Probing cosolvent effects on film formation in waterborne 2K polyurethanes using Förster resonance energy transfer

Two-component waterborne polyurethane (2KWBPU) coatings often require organic co-solvents to reduce polyisocyanate (PIC) viscosity for effective mixing with aqueous polyol dispersions. While these co-solvents are typically considered passive diluents, emerging evidence suggests they actively influence film formation and final coating properties. This study systematically investigates how co-solvent molecular structure affects film formation mechanisms in 2KWBPU systems using Förster resonance energy transfer (FRET). Five co-solvents with varying polarity, volatility, and water miscibility were examined: dipropylene glycol dimethyl ether (DPGDME), 2-(2-butoxyethoxy)ethyl acetate (BGDA), propylene glycol monomethyl ether acetate (PGMEA), propylene carbonate (PC), and tetrahydrofuran (THF). FRET studies with dual-labeled polyol latex particles revealed that co-solvent effects on film formation follow distinct structure-property relationships. Volatile solvents (THF, ethyl acetate) created vertically heterogeneous films, while high-boiling polar solvents (PC, PGMEA, BGDA) promoted uniform, extensive particle swelling ( Φ ET ≈ 0.59–0.62). Low-polarity solvents (DPGDME) enabled moderate, uniform swelling ( Φ ET ≈ 0.67) limited by reduced polyol compatibility. Hansen solubility parameter analysis provided quantitative structure-property correlations, while atomic force microscopy confirmed solvent-dependent particle deformation predicted by FRET measurements. FTIR spectroscopy revealed that co-solvent retention significantly influences crosslinking kinetics, with high-boiling solvents extending cure times through reactant dilution. These findings demonstrate that co-solvent selection critically influences both film formation and cure times, providing a framework for rational formulation design in waterborne polyurethane coatings. • Co-solvents actively govern film formation in waterborne 2K polyurethane coatings. • FRET directly quantifies solvent-dependent polyol chain interdiffusion and swelling. • Volatile solvents induce vertical film heterogeneity during drying. • High-boiling polar solvents enable uniform, extensive intra-particle swelling. • Co-solvent retention significantly alters polyurethane cure kinetics.