Preparation and properties of waterborne bio-based polyurethane/siloxane cross-linked films by an in situ sol–gel process

•Two castor oil-based polyols were synthesized by thiol-ene coupling.•A series of waterborne bio-based polyurethane/silica hybrid dispersions were prepared.•The formation of nano-silica through an in situ sol–gel process was detailed.•Particular focus was placed on the effect of siloxane content on the properties of the hybrid films.•The bio-based nanocomposites had good thermal, transparency and water-resistance properties. Waterborne castor oil-based polyurethane-silica hybrid materials with chemically bonded polymer matrix and silica nanoparticles were designed and prepared. The formation of cross-linking structures in the waterborne polyurethane system was confirmed by Fourier transform infrared spectroscopy. Transmission electron microscopy images indicated that nano-silica was encapsulated by polyurethane and exhibited an apparent core–shell structure. The nano-silica in the polyurethane matrix was vital in improving both the hydrophobicity and thermal stability of the resulting hybrid polyurethane films. With increased silicone content, the roughness, hydrophobicity, and thermal stability of the films were enhanced, but the transparency of the films in the 300–800nm region was decreased. These results can aid in the design of bio-based hydrophobic waterborne polyurethane films with favorable thermal stability and optical transmittance.