Papers by Author: Hyoung Jin Choi

Abstract: Large density of the dispersed phase in magnetorheological (MR) fluids has hindered operating MR test and their industrial application. This present work adopted Fe3O4 nanoparticles to prepare MR fluid because of their moderate density and good magnetic property. Furthermore, in order to resolve aggregation problem of Fe3O4 particles due to their nano-scaled size, conducting polypyrrole (PPY) was synthesized around naono-sized Fe3O4 particles via conventional oxidation polymerization. Weight ratio of PPY to Fe3O4 was adjusted to be 5% to avoid possible deterioration of the magnetic property of Fe3O4 particles. TEM images described the morphology for PPY-Fe3O4, and XRD pattern provided information on structural characterization and particle size. Finally, MR performances of pure Fe3O4 and PPY-Fe3O4 nanocomposites were investigated via rotational and oscillatory tests.

Abstract: Nanocomposites of conducting polymers of polyaniline (PANI), poly(oethoxyaniline) (PEOA) and polypyrrole (PPy) with clay prepared via either in-situ emulsion polymerization or solvent intercalation were investigated especially for electrorheological fluid (ER) application. Internal structures of these nanocomposites were examined via wide angle X-ray diffraction (WAXD), and transmission electron microscope (TEM). The intercalated nanostructures analyzed via WAXD and TEM were correlated with the electrical property change originated from the nanoscale interaction between clay and conducting polymer. Moreover, their ER behaviors were measured via rotational rheometer with external electric field controller.

Abstract: As an effective way to enhance various properties of polymer, polymer/clay nanocompoiste is being adopted since it can hybrid the properties of the two components, showing superior physical and mechanical properties. In this study, poly(ε-caprolactone) (PCL) nanocomposites using an organoclay were prepared by a solution intercalation method, and their unique internal structures and rheological properties induced by the presence of organoclay at nano level were studied using WAXD and a rotational rheometer.

Abstract: To enhance dispersion stability of magnetorheological (MR) fluids, hybrid magnetic particles of carbonyl iron (CI)/ poly(vinyl butyral) (PVB) with core/shell microstrcutre (CI-PVB) were prepared, since pure magnetic CI based MR fluid systems show severe sedimentation of the CI particles due to the large density mismatch with the carrier liquid and difficulties in redispersion after caking. The composite particles of CI-PVB have a lower density than that of the pure CI particles, while exhibiting almost original magnetic property of the CI. Both CI and CI-PVB particles were dispersed in mineral oil (20 vol%) and their MR characteristics were examined via a rotational rheometer with a magnetic field supplier. Various characterizations of the CI-PVB particles were performed via SEM, TEM and FT-IR. Both yield stress and flow curve of shear stress as a function of shear rate of the MR fluids were investigated under applied magnetic field strengths.

Abstract: Polypropylene (PP)/organoclay nanocomposites were prepared via a melt-mixing method through two-step melt compounding using a co-rotating intermeshing twin screw extruder. Maleic anhydride grafted polypropylene (PP-g-MA) was adopted during compounding as a compatibilizer. Structural investigations via X-ray diffraction (XRD) and transmission electron microscopy (TEM) revealed the intercalated morphology of the nanocomposites. The use of PP-g-MA led to not only good clay dispersion but also heterogeneous intercalation. Thermal degradation of the PP nanocomposites was studied by thermogravimetric analysis (TGA). Rheological properties of the PP/organoclay nanocomposites were also investigated via a rotational rheometer in which linear viscoelastic measurements were performed in oscillatory shear with small strain amplitude. Storage (G’) and loss (G”) moduli were found to be increased at all frequencies with increasing clay contents.

Abstract: Surface-conductive microspheres consisting of poly(methyl methacrylate) (PMMA) (6.5 μm) core and carbon nanotubes (CNTs)-adsorbed shell were prepared using a simple process involving the blending of two colloidal solution; an aqueous CNT dispersion with surfactants and an aqueous PMMA microsphere colloid. These were adopted as the suspended particles for electrorheological (ER) fluids, in which the electrical conductivity originated primarily from the surface-coated conducting CNT layers. The CNT-adsorbed polymeric microspheres were monodisperse and spherical in shape. The CNT-PMMA composite suspensions in silicone oil showed the typical ER characteristics of forming a chain-like structure under an applied electric field. The CNT-PMMA composite microspheres exhibited a conductivity ranging from 5.2×10-4 to 6.3×10-5 S/cm, which is an acceptable conductivity range for ER fluids. This phenomenon can be explained by the interfacial polarizability of CNTs adsorbed on the surface of the polymeric microspheres.

Abstract: Polystyrene/clay nanocomposite with organically modified montmorillonite was synthesized via emulsion polymerization. Organic styrene monomer was first intercalated into intergalley of the clay hosts, followed by a typical emulsion polymerization with surfactant and initiator. To investigate the effect of clay loading, we also prepared PS/clay nanocomposites with different clay contents. Wide angle X-ray diffraction (XRD) and transmission electron microscopy (TEM) showed that the emulsion polymerization of PS in the presence of clay produces partially exfoliated and intercalated nanocomposites. In order to study their rheological properties, the melt pallets of composite materials were prepared and measured via a rotational rheometer with a parallel plate geometry.

Abstract: Radially extruded tubular components are adopted for the deformation analysis by rigid-plastic finite element method. FE Simulations were conducted to investigate the influence of different geometric parameters and process condition, such as the ratio between inner and outer diameter of tubular components, gap height, die corner radius and friction factor, on metal flow into radial direction. The results of the simulations are discussed in terms of separation length, defined as the length that the material flow is separated away from the die in gap height, and maximum force requirements for the radial forming process. Furthermore the pressure distributions exerted on the die-wall interfaces and deformation pattern was shown to obtain the features in producing sound radial extruded components. Finally some guidelines for basic design data in the radial extrusion process due to this simulation work might be drawn up.

Abstract: Conventional multi-step extrusion processes with solid billet are examined by the rigid-plastic finite element method in order to provide criteria for new process sequence for hollow parts. Two examples are taken for the analyses such as the current three-stage cold extrusion process for a hollow flange part and five-stage process for manufacturing an axle housing. Based on the results of simulation of the current three-stage and five-stage manufacturing processes, new design strategy for improving the process sequences is developed simply by replacing the initial billet from solid to hollow one. The developed new process sequences are applied for simulation by FEM and they are compared with the existing processes to confirm the usefulness of new process sequences with hollow initial billets. The results of simulation show that the newly proposed process sequences with hollow billet instead of solid one are more economical way to manufacture required parts, respectively.

Abstract: New manufacturing processes for a valve-spring retainer (VSR) are proposed by replacing the initial solid billet for commercially available thick-walled pipes. The rigid-plastic FEM has been applied to simulate the conventional five-stage manufacturing process for VSR component. The existing process includes mainly backward extrusion and heading operations. A process design methodology is proposed and applied for the analysis. The process design criteria are the maximum force requirement within the available press limit, and the material saving by reducing the wastes from the process. As a result, several simulations of one-step process from selected stocks to the final product shape are performed for a possibly better process than the conventional one. Statistics among different processes are summarized and compared each other in terms of number of required operations for final product, forging load, material waste, number of individual die, process time, and even the possibility of fracture during service. Experiment also has been conducted to ensure that the proposed one-step process is safe operation without geometrical defects.