Mechanical Properties of Waterborne Polyurethane Latex Film Formed From Acrylic and Blocked Polyisocyanate Particles

ABSTRACT Waterborne latex films are promising environmentally benign materials for coating and adhesive applications; however, achieving sufficient mechanical strength remains a challenge. We previously developed a reactive latex system composed of hydroxyl group‐containing acrylic latex (AL) particles and blocked polyisocyanate (BPI) nanoparticles, which undergo in situ urethane crosslinking through the reaction between hydroxyl and isocyanate groups upon heating. In this study, we systematically examined the effect of the NCO/OH molar ratio, defined as the ratio of isocyanate groups in BPI nanoparticles to hydroxyl groups in AL particles, on the structure formation, thermal transitions, and mechanical behavior of the resulting polyurethane films. Cross‐sectional electron microscopy revealed interdiffusion between AL and BPI nanoparticles above their respective glass transition temperatures, leading to homogeneous polyurethane networks. Films prepared with NCO/OH ratios between 0.75 and 1.0 exhibited a well‐balanced combination of flexibility and surface hardness, whereas deviations from this range resulted in brittle or excessively soft structures. Films at an NCO/OH ratio of 1.0 also showed high optical transparency and excellent solvent resistance. These results elucidate the structure–property relationships governing crosslinked polyurethane latex films and provide molecular‐level insight into designing waterborne coatings.