Papers by Author: Jae Sik Na

Abstract: The epoxy composites with high thermal conductivity for metal-core printed circuit board (MCPCB) can be prepared by varnish coating and a hot press method. Alumina filler of plate-like shape was used as primary micro-filler, while plate-like alumina filler, h-BN, a-BN and s-BN filler were used for blending into the plate-like alumina filler as the secondary filler. Results showed that the secondary fillers a-BN and s-BN loaded epoxy composites have higher thermal conductivity than alumina filler single-loaded composites. Also, BN filler has high thermal conductivity, but h-BN filled epoxy composite has lower thermal conductivity than alumina filled epoxy composite. The decrease of voids in epoxy composite are very important, and the filler shape and surface modification is also necessary to achieve high thermal conductivity in epoxy composite for MCPCB

Abstract: 1-Benzyl-3-methyl-imidazolium hexafluoroantimonate (BMH) was newly synthesized and characterized with FT-IR, 1H-NMR. We synthesized catalysts fulfill requirements for a rapid cure at a moderately elevated temperature in curing the epoxy resin for neat diglycidyl ether bisphenol A (DGBEA). The cure behavior of this resin was investigated at elevated temperature and cure temperature in the presence of 0.5, 1.0, 2.0 wt% of 1-benzyl-3-methyl-imidazolium hexafluoroantimonate (BMH) by mean of differential scanning calorimeter (DSC). Chemical conversion as function of temperature and amount of BMH (0.5, 1.0, 2.0 wt%) were determined from DSC. It was found that BMH were superior latent thermal catalyst for catinonic curing which have a good thermal stability.

Abstract: Bisphenol of 4,4’-dihydroxy-N-benzylidene-4-aminophenol containing azomethine group was synthesized via condensation of an aromatic amine namely 4-aminophenol with p-hydroxybenz aldehyde. It’s epoxy derivate and epicholorohydrin produced the new liquid crystalline diglycidyl ether of 4,4’-dihydroxy-N-benzylidene-4-aminophenol. A new type of liquid crystalline epoxy and azoemthine hardening agent are characterized at 300°C for 10 min, using c11z as a catalyst. The chemical structures were confirmed by FT-IR, ¹H-NMR. The curing reaction with 4,4’-dihydroxy-N-benzylidene-4-aminophenol were carried out in the ratio of glycidyl ether of 4,4’-dihydroxy-N-benzy lidene-4-aminophenol/azomethine hardening 1:0.8. The degree of crystallinity studied by X-ray diffractometer, differential scanning calorimetry (DSC) and polarized optical microscope (POM).

Abstract: The emulsion stabilizing potential of chitosans was compared in the presence of organic additives. The 4 types suspension of 0.1 wt% of chitosan flocculant were obtained by mixing of chitosan colloidal dispersion with three kinds of additives; sorbic acid, benzoic acid and dibutylhydroxytoluene (BHT). The viscosity of emulsion revealed the following order of stabilizing potentials; sorbic acid > benzoic acid > BHT. As a bio-adsorbent for the treatment of biomedical wastewater, the results were capable of adsorbing more than 30% of pure chitosan. The chitosan emulsions represented that the removal efficiency were increased by COD 59.2%, BOD 70.1%, Zn 77.1%, Cu 93.7%, E. coli 99.4%. As a result of this investigation, it is remarked that the high stabilizing potential of chitosan solution is explained by higher adsorption efficiency with organics, heavy metals and microorganism, and that the effectiveness of chitosan solution for coagulating biomedical wastewater suspension could be improved due to stabilization of the viscosity in the presence of organic additives.

Abstract: The hydrogels composed of chitosan and carboxylmethyl-β-cyclodextrin (CMCD) were prepared to enhance and sustain the antioxidant activities. The carboxylic groups of CMCD were directly grafted and cross-linked on the amino groups of chitosan. The swelling rate decreased with increasing the CMCD contents in the hydrogel and the swelling ratio of CMCD-grafted chitosan hydrogels was improved due to breaking the crystallinity of chitosan caused by introducing the bulky group such as cyclodextrin. The scavenging activity of the tested hydrogels increased with CMCD contents, because antioxidants entrapped in the CMCD were potent free radical terminators.

Abstract: This study is to develop a novel method for preparation of the chitosan scaffold having interconnected open pore structure and controlled pore distribution. For this, the effects of addition of non-solvent on chitosan solution were estimated. The porous scaffolds were typically prepared by solid-liquid separation and subsequent sublimation of solvent. Alcohol was used as non-solvent for chitosan. The difference of freezing temperature of each of the components induced the liquidliquid and the liquid-solid phase separation via demixing solution (solvent/non-solvent/chitosan). The morphology, heterogeneous pore distribution and mechanical properties of the scaffolds were examined. The addition of non-solvent in chitosan solution was to make the controlled homogeneous micropores and improved interconnectivity between pores without any surface skin layer. For control chitosan scaffold, the pore size was mainly about 80~100 μm. On the contrary, Pore diameters could be controlled mainly within the range 30~100 μm, with a variation of solvent/non-solvent ratio. The number of minute pore (4~25 μm) over chitosan scaffold increased with increasing ratio of non-solvent. New prepared scaffold exhibited larger value of breaking elongation, more elasticity, but less tensile strength than that of control scaffold.

Abstract: Biodegradable chitosan/clay nanocomposite membranes were prepared by solution casting method for the application of dehumidification. The cationic biopolymer, chitosan was intercalated into clay through cationic exchange and hydrogen bonding process. Diluted acetic acid was used as a solvent for dissolving and dispersing chitosan into clays. Chitosan was successfully intercalated into clay and it was confirmed by X-ray diffraction method. Thermal stability and the mechanical properties of the nanocomposites are characterized by TGA and Universal Testing Machine. Thermal stability and mechanical properties were enhanced by increasing clay contents in chitosan/clay nanocomposites. Gas permeation and water vapor permeation properties of the nanocomposites were measured by time-lag methods. Permeability of N2 gas and water vapor through chitosan/clay nanocomposite membranes decreased when the content of clay in the nanocomposite increased.