UV-curable zirconia (ZrO2) nanoparticle coatings, prepared by dispersing highly-crystalline zirconia nanoparticles (4 nm) in tetrahydrofuran with the aid of 3-glycidoxypropyltrimethoxysilane (GPTMS) and following addition of a cationic photoinitiator, were cast on silicon wafers (or glass substrates) by dip-coating or spin-coating and photopolymerized to get transparent ZrO2 nanoparticle films. Ellipsometrical characterization indicates that the refractive index of the film changes from 1.63 to 1.77 at wavelength of 632nm when the molar ratio of GPTMS-to-ZrO2 decreases from 0.30 to 0.15. Nano-indentation tests show that the films exhibit robust mechanical performance though they are not heat-treated.

The photopolymerization kinetics of poly(urethane-acrylate)/zirconia (PUA/ZrO2) nanocomposite coatings were monitored on-line and in-real-time with microdielectrometry (DEA) in terms of logarithmic ion viscosity (log). For comparison, real-time infrared spectroscopy (RTIR) was also employed to determine the evolution of C=C bond conversion (C) with irradiation time (t). Coupling log-t curves with C-t curves indicates that the relationship between log and C for all the coatings with different formulations (i.e. the type of photoinitiator, ZrO2 content) and curing condition (i.e. irradiation intensity) can be correlated to one linear equation at conversion less than ∼85%. Moreover, DEA monitoring was longer time to reach polymerization equilibrium than RTIR for those cases with high limited conversion, suggesting that the former is more sensitive to C=C bond conversion than RTIR at high conversion stage (C>∼85%). DEA monitoring on the photopolymerization kinetics of PUA/ZrO2 coatings as a function of various parameters further demonstrates that DEA results are reliable and reasonable to conventional radical photopolymerization. It is believed that DEA technique will be useful not only on the optimization of curing condition but also for the formulation of UV-curable coatings.

BACKGROUND: Polyacrylate/silica nanocomposite latexes have been fabricated using blending methods with silica nanopowder, in situ polymerization with surface-functionalized silica nanoparticles or sol-gel processes with silica precursors. But these approaches have the disadvantages of limited silica load, poor emulsion stability or poor film-forming ability.

Highly crystalline and dispersible zirconia nanoparticles, ex situ synthesized from a solvothermal reaction of zirconium(IV) isopropoxide isopropanol complex in benzyl alcohol, were functionalized with 3-(trimethoxysilyl)propyl methacrylate and blended with UV-curable urethane-acrylate formulations to fabricate poly(urethane-acrylate)/zirconia (PUA/ZrO2) nanocomposite coatings. A critical ZrO2 concentration of 20 wt% was observed for the evolutions of both the structure and properties of the nanocomposites as a function of ZrO2 content. Below the critical concentration, completely transparent nanocomposite film was obtained and the nanocomposites exhibited increasing final carbon-carbon double bond conversion, refractive index, hardness, elastic modulus, and thermal stability as ZrO2 content increased. However, serious agglomeration of ZrO2 nanoparticles occurred at 25 wt% of ZrO2, which decreased final conversion, transparency and hardness, and thermal stability of the nanocomposite film. These results clearly reveal that the performance of UV-curable anocomposites is strongly dependent on the dispersion of nanoparticles.

Poly(methyl methacrylate)/zirconia (PMMA/ZrO2) nanocomposites with ZrO2 content as high as 15 wt% were prepared by modifying non-aqueous synthesized ZrO2 nanoparticles with methacryloxypropyltrimethoxysilane (MPS) in tetrahydrofuran, dispersing MPS-functionalized ZrO2 nanoparticles in MMA and following in situ bulk polymerization with controlled pre-polymerization time. The MPS-functionalized ZrO2 nanoparticles showed an efficient crosslinking role in the polymerization, leading to a complete gel of PMMA at 5 wt% of ZrO2 content. Homogeneous dispersion of the ZrO2 nanoparticles at primary particle size level was observed in all nanocomposites, which results in good clarity of the obtained nanocomposites. Hardness tests (pendulum hardness tests and indentation tests) and antiscratch tests (abrasion tests and nano scratch tests) were employed to probe the surface mechanical properties of the nanocomposites. The properties of nanocomposites as a function of ZrO2 content, revealing from various characterization techniques, are not consistent and discussed in detail. At low ZrO2 content, the mechanical properties are enhanced by the formed crosslinking structure. However, remarkable improvements of hardness and scratch resistance of PMMA were achieved when 15 wt% of ZrO2 content was embedded.