Water-Soluble POSS Promotes Concurrent Coalescence and Crosslinking for Multiscale-Reinforced, Coalescent-Free...

Waterborne thermoset coatings face a fundamental formulation dilemma, whereby premature crosslinking restricts latex particle deformation and polymer diffusion, thus necessitating the use of VOC-emitting coalescing agents to achieve continuous film formation. Here, this dilemma may be resolved using a water-soluble, amine-functionalized polyhedral oligomeric silsesquioxane (NPOSS) to promote cooperative coalescence and interparticle crosslinking in a latex of epoxy-functionalized microspheres with two-stage seeded core-shell architecture. Dissolved in water, NPOSS appeared to transiently plasticize the latex particles to facilitate coalescence while exhibiting a moderate reaction rate that permits interparticle diffusion prior to extensive crosslinking. Cryo-TEM and DLS reveal wet-state particle association, deformation, and fusion, while ATR-FTIR confirms continued post-drying network densification. This process yields a multiscale reinforcement architecture comprising a molecular-scale covalent network, nanoscale chemical heterogeneity, and micrometer-scale compositional homogeneity. Driven by this multiscale reinforcement, the NPOSS network simultaneously delivers enhancements in both macroscopic strength and toughness. Transparent coatings prepared without added coalescing agents or co-solvents exhibit 2H hardness, 6.6 N scratch resistance, >89% visible transmittance, and strong chemical resistance under laboratory spotting conditions.