Interfacial Energy and Composition Controlled Self-Stratification in Polyurethane Coatings

Self-stratifying polymer systems are of great interest for coatings, as such systems reduce the time, cost, and environmental impact associated with the application of multilayered coatings by providing several layers in a single coating step. We have developed an understanding of self-stratification in polyurethane systems that occurs when hydrophobic and hydrophilic polyols containing ethylene oxide, propylene oxide, and butylene oxide mers and prepolymers containing toluene diisocyanate and methylene diphenyl diisocyanate are mixed and cured. When these components are mixed in appropriate proportions, self-stratification occurs where the hydrophobic component migrates to the air interface and the hydrophilic component to the substrate interface, with a thin hydrophobic layer present at the substrate walls when the substrate is hydrophobic. Self-stratification requires less than 60 min, significantly less than the time required for the storage modulus to crossover the loss modulus (∼5 h). SIMS, XPS, and confocal Raman show that the stratification process at the air and substrate interfaces is dependent on interfacial surface energies, with the thickness and composition of the up to 10 µm thick interfacial region at the substrate controlled by the substrate surface energy. Self-stratification is observed in both the bulk and thicknesses conventionally associated with coatings (10s of µm).