Phase separation and properties of UV-curable polyurethane/zirconia nanocomposite coatings

Transparent zirconia (ZrO(2)) nanocrystal dispersions in butyl acetate (BAc) or tetrahydrofuran (THF) were prepared by functionalizing zirconia nanocrystals, synthesized from a solvothermal reaction of zirconium (IV) isopropoxide isopropyl alcohol complex in benzyl alcohol, with 3-methacryloxypropyltrimethoxysilane (MPS), and were embedded into UV-curable polyurethane (PU) coatings. It was found that the ZrO(2) concentration was the dominating factor influencing the transparency of the ZrO(2) dispersion, while the molar ratio of MPS to ZrO(2) was rather efficient in adjusting the amount of MPS attached. Phase separation of the ZrO(2) nanocrystals in polyurethane coatings was observed, and was strongly related to the MPS-modified ZrO(2) (MPS-ZrO(2)) content and the amount of MPS chemically bonded. The phase separation of the PU/ZrO(2) nanocomposite coatings appeared more seriously for higher MPS-ZrO(2) content and higher amounts of MPS attached; thus the transparency of the nanocomposite coatings was lower. However, completely transparent PU/ZrO(2) nanocomposite coatings could be still achieved even with a ZrO(2) content up to 7.5 wt.-% using a ZrO(2)/BAc dispersion. Improvements in pendulum hardness, microhardness and scratch resistance of the polyurethane coatings via addition of MPS-ZrO(2) were demonstrated, but were not remarkable because of the low cross-linking efficiency of the MPS-ZrO(2) nanoparticles resulting from the poor compatibility of the MPS-ZrO(2) nanoparticles with the pure PU oligomers.