Hard and transparent hybrid polyurethane coatings using in situ incorporation of calcium carbonate nanoparticles

Highlights: → In situ mineralization via gas diffusion was adopted for a good dispersion of calcium carbonate nanoparticles in the polymeric PU matrix. → Hybrid films with high dispersion, transparency, robust and thermal stability can be obtained by controlling the CaCO3 loading. → The hybrid films display a significant improvement in its water resistance, surface hardness, scratch resistance and flexibility, with the introduction of CaCO3, and all coatings exhibited excellent chemical resistance and adhesion. - Abstract: The combination of hardness, scratch resistance, and flexibility is a highly desired feature in many coating applications. The aim of this study is to achieve this goal through the in situ introduction of an unmodified calcium carbonate (CaCO3) into a water-soluble polyurethane (PU) matrix. Smooth and (semi-) transparent films were prepared from both the neat PU and the CaCO3-filled composites. As evidenced by the measurements from scanning electron microscopy (SEM), optical microscopy, dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA), hybrid films with high dispersion, transparency, robustness and thermal stability could be obtained by controlling the CaCO3 loading. The storage modulus could increase from 441 MPa of neat PU matrix to 1034 MPa of hybrid film containing 2% (w/w) CaCO3. In addition, the same hybrid films displayed a significant improvement in its water resistance. In this case, the water-uptake ratio decreased from 41.54% of PU to 2.21% of hybrid film containing 2% (w/w) CaCO3. Moreover, with the introduction of CaCO3, conventional coating characterization methods demonstrated an increase in the surface hardness, scratch resistance and flexibility, and all coatings exhibited excellent chemical resistance and adhesion.