Papers by Author: Dae Sung Kim

Abstract: The reinforced clear coat on glass substrate was prepared by coating waterborne-polyurethane hybridized by silica sol or aluminate modified silica sol. To enhance the physicochemical property of the film, aluminate modified silica particle was prepared by the interaction between sodium aluminate and cation-exchanged silica. The hybrid thin films were developed in the range of the modified silica content varying from 0 to 20 wt%. The film, containing the modified silica content of 16 wt%, exhibits markedly improved properties as compared to the neat polyurethane and the polyurethane containing silica particle; e.g., an improved hardness, chemical and abrasion resistance, with maintaining the transparence. However, the film transparence was decreased with modified silica content above 18 wt%, and its hardness was not improved with its content below 14 wt%.

Abstract: TiO₂ coated mica (TiO₂/Mica) as solar reflective pigment was prepared under hydrothermal treatment. Dispersed TiO₂ nanosol, having an average particle size of about 25-30nm, was coated on mica flake by the difference between the surface charge of the particles at pH 2 and then calcined at 850°C to stabilize the coated layer on mica. The CIE color coordinate and total solar reflectance (TSR) properties of these pigments were investigated in relation to thickness variation of TiO₂ layer coated on mica having various lateral sizes. Dense and uniform TiO₂ layers were tightly coated on the surfaces on mica substrates. Isolation-heat paints were prepared with 20wt% pigments fully dispersed in acryl-urethane resin and several additives to coat the film uniformly. Thermal property of these films, recorded by an isolation-heat measuring system, was observed for the relationship of TSR value according to the thickness of TiO₂ layer and mica’s lateral size, compared to TiO₂ itself.

Abstract: MgO/XLPE composites, composed of micro-and nanosized MgO material having different particle size as inorganic filler and cross linked polyethylene (XLPE), were prepared and investigated to the space charge dynamics of these composites for high voltage DC insulation. Added MgO material, having 50nm to 1μm in size, were synthesized from the calcination of Mg (OH)₂ nanoplatelets which was controlled by potassium content in Mg (OH)₂, varying the dilution volume ratio of distilled water/ Mg (OH)₂ suspension. MgO/XLPE composites were obtained by thoroughly mixing LDPE and dicumyl peroxide (DCP) as the cross-linking agent and silane modified MgO before kneading to enhance the dispersion degree of the composites. The space charge distributions of the composites were observed by controlling the particle size and content of a dispersed MgO in XLPE matrix. To prevent the breakdown under high DC stress, it was found that the reduction of space charge of MgO/XLPE composite depends upon the smaller size of MgO particle and the its content above 0.5 phr (per hundred part of resin, wt%). MgO material and MgO/ XLPE composites were characterized with TEM, XRD, BET, ICP, and the space charge properties of the composites were measured by a pulsed electro-acoustic method (PEA).

Abstract: Highly dispersed ZnO nanosol, having an average particle size of about 40nm based on Particle Size Analysis (PSA), was prepared under aqueous solution without the removal of large particles by centrifugation. The ZnO nanosol was investigated on the effect of various dispersion parameters, i.e. milling time, dispersant content, pH, etc. The nanosol was effectively dispersed at 20~30 wt% of dispersant amount compared to ZnO content under ball-milling for 10 hours at pH 10. The dispersion characteristic of the nanosol was investigated into particle size and zeta potential. We discussed on the dispersion behavior of (-) charged ZnO particle surrounded by dispersant together with the variation of (-) charged dispersant's amount in aqueous medium at pH 10. The ZnO powder and nanosol were characterized by SEM, TEM, TGA, FT-IR, PSA and Zeta-potential.

Abstract: Zinc oxide(ZnO) has been widely applied in the fields of cosmetics, paint, coating and gas sensor. Size down of ZnO is essential for the preparation of sol or film of ZnO in the application. The size and shape of ZnO particles are controlled by varying drying method. The ZnO was prepared by mixing of zinc acetate and sodium hydroxide at 0°C. Then it was freeze dried or 20°C vacuum dried. After washing the residual ions, it was further compared by drying at 20°C vacuum drying or 200 °C oven drying. The ZnO powder was characterized by XRD, Brunauer-Emmet-Teller analysis(BET) and transmission electron microscope(TEM). The ZnO have wurzite structure with ~10 nm particle size when it was prepared by freeze drying and then 20°C vacuum drying after washing the residual ions. On the other hand, without freeze drying, the ZnO particles observed to be aggregated to 200~400 nm with primary particle size of ~20 nm. The smaller size of ZnO particle in freezing drying compared to 20°C vacuum drying may be explained by frozen water preventing the growth of ZnO particles.

Abstract: The coatings composed of colloidal silica, methyltrimethoxy silane (MTMS), water, and acid catalysts have been prepared with variation of colloidal silica contents from 0 to 20 %. All the coatings were prepared and coated on the aluminum plate under the same condition except the contents of colloidal silica, and the compensating contents of MTMS and water with fixed molar ratio of MTMS and water to be 3. The physical properties were compared in terms of hardness, adhesion strength, corrosion, wear resistance, and thermo gravimetry analysis, etc. The pencil hardness was drastically increased with the addition of 5% colloidal silica particles and was further increased slightly by increase of additional silica. The corrosion resistance of the coating film against alkali solution was improved by increasing colloidal silica content, however, the acid resistance was less affected by the content of colloidal silica.

Abstract: TiO2/SiO2 hybrid film, having water resistance, for water purification photocatalyst was successfully and firmly coated on stainless steel with one dip coating and heat curing method. By controlling relative content of the TiO2 sol and the SiO2 based binder, these films were found to have a significant influence on the hardness, water-resistance, and photocatalytic activity.

Abstract: The application of VOC-free materials for building interior is the first step to reduce the contamination level, however, it is practically unavoidable to find a way to eliminate existing pollutants from furniture, adhesives, etc. As a positive way of controlling the air indooors, the air conditioner may be applied with high functional filter, which uses electricity for compulsory air circulation. As an alternative, we modified natural zeolite powder by ion exchange and also by addition of TiO2 photo-catalyst for multifunctional powder to contribute to controlling the indoor air quality. The modified zeolites were applied on the interior non-woven polypropylene sheet to reduce indoor air pollutants not only by adsorption mechanism but also by decomposition mechanism. We studied deodorization of the multi-functional powders and also their coating on non-woven textiles by a Gastec method.

Abstract: Estrogen-related receptors (ERRs), orphan nuclear receptors, share a significant amino acid sequence homology with estrogen receptors (ERs), yet their ligands do not respond in the same manner. In fact, some of the ligands that are known as agonists of ERs show antagonistic effect in ERRs. Accordingly, the current study investigated the structures of the ligand-binding domains using homology model building and docking studies. The results showed clear differences between the ligand-binding pockets of ERRs and ERs, thereby providing structural insights into the activities related to the ligands.