Abstract
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.
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