Understanding and Controlling the Morphologies of Polyurethane-Based Particles in Aqueous Dispersions

Aqueous polyurethane modified acrylics (PUMAs) offer a low solvent route to environmentally friendly, high-performance coatings for paint producers like AkzoNobel. For effective product development, it is necessary to understand the control of the morphologies in dispersions.
The structural morphology of the polyurethane (PU) component is studied by small angle X-ray scattering (SAXS) on aqueous dispersions with varying compositions. This reveals a major population of spherical particles, and a minor population of supramolecular structures, formed by hydrogen-bonding of acidic fragments, and controlled by the acid content of the composition. Particle radius follows a surface charge model, developed for similarly amphiphilic acrylic statistical copolymers. In situ grazing incidence SAXS reveals that particles do not coalesce during drying, remaining discrete, embedded in a matrix comprising the minor population. Both the particle interfaces, and microphase separation of hard blocks and soft segments, form short-range periodic structures in the films.
Various synthetic routes to aqueous PUMA dispersions are considered. It is found that adding acrylic monomer as the solvent for PU prepolymer synthesis yields the most uniform PUMA particles with the least scattering contribution from interparticle interactions. SAXS analysis is carried out on 16 PUMAs (or PU/polystyrenes) and their precursors, made with 3 methacrylic and styrene monomers, at 4 PU/monomer ratios. Structural models, featuring homogeneous spheres, core-shell particles and dissolved supramolecular structures, are applied to analyse SAXS profiles and reveal that precursor particle morphology depends on monomer/PU compatibility. For PUMAs and PU/polystyrenes, hydrophobic butyl methacrylate and styrene generate core-shell particles, while methyl methacrylate's higher water solubility results in more intricate morphologies. Morphology evolution during polymerisation of precursors to PUMAs is studied by time-resolved in situ SAXS. Application of structural models to the time-resolved data, on a representative composition, is interpreted in terms of monomer migration from reservoir droplets, and the polymerisation and phase separation kinetics.