| Ultraviolet(UV)curable coatings are a type of green paint products that are under active development in recent years.The products possess characteristics of being environment-friendly,energy saving,economical,efficient and enabling.At present.UV curable coatings with zero volatile organic compound(VOC)emission exhibited its own distinct advantages,particularly on the background of energy saving and emission reduction advocated by all over the world.In academia,it is approved to have extensive application prospects.Moreover,the coatings can be improved with respects to mechanical and corrosion-resistant properties by introducing inorganic additives.This study aims to combine the advantages of nano-composite and UV curing techniques and provide the theory fundamental and experimental basis for application of the coatings on the metallic substrates.This work was devoted to a study on a series of EA-SiO2 and EA-Si composite coatings based on epoxy arcrylate(EA)as an oligomer.The used approaches included simple blending,sol-gel and hybrid UV curing techniques.The structures,morphologies and properties of composite coatings were investigated systematically.When prepared from basic hydrolysis of TEOS,the size of SiO2 particles are found to increase with the decrease of dielectric constants and polarities of the three solvents of methanol,ethanol and acetone.For the EA-TPGDA(tripropylene glycol diacrylate)system,the temperature dependence of the viscosity was studied.The iso-viscosity curves and Andrade equations were obtained,which can be used for estimation of viscosities of the EA-TPGDA system within a temperature range from 298 to 323 K.Due to different generation mechanisms of active free radicals.the photo-initiating efficiency of TPO(2,4,6-trimethylbenzoyldiphenyl phosphine oxide)and Irgcure 184 as cleavage type photo-initiators was found to be higher than that of BP(benzophenone)as hydrogen-abstracting type photoinitiator.The optimal concentrations of the photo-initiators were found to be 1 wt%for TPO,2 wt%for Irgcure 184 and 3 wt%for BP,respectively.FTIR spectra of the blended EA-SiO2 composite coatings exhibited vibration characteristic peaks of the Si-O-Si bonds,indicating that SiO2 inorganic particles were embedded into the EA matrix.As evidenced by SEM,the surface morphology of the composites showed good compatibility with no phase separation.However,agglomerations of SiO2 particles were observed.The higher content of the SiO2,the more agglomerations.Addition of SiO2 nanoparticles an inorganic additive was found to enhance the thermal resistance of the UV cured coatings.The best effect was achieved at the content of 5 wt%SiO2.Addition of SiO2 was also found to improve the hardness of the composite coatings and the greatest hardness was obtained when the contents were in a range of 3?5 wt%SiO2.Following the photochemical method for preparing gold nanorods,S iO2 nanoparticles were synthesized by photochemical route.Formation mechanisms of the SiO2 sol were briefly discussed.Using thus obtained SiO2 nanoparticles,UV curable EA-SiO2 composite coatings were prepared by surface modification and following enrichment.Focusing on the surface morphology of the composite coatings,addition of SiO2 nanoparticles prepared by the photochemical sol-gel method can reach a content of 10 wt%,much higher than that by the blending method.The thermal stability of coatings containing SiO2 additives was found in the order of EA-Si-3<EA-Si-6<EA-Si-10>EA-Si-15>EA-Si-20,i.e.peaked for the EA-Si-10 sample with 10%of SiO2.It was apparent that agglomeration of SiO2 particles occurred at higher SiO2 contents.In addition,the presence of SiO2 nanoparticles could reduce the conversion rate of C=C double bonds during the curing process and therefore prolong the tack free time.The coatings with 6?10 wt%SiO2 showed the highest hardness of 4H.During the free radical-cationic hybrid curing process,the dissociation of an iodonium salt led to formation of active free radicals and cations,which,in turn,induced ring-opening polymerization and polycondensation of the GPTMS(y-Glycidoxypropyltrimethoxysilane).At the same time,the free radicals can also promote the polymerization of the EA.FTIR and Raman spectra showed that the inorganic Si-O-Si network ywas formed in the EA-Si composite coatings.Addition of the GPTMS as a cationic curing component led to a reduced conversion of C=C double bonds.The resultant hybrid EA-Si coatings exhibited good transparence,smooth defect-free surface,and uniform distribution of the Si element.The formation of Si-O-Si networks could enhance the flexibility of the polymeric chain and raise the decomposition temperature of the soft segment.The transmittance of the EA-Si coatings at a wavelength of 346 nm was found to increase with the content of the cationic curing component.Under the same curing condition,the tack free time of the hybrid EA-Si coatings was increased with the content of the cationic curing component.However.little effect was found of the cationic curing component on the hardness.UV curable composite coatings could be applied onto the pretreated galvanized sheets.The adhesion was found to have reached the requirements of national standards.Compared to the chromizing process,the effect of silanizing process with little environmental consequence was more excellent.The corrosion resistance in NaCl solution were tested for three types of coatings:the pure EA coating,EA-SiO2 coating and EA-Si coating.In the EA-SiO2 composite coatings,the presence of inorganic SiO2 particles could improve the barrier effect against the water permeation.In the EA-Si composite coating,the Si-O-Si network structure effectively hindered the water transport and therefore enhanced the corrosion resistance of the coating.As a summary the corrosion resistance of the three coatings on metal substrates prepared in this study was found to be in the order of EA-Si coating>EA-SiO2 coating>pure EA coating. |
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