Waterborne polyurethane: the effect of the addition or in situ formation of silica on mechanical properties and adhesion

A series of composites consisting of commercial waterborne polyurethane (PU) and silica were prepared by in situ synthesis (sol-gel method) and compared to those prepared by the addition of commercial silica (blending method). Adhesion resistance, mechanical resistance, small-angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC) measurements were performed in order to evaluate the effect of silica addition. The adhesion resistance reached a maximum at 3 wt.% of added silica. Comparing the sol-gel composites with the analog composites obtained by the addition of commercial nanosilica, it was observed that although the composites containing commercial silica displayed higher mechanical resistance, better adhesion was obtained with the in situ method. The DSC results showed increasing crystallinity with increasing silica addition.