A Study of Phase Composition Effect of Hollow Titania Photocatalyst and its Application to Infrared Reflective Coating

Abstract

In recent years, since polymer films containing thermal blocking and near-infrared (NIR)-reflective pigments have received much attention for their potential applications in energy-saving fields, it serves energy saving efficiently as not only blocks the transmission of solar heat rays from the outside, but also reduces the consumption of cooling and heating energy by the insulation effect that minimizes indoor heat loss. However, in practical environments, dust present in the air is easily adsorbed and adheres to the surface of these films, thus gradually reducing their NIR reflectance. To overcome drawbacks, there is an increasing need to develop a thermal blocking system along with long-term periods. Thermal energy control can be roughly achieved by two methods: thermal insulation and thermal reflection. Insulation means a function of preventing heat from being transferred to the inside by using a heat insulating material having a low thermal conductivity. Hence, it is obvious that the thermal conductivity coefficient of the material is a major factor indicating the adiabatic effect. On the other hand, IR reflection means to selectively and efficiently reflect infrared rays, which account for more than 50% of sunlight, to prevent the temperature rise of the roof or exterior walls, which is the cause of the increase in the internal temperature of the building. It can be seen more appropriate that reduction of thermal factors through light reflection. In this present work, it is intend to research and develop coated steel sheets with thermal control characteristics such as heat insulation and heat reflection together without surface contamination. In addition, it is also intended to develop a single/thermal coated steel sheets which can be applied to high-speed thin film coating for commercialization and high productivity. In general, since the rate of heat transfer by conduction and convection in the closed micro-sized void is very low, in the case of micro hollow spheres having a high porosity of 80% or more, the effective thermal conductivity is expected to be considerably low. Therefore, hollow silica spheres having a lower thermal conductivity have already been attempted to develop with improved mechanical properties by others. In this present work, nanocrystalline titania have a role as a photocatalyst when absorbing light. It can absorb sunlight and remove contaminants from the particle surface. In addition, it has the advantage of higher reflectivity than silica. So, it was designed to maximize the effect after applying it to the reflective coating by combining the advantages of the hollow structure and the advantages of the titania photocatalyst. Hollow structured titania was prepared by direct chemical deposition method as follows: (1) preparation of hard templates; (2) functionalization/modification of template surface to achieve favorable surface properties; (3) coating the templates with designed materials or their precursors by various approaches, possibly with post-treatment to form compact shells; and (4) selective removal of the templates to obtain hollow structures. Anionic polymer seed were prepared by multi stage emulsion polymerization to assign electronegative characteristics. With help of electrostatic attraction and controlling hydrolysis and condensation reaction, 200-300 nm coreshell titania was successfully produced. The, sintering process at high temperature was carried out to remove seed and form hollow titania. The titania has high porosity, high surface area by controlling shell thickness. Moreover, the hollow spheres showed excellent photocatalytic performance compared with commercial titania nanopowders. It was observed that the photocatalytic powder correlated with sintering temperature. Optimized manufacturing hollow titania that has photocalytic acitivity has been deduced by calicination processing control. IR reflective coating was applied on steel sheets with the synthesized hollow titania to confirm their photocatalytic and reflection performance. Interestingly, most of the particles located near surface with help of hollow structure in a coating. It is important to the position of particles in coating especially for photocatalytic ability implementation. When the photocatalayst was immobilized deeply in coating, it was hard to observed sun light and act organic degradation. In my research, it was observed that the hollow particles were induced to float to the surface by the hollow structure. No more additional coating is needed using this technology that can decrease production cost. Moreover, hollow titania provides enhanced reflection to coating due to both high reflectance nature of titania and existence of void which is able to give more light scattering probability. In the ends, accomplished performance was achieved in terms of thermal blocking (due to increase in IR reflection) and surface cleaning (due to photocatlaytic characteristic) within one coating.