Prepartion And Properties Of Waterborne TiO₂-based Self-cleaning Coatings

Titania-based self-cleaning coatings have great potential applications in architecture, transportation and new energy field. There have been many methods to fabricate these coatings. However, the existed TiO2-based coatings, including inorganic TiO2coatings, TiO2-based nanocomposite coating with organic or inorganic binder, have limited applications because of their inherent disadvantages. Inorganic TiO2coatings can be fabricated only using expensive equipments or under harsh conditions despite their good transparency and self-cleaning effect. TiO2-based nanocomposite coatings with organic or inorganic binder have short service life or low self-cleaning performance. Therefore, TiO2-based self-cleaning coatings need further development. In this thesis, two routes were employed to fabricate waterborne self-cleaning coatings based on silica/polymer binder. In one route, TiO2nanoparticles were directly mixed with silica sol/polymer latex blends. Prior to fabrication of self-cleaning coatings, the film formation behavior of silica sol/polymer latex blends and the mechanical properties of the corresponding coatings were investigated. In other route, TiO2nanoparticles were first encapsulated with poly(methyl methacrylate)(PMMA) via miniemulsion polymerization, and afterthat, mixed with silica sol, in order to control the residence of TiO2nanoparticles merely in polymer phase. Evolution of the surface morphology, wettability, and transparency with UV-irradiation time was monitored for the coatings from both routes, The formation mechanism of self-cleaning surface were discussed and the performance of the coatings at outdoor were examined. The detailed research contents and the main results are as follows:Silica sol (diameter:8-100nm) and polymer latex (Tg<25℃) were mixed and dried at room temperature to prepare nanocomposite films with high silica load (≥50wt%). Effects of silica size, silica load, and the Tg of the polymer on the film-forming behavior and mechanical properties of the silica/polymer latex blend films were investigated. Thirty nanometers was found to be the critical silica size for the evolution of film-forming behavior, surface morphology, and mechanical properties. Colloidal silica particles smaller than this critical size act as binders to form strong silica skeleton. This gives the final silica/polymer nanocomposite film its porous surface and high mechanical strength. However, silica particles with sizes of30nm or larger tend to work as nanofillers rather than binders, causing poor mechanical strength. Critical silica load was also revealed for the mechanical strength of the coatings as a function of silica load. As silica load increases, the mechanical strength increases very slowly below the critical silica load, however, remarkably increases above the silica load due to the formation of silica skelton.SiO2sol or modified SiO2(M-SiO2) sol, polymer latex, and the aqueous TiO2nanoparticle dispersion were mixed to prepare waterborne self-cleaning coatings. Modification of silica, silica size, polymer type and particle size, SiO2/polymer ratio, and TiO2load were found to influence coating properties greatly. Coatings with relatively bigger colloidal silica particles, polymer latex with nanosizeand silica load close to50-60wt%showed better durability and transparency. Coatings with unmodified SiO2sol have the advantages of simplicity in fabrication and application, however, poor mechanical strength than that with M-SiO2sol. Outdoor exposure tests showed that of the optimized self-cleaning coating has excellent self-cleaning performance outdoors without sacrificing its decoration when it was covered on the conventional exterior architecture coating.TiO2nanoparticles were first modified with y-methacroxylpropyltrimethoxysilane in the presence of low quantity of MMA, and subsequently used to fabricate TiO2@PMMA core-shell latex by mini-emulsion polymerization. M-SiO2/TiO2@PMMA self-cleaning coatings were prepared by mixing M-SiO2sol and TiO2@PMMA core-shell latex. Accelerated weathering tests showed that it took shorter time to obtain suprahydrophilic property for M-SiO2/TiO2@PMMA coatings relative to M-SiO2/PMMA/TiO2coatings. This may result from the mere distribution of TiO2nanoparticles polymer phase in the coatings. Moreover, the obtained suprahydrophilic coatings exhibited good transparency, mechanical property and self-cleaning effect.