Hyun-Sik Hahm

Influence of aspect ratio and skin effect on EMI shielding of coating materials fabricated with carbon nanofiber/PVDF

The effectiveness of electromagnetic interference (EMI) shielding was measured for poly vinylidene fluoride (PVDF) coating materials containing carbon nanofibers. Carbon nanofibers produced from the C2H4/NC73 system exhibited higher shielding effectiveness (SE) by relatively large specific surface area and high aspect ratio than those from others. When the thickness of carbon nanofibers filled PVDF coating materials varied from 25 to 50 μm, the electrical conductivity of coating materials increased sharply from 1.34 to 1.91 S/cm. However, the electrical conductivity approached a certain value with further raise of the thickness. This phenomenon denotes that a critical thickness of coating materials exists around 50 μm. The electrical conductivity and SE of coating materials decreased dramatically as the carbon nanofiber fillers were milled. It could be concluded that the decrease of the shielding effectiveness of carbon nanofiber filled composite was due to the decrease of the fillers aspect ratio by ball milling.

Preparation and characteristics of two-component polyurethane flame retardant coatings using 2,3-dibromo modified polyesters

Two-component polyurethane flame retardant coatings were prepared by blending 2,3-dibromo modified polyesters (2,3-DBPOs) and polyisocyanate. 2,3-DBPOs were synthesized by polycondensation of 2,3-dibromopropanoic acid, a flame retardant aliphatic carboxylic acid, with 1,4-butanediol, trimethylolpropane, and adipic acid. The content of 2,3-dibromopropanoic acid was varied at 10, 20, and 30 wt % for the polycondensation reaction. Various physical properties of these new flame retardant coatings were comparable to non-flame-retardant coatings. They showed desirable properties for a flame retardant coating such as rapid drying and 9–12 h of pot life. Coatings with 30 wt % 2,3-dibromopropanoic acid did not burn using the vertical burning test.

Direct synthesis of dimethyl ether from synthesis gas over metal-pillared ilerites

The capability of metal (Cu, Zn)-pillared ilerites and metal oxide (CuO, ZnO)-impregnated metal-pillared ilerites for direct synthesis of dimethyl ether (DME) from synthesis gas was explored. The metal-pillared ilerites were synthesized and characterized by XRD, BET, ICP-AES and SEM. The reaction was carried out in a fixed bed reactor with the prepared catalysts at different temperatures (200, 250, 300°C), 20 bar and H2/CO ratio of 2. For CuO/Zn-ilerite catalyst, CO conversion was about 62% and selectivity to DME was about 89% at 250°C.

Preparation and characterization of weather resistant silicone/acrylic resin coatings

Preparation and characterization of weather resistant silicone/acrylic resin coatings were conducted. In order to prepare these coatings, a silicone/acrylic resin (KLD) was first prepared by an addition polymerization reaction of monomers, including n-butyl acrylate, methyl methacrylate, n-butyl methacrylate, and 3-methacryloxypropyltrimethoxysilane (MPTS). In the preparation of the silicone/acrylic resin, Tg of the acrylic copolymer was fixed at 40°C and the contents of MPTS were varied to be 10, 20, and 30 wt%. The weather resistant silicone/acrylic resin coatings were then prepared by blending the synthesized silicone/acrylic resin and TiO2. The viscosity of the synthesized resin decreased with the content of MPTS, whereas the thermal stability at high temperature increased. The prepared coatings exhibited excellent adhesion to various substrates, and various physical properties of the coatings were satisfactory. The weatherability of the coatings was tested three ways: outdoor exposure test, Weather-Ometer (WOM), and QUV accelerated weatherability tester (QUV). The gloss retention, yellowness index difference, color difference, and lightness index difference were improved at high MPTS concentration. The coatings containing 30 wt% MPTS have especially good weather properties.

Reaction mechanism of partial oxidation of methane to synthesis gas over supported ni catalysts

A mechanistic study on the partial oxidation of methane to synthesis gas (H2 and CO) was conducted with supported nickel catalysts. To investigate the reaction mechanism, pulse experiments, O2-TPD, and a comparison of the moles of reactants and products were carried out. From the O2-TPD experiment, it was observed that the active catalyst in the synthesis gas production desorbed oxygen at a lower temperature. In the pulse experiment, the temperature of the top of the catalyst bed increased with the pulses, whereas the temperature of the bottom decreased. This suggests that there are two kinds of reactions, that is, the total oxidation of methane (exothermic) at the top and reforming reactions (endothermic) at the bottom. From the comparison of the moles of reactants and products, it was found that the moles of CO2, CH4 and H2O decreased as the moles of H2 and CO increased. The results support the mechanism that synthesis gas is produced through a two-step reaction mechanism: the total oxidation of methane to CO2 and H2O takes place first, followed by the reforming reaction of the produced CO2 and H2O with residual CH4 to form synthesis gas.

Crystallization of [Ga]-MFI Under Atmospheric Pressure

The crystallization of [Ga]-MFI was investigated as a function of synthesis time under atmospheric pressure. The molar composition of the reactants was 100SiO2–Ga2O3–11Na2O–11TPABr–3500H2O. The crystallinity of [Ga]-MFI was examined using several analytical instruments, such as XRD, XPS, XRF, FT-IR, solid state MAS-NMR, and DTG/DTA. [Ga]-MFI was successfully synthesized under atmospheric pressure at 97°C in 72 h. It was found that the nucleation of [Ga]-MFI took quite a long time, but the crystallization took place very fast. It is supposed that the nucleation is the rate-controlling step in [Ga]-MFI synthesis under atmospheric pressure. Consequently, if the induction period of the nucleation can be shortened, it would be possible to synthesize [Ga]-MFI commercially under atmospheric pressure.

Propylene aromatization on Ga-MFI Zeolites

Propylene aromatization reaction was performed on various MFI type zeolites containing Ga species. The Ga was introduced into the zeolites by substitution (Ga-MFI), ion exchange (GIZ) and physical mixing (GPZ). A com-mercialized zeolite (PQZSM-5) was also used for comparison. The catalysts prepared were characterized by using XRF, XPS, surface area measurement, NH3-TPD, and H2-TPR. Through the Ga substitution, the acidity of the modified catalysts was decreased, and dehydrogenation and aromatization reactions occurred more easily. The lattice Ga did not react well with hydrogen contrary to the Ga located at the outside of the lattice. It was also found that Ga-MFI catalysts facilitate alkylation reactions.

Synthesis of the [Ga]-MFI under atmospheric pressure

The crystallization of the [Ga]-MFI was investigated as a function of synthesis time under atmospheric pressure. The molar composition of the reactants was 100SiO2-Ga2O3-llNa2O-llTPABr-3500H2O. The crystallinity of the [Ga]-MFT was examined by using several analytical instruments, such as XRD, XPS, XRF, FT-IR, solid-statemas-NMR, DTG/DTA, and SEM. The [Ga]-MFI was successfully synthesized under atmospheric pressure at 97 ‡C in 72 h. It was found that the nucleation of the [Ga]-MFI took a quite long time, but the crystallization took place very fast. It is supposed that nucleation is the rate-controlling step in the [Ga]-MFI synthesis under atmospheric pressure. Consequently, if the induction period of the nucleation can be shortened, it would be possible to synthesize the [Ga]-MFI commercially under atmospheric pressure.