Preparation And Properties Of UV-curable Poly(urethane-acrylate)/ZrO₂ Nanocomposite Coatings

Organic-inorganic hybrid materials combine both the advantages of organic polymer(flexibility,light weight,good impact resistance,good processability,etc.) and inorganic materials(high mechanical strength,high hardness,good chemical resistance,thermal stability,optical properties,etc.) and have been widely applied in various fields.Especially,the UV curable hybrid coatings have been widely concerned since they possess both the advantages of UV-curing process and of hybrid nanocomposite materias,namely,environmental-friend and high performance(or new function).Up to now,some UV curable nanocomposite coatings were reported. However,the study was still limited and mostly focused on the systems containing SiO₂,TiO₂,and clay.The application of ZrO₂ nanoparticles was rarely reported.ZrO₂ nanoparticles would be an ideal building block for nanocomposites because they offer several advantages such as chemical inertness,excellent thermal stability,high refractive index,and high hardness.In this study,commercial ZrO₂ nanopowder or non-aqueous synthesized ZrO₂ nanocrystals was incorporated into poly(urethane-acrylate) oligomer(PUA),monomers to fabricate UV-curable nanocomposite coatings.The photopolymerization kinetics of PUA/ZrO₂ nanocomposite coatings was on-line and in-real-time monitored with microdielectrometry(DEA).Their mechanical properties,thermal properties and optical properties of the nanocomposite coatings as well as the phase behavior of ZrO₂ nanoparticles in different monomers and PUA oligomerr was studied.The dependence of the improving efficiency of nano-ZrO₂ on the properties of organic coatings was deeply investigated.All the results are expected to provide some concrete evidence for the development and application of nanocomposites.The detailed research contents and results are detailedly described as follows:Commercial ZrO₂ nanoparticles were functionalized by 3-glycidyloxypropyltrimethoxysilane(Z-6040),After that,the modified nano-ZrO₂ were incorporated into PUA oligomer(UVU6219) and tripropylene glycol diacrylate (TPGDA) to prepare UV-curable PUA/ZrO₂ nanocomposite coatings.Effect of the zirconia content on the UV-curing kinetics of the coatings was investigated by dielectric analysis(DEA).The influence of photoinitiator types,photoinitiator content, UV irradiation intensity,atmospheric character and film thickness were also investigated.For comparison,real-time infrared spectroscopy(RTIR) was also employed.Linear relationship between logηand C at conversion lower than about 85%is acquired via a large numbers of comparisons between DEA and RTIR results for those PUA/ZrO₂ nanocomposite coatings.DEA monitoring has 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%).Due to the good reproducibility and sensitivity,DEA will be a highly efficient tool for the optimizations of both UV-curable coatings formulation and curing condition.In addition,the final double bond conversion decreases with increasing nano-ZrO₂ content because the transparency of the coatings unavoidably reduces after ZrO₂ nanopowder is incorporated.Highly-dispersible zirconia nanocrystals,ex situ synthesized from a solvothermal reaction of zirconium-(â…£) isopropoxide isopropanol complex in benzyl alcohol,were functionalized with 3-(trimethoxysilyl)propyl methacrylate(MPS) and blended with UV-curable polyurethane oligomer(6148J-75) to fabricate transparent PUA/ZrO₂ nanocomposite coatings.Their photopolymerization kinetics,mechanical properties, thermal properties and optical properties were investigated.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 critial concentration,completely transparent nanocomposite film was obtained and homogeneous dispersions of ZrO₂ nanoparticles with primary particle size level were achieved.As a consequence,the nanocomposites exhibit increasing final C=C double bond conversion,refractive index,hardness,modulus,and thermal stability as ZrO₂ content increases.Particularly,the nanocomposite with 20 wt%of ZrO₂ has extremely high performance,namely,a high refractive index of 1.63,82%increment on hardness and about 60℃higher thermal decomposition temperature relative to pure PUA film.However,beyond this critical ZrO₂ loads,ZrO₂ nanoparticles tend to agglomerate in the nanocomposite,leading to serious reduction of the transparency and thus lower final conversion,nearly no improvement on properties in comparison with pure PUA coating.Based on the above study,the phase behavior of MPS-functionalized ZrO₂ was further investigated in monomers or PUA/monomer mixtures.Their mechanical properties and optical properties were investigated as well.It's found that the existence of reactive monomers with low molecular weight facilitates the compatibility between the PUA and MPS-functionalized ZrO₂.And the higher the amount of MPS attached to ZrO₂ nanoparticle is,the lower the transparency of nanocomposite coating is.The coatings keep transparent even at ZrO₂ content of 62 wt%in pure TPGDA.Thus,the critical ZrO₂ concentration can be adjusted by the composition of organic matrix.Moreover,the better the mechanical properties of the organic matrix are,the higher the improving efficiency of ZrO₂ on mechanical properties of the coatings is.In addition,preparation of thermosetting acrylic coatings with titanium-oxo-cluster as a curing agent is primarily confirmed.The effects of the titania content,the preparation condition on the structures and properties of the hybrid materials were studied.DEA was employed to monitor the curing reaction.The results indicate that the resulted acrylic/titania coatings have better thermal stability, mechanical property and UV-block property.Anyway,this method opens another way for fabrication of thermosetting coatings,which can eliminate the toxicity of common organic curing agent and meanwhile endow the coatings with the high performance of hybrid coatings.