Stratification of polyisocyanate in two-component waterborne polyurethane films

The coatings industry continually seeks advancements in waterborne two-component polyurethane (2 K WBPU) systems, given their potential for environmentally friendly applications and superior film properties. This study explores the mechanistic aspects of film formation in 2 K WBPU coatings formulated with an acrylic polyol and a polyisocyanate (PIC) derived from a trimer of hexamethylene diisocyanate (HDI-trimer). To enhance water dispersibility—a key parameter in industrial applications—the HDI-trimer is modified with a short polyethylene glycol, producing a hydrophilically modified PIC (hmPIC). While prior studies identified hmPIC as an effective reactive plasticizer for the polyol, the common practice of adding unmodified HDI-trimer (uPIC) to hmPIC for moisture sensitivity reduction necessitates a deeper understanding of film properties. Using fluorescence resonance energy transfer (FRET) and confocal fluorescence microscopy, we investigated the effects of blending uPIC with hmPIC on coalescence, phase separation, and film stratification in 2 K WBPU systems. Notably, environmental factors like relative humidity were found to influence film stratification significantly. This finding has critical implications for coating performance, such as durability and water resistance. A novel method was also introduced to monitor water permeation within stratified films, offering a valuable tool for optimizing film properties in real-world applications. This study underscores the importance of strategic component interactions in polymer coatings, suggesting that controlled film stratification can significantly enhance film performance in industrial settings.