC). Interactions between the PIC and the polyol take place both in the aqueous medium and after application of the coating to a substrate. A deeper understanding of this mechanism should guide improvements in formulations to improve WB 2K-PU coating performance. Our long-term goal is to investigate the film formation mechanism of a series of different polyol nanoparticles with aqueous dispersions of PICs based on a hexamethylene diisocyanate (HDI) trimer; as a first step in this direction, we examined the nature of the nanoparticles formed by dispersing an oligo-ethylene glycol (OEG)-modified HDI trimer in water. Typical techniques for nanoparticle size characterization (dynamic light scattering (DLS) and capillary hydrodynamic fractionation chromatography, (CHDF)) led to misleading information. These techniques indicated the presence of uniform nanoparticles with a diameter of ca. 100 nm that persisted for days. In contrast, DOSY H-1 NMR gave a diameter of 20 nm. Nanoparticle tracking analysis (NTA) confirmed the presence of the larger particles, but showed that they represented only 18 wt % of the dispersion. Over time, the isocyanate groups in the particles reacted with water, and the nanoparticles evolved from liquid droplets to mechanically robust polyurea particles. At this stage, imaging by electron microscopy showed a predominance of 20 nm particles. The kinetics of this chemical transformation was monitored by Fourier transform infrared (FT-IR) spectroscopy. The reaction of the isocyanate groups followed pseudo-first-order kinetics with a half-life of 11 to 14 h.