Influence of the airflow and humidity on the chain aggregation during the film-formation in a flexible waterborne polyurethane formulation

Statement of objectives: Soft, waterborne polyurethane dispersions are indispensable components in many state-of-the-art materials, with applications ranging from binders for coatings and adhesives to matrixes for flexible devices. While the static bulk nanostructure of such systems is widely studied, the influence that environmental conditions such as relative humidity and airflow have on their film formation and phase segregation behavior in supported films is unknown. Experiments: Here, we elucidate the nanostructure evolution occurring during drying of an industrially relevant, soft polyurethane, utilizing real-time, non-destructive grazing incidence X-ray scattering analysis. Using an environmental-controlled casting cell, we highlight the differences between the drying mechanism under different conditions generated by tuning the airflow and the relative humidity. Findings: Our results show how the environment's relative humidity strongly influences chain aggregation and chain interdiffusion due to extended plasticization of the hard segment at high humidities, while accelerated air flows are responsible for the occurrence of (partial) skinning. Interestingly, despite changes in the chain aggregation behavior and occurrence of skinning and skin breakup during drying resulting in higher roughness at the film surface, minor influence is registered on the bulk tensile properties of the films, revealing the resilient nature towards environmental conditions of these soft weakly phase segregating polyurethane systems.