Fabrication Of Durable UV Nano-absorbers And Their Application In Transparent UV-shielding Polymer Coatings

UV component of the sunlight have attracted considerable attention, because it causes serious damage to huam skin, woods and various kinds of polymer materials. Recently, the dose of UV radiation reaching the earth surface is still increasing as the ozone layer is gradually depleted by human’s industrial activities. Thus, it is highly desirable to develop the UV-shielding materials to protect human, woods and polymer materials against UV light damage. UV-shielding materials can be divided into two types:organic UV-absorbers and inorganic nanoparticles. Orgainc UV-absorbers usually experience photostability and migration problems, and only absorb certain wavelength range of UV light. The inorganic nanoparticles also have several drawbacks such as poor dispersibility and high photocatalytic activity. These disadvantages greatly affect their performance in application, and thus, development of new durable UV-shielding materials is strongly desired. In this thesis, several durable UV-shielding materials were prepared through fabrication of core-shell structures and host-guest assembly based on the common UV-shielding products, such as avobenzone (BMDM), nano-ZnO, and nano-TiO2. In addition, a new UV-shielding nano-material, Mg/Al+Fe LDHs, was also developed. All these UV-shielding materials were embedded into polymer matrices to fabricate the highly transparent and durable UV-shielding polymer coatings. The preparation, structures and properties of the UV-shielding materials developed were fully studied in this thesis, and the results can theoretically promote the development of new UV-shielding materials. The research details and results are listed as follows:(1) Fabrication of Eu(BMDM)3@PSi nanoparticles and its application in waterborne polyurethane coatingsAvobenzone (BMDM), one of the most commonly used sunscreen agents, was employed as the organic ligand to fabricate the europium β-diketone complex (Eu(BMDM)3). The Eu(BMDM)3 was further coated with a polysiloxane layer by the hydrolysis-condensation reactions of octyltrimethoxysilane (OTS) to produce the Eu(BMDM)3@PSi nanoparticles. The relationship between the OTS amounts and the photo- and thermostability of the obtained nanoparticles was investigated. All the Eu(BMDM)3@PSi nanoparticles exhibited better photo- and thermostability, and moreover, better water dispersibility than that of Eu(BMDM)3. The Eu(BMDM)3@PSi nanoparticles were further embedded into waterborne polyurethane (PU) latex to fabricate the UV-shielding polymer coating. The coating was highly transparent, and highly effective on blocking the UV light within 300-400 nm. Moreover, the UV-shielding coating also showed efficient light conversion property, which could convert the harmful UV light to nutritious red light for plants, and thus may find some potential applications in agriculture field for promoting the photosynthesis of plants.(2) Facile preparation of ZnO QDs@SiO2 and its application in waterborne UV-shielding polymer coatingsZnO quantum dots (ZnO QDs) were firstly prepared via a sol-gel process using LiOH·H2O and Zn(Ac)2·2H2O as the precursors. Then, a silica layer was in situ coated on the surfaces of ZnO QDs through hydrolysis-condensation reactions of TEOS in a mixture of ethanol and water to fabricate ZnO QDs@SiO2 nanoparticles. The relationship between TEOS amounts and photocatalytic activities of the ZnO QDs@SiO2 were investigated. It was found that the as-obtained ZnO QDs@SiO2 nanoparticles have the following properties:(a) They are small enough not to scatter visible light and can strongly absorb the UV light below 340 nm; (b) They are highly dispersible in water, and successful in fabrication of waterborne UV-shielding polymer coatings with very ultrahigh transparency; (c) When the silica shell is thick enough, the photocatalytic activities of ZnO can be considerably depressed, and their polymer coatings have better durability than that of polymer/ZnO nanocomposites. Moreover, the ZnO QDs@SiO2 nanoparticles emitted strong blue light and showed higher quantum yield (QY,27%) than that of ZnO QDs (13.4%). Interestingly, the QY of the ZnO QDs@SiO2 aqueous dispersion was still comparable to that of the freshly-prepared ZnO QDs@SiO2 even after storage for 4 months. Thus, the ZnO QDs@SiO2 can be used as bifunctional nanoparticles to fabricate the UV-shielding coatings with excellent luminescent properties.(3) Preparation of TiO2@Mg/Al LDHs and its application in UV-shielding polymer coatingsA watersoluble Ti precursor prepared by the reaction between titanium powder and H2O2 was intercalated into Mg/Al-NO3 LDHs to produce the Ti(OH)3O2-@Mg/Al LDHs, which then experienced a hydrothermal process to fabricate the TiO2@Mg/Al LDHs. Due to the confinement effect of the LDHs layers, the TiO2 nanoparticles within LDHs were very small (2-3 nm), and have far weaker photocatalytic activities than that of free TiO2. The TiO2@Mg/Al LDHs were dispersible in alcohol and readily embedded into polymer matrices to prepare transparent UV-shielding coatings. The TiO2@Mg/Al LDHs based polymer coatings could effectively block the UV light below 320 nm and protect the UV-sensitive materials against UV damage. Moreover, the coatings containing TiO2@Mg/Al LDHs had excellent durability and remained stable after accelerated weathering test for 200 h, while the coating containing free TiO2 nanoparticles experienced photodecomposition and transparency loss.(4) Fabrication of Mg/Al+Fe LDHs and its application in UV-shielding coatingsFe3+ doped Mg/Al layered double hydroxides (Mg/Al+Fe LDHs) were prepared by a facile solvothermal method. Incorporation of Fe3+ into Mg/Al LDHs can efficiently shift the UV-absorption band of Mg/Al LDHs to cover all UV range (200-400 nm). The as-obtained Mg/Al+Fe LDHs were intrinsically dispersible in water (hydrophilic), and inorganic UV-shielding coatings based on its aqueous dispersion were fabricated due to its self-supporting behavior. The Mg/Al+Fe LDHs could also be dispersed into organic solvent (hydrophobic) after sequentially washed with ethanol and acetone. The hydrophilic and hydrophobic Mg/Al+Fe LDHs were embedded into their corresponding polysiloxane to fabricate the UV-shielding coatings, respectively. Both the hydrophilic and hydrophobic coatings were highly transparent, long-term durable and remained stable even after accelerated weathering test for as long as 800 h.