Synthesis And Properties Of Hydrophobic Composited Coatings For Historical Sandstones

There are many precious historic and cultural heritages in the 5000 years history of china. As an outstanding example of outdoor immovable stone monuments, Yungang Grottoes in Shanxi are precious cultural heritage and were listed as an UNESCO world heritage. In recent years, in addition to the natural factors, the stone monuments are dramatically damaged due to attack of air pollutants present in atmosphere. If no measures are taken, these precious cultural heritages will disappear in the near future. However, the requirements of materials for conservation of historical stone are becoming much more concerning now. The inorganic and organic composite coatings are considered to be promising with applications in stone conservation due to their anti-aging, sterilization, resistance to UV, corrosion, and oxidation.In the present dissertation, the composite coatings consisting of the TiO₂-coated SiO₂ nanoparticles and inorganic with low surface energy were synthesized. These coatings present significantly improved ability as anti-weathering coatings and their properties are evaluated.Firstly, SiO₂ nanopowder coated by a thin lay of TiO₂ which are synthesized by wet chemical method. Using SEM and high resolution TEM, we observe that the morphology and dispersibility of the TiO₂-coated SiO₂ nanoparticles are affected significantly by the content of SiO₂, the amount of water, the concentration of TBOT, the feed speed, and the feed time. The experiment of photodegradation MO shows that the TiO₂-coated SiO₂ nanoparticle has very good photocatalytic property, which is closely related to the annealing temperature.Secondly, the composite coatings are prepared by mixing the nanoparticles with the organic solution with the low surface energy (BYB1002, BYB1008, and fluorine silicone) by high energy ball milling and ultrasonic dispersion technique. These coatings are subsequently sprayed onto the sandstone surface or glasses. We observe that the hydrophobicity of the coatings is affected by the content of the nanoparticles. However, the hydrophobicity of coatings sprayed on the surface of glass is better than that on the surface of stone since the surface of stone is much rougher than that of the glass. As the content of the nanoparticles increases, the hydrophobicity further increases. The superhydrophobicity with water droplet contact angles >150o could be achieved while the certain content of the nanoparticles is arrived. This phenomenon is due to the the presence of micron-sized protrusions formed by nanoparticle aggregates and the siloxane polymer with low surface energy.Given the applications of as-prepared coatings as an anti-weathering coatings for sandstone monuments, the hydrophobicity is not the sole parameter to be used to evaluate their ability as protective coatongs. Furthermore, the water absorption, breathability, capillary absorption, anti-weathering ability, UV-resistant ability, and freezing-thawing resisting ability of the materials had been investigated. These results show that the use of BYB1002 as coatings improves significantly the hydrophobicity, capillary absorption, and anti-weathering ability. Although the water absorption, breathability, and capillary absorption can not be improved for coatings with addition of nanoparticle, their anti-weathering ability, especially the acid-resistant ability, is improved. However, we observe that the UV-resistant ability of the composite coatings with addition of TiO₂ coated SiO₂ nanoparticle is degraded evidenced by significantly decreased water droplet contact angles after continuous irradiation of ultraviolet light. It is believed this decrease is ascribed to non-selective photodegradation of TiO₂ that results in the decomposition of the organic.