Ki-Jun Lee

Growth of gas bubbles in the foam extrusion process

In comparison to other kinds of crosslinked thermoset foam, recyclable polyolefin foam is an environmentally friendly material, but it increases the degree of foaming in the foam extrusion process. In order to overcome this shortcoming, the effect of material and process variables on the degree of foaming and the growth behavior of the gas bubbles was studied by using polyolefins, such as polypropylene (PP), low density polyethylene (LDPE), and high density polyethylene (HDPE). These materials were foamed in extrusion process with a blowing agent consisting of sodium bicarbonate and citric acid. A higher degree of foaming was obtained with a low temperature of cooling water and high melt temperature. The take-up speed did not influence the degree of foaming. It was also observed from cell morphologies that the size of the gas bubbles was reduced with increased take-up speed and decreased melt temperature, but the size of the gas bubbles was nearly invariant as the temperature of the cooling water increased. In addition, the effects of the molecular weight, the molecular structure, the melt viscosity, and the melt tension on the degree of foaming and the growth of the gas bubbles were investigated. It was revealed that the degree of foaming and the size of the gas bubbles were directly related to the melt tension of the polymer melt, regardless of the structure and the molecular weight of the polymer resin and the temperature of the polymer melt. This implies that the ability to sustain the cell in the melt state is a critical foaming parameter in the extrusion process.

Crystallization kinetics of glass fiber reinforced PBT composites

The effect of glass fibers on the crystallization of poly(butylene terephthalate) (PBT) was investigated by crystallization kinetics analysis under isothermal and nonisothermal conditions. From the crosspolar optical micrographs of melt- and solvent-crystallized PBT composites, the glass fibers were found to increase the number density and decrease the size of crystallites. The glass fibers provided heterogeneous nucleation sites, and thus enhanced the overall rate of PBT crystallization in isothermal experiments. However, the Avrami exponent and the regime transitions were not significantly affected by the presence of glass fibers. For the nonisothermal kinetics of PBT composites, the model prediction was excellent in most ranges of crystallization, but it deviated above 70% of crystallization especially at fast cooling rates (>40°C/min). This discrepancy of the model seemed to result from the growth regime transitions, which were clearly observed especially at high undercoolings.

Composite cure kinetic analysis of unsaturated polyester free radical polymerization

Curing kinetics of unsaturated polyester resin system exhibiting apparent induction periods was investigated by modeling free radical initiation and propagation processes. The isothermal curing induction time as well as the maximum-rate time provided the same activation energy in the Arrhenius relation, and therefore, the isothermal curing master curve was constructed by using the reduced time method. Two model elementary rate equations for radical and monomer were proposed to describe the free radical polymerization of unsaturated polyester resin systems. The power law was adopted to express the conversion dependence function of the initiation efficiency and the monomer reaction rate. Demonstrating the capability of the developed model, the agreement between experimental and predicted data was excellent in both isothermal and dynamic-heating conditions, even with the same model parameters in different thermal conditions.

Healing of fractured polymers by interdiffusion

The healing process of fractured polymers is analysed on the basis of the minor chain model. The healing at the interface by interdiffusion is described in terms of the concentration of minor chains. Chain scission would occur at the surfaces of fractured polymers and the resulting minor chains play an important role in the healing process. The localized minor chains at the interface dominate over the flat distribution of the chains in the bulk of the polymer. The molecular quantities derivable from the concentration profile in the fracture case show drastically different behaviour compared with two pieces of a polymer that are put in contact for interdiffusion, the latter corresponding to the flat chain distribution. The result suggests an alternative to the usual correlation for the time dependence of the fracture toughness.

Mechanism of void formation in composite processing with woven fabrics

A mechanism for void formation is suggested, involving an analysis of the resin flow at macro and micro levels. Both these types of flow arise from the characteristic structure of woven fabrics. In order to investigate the mechanism of void formation in woven fabrics, a new dimensionless number was defined as the ratio of the superficial velocity to the initial spreading rate. The prediction of void formation using this dimensionless number had a good correlation with the results from the cross sectional observation and void content measurement. It was confirmed that the mechanism introduced in this study is very desirable and that the morphological structure of the woven fabric plays a key role in void formation.

Process analysis of resin transfer molding with autoclave‐assisted laminate consolidation

An improved resin transfer molding process was implemented by the assistance of the autoclave in epoxy/carbon fiber composite manufacturing. In the autoclave-assisted resin transfer molding (ARTM) process, the mold-filling pressure and the consolidation pressure were controlled by the autoclave in a continuous fashion during the mold filling and consolidation stages. Using a smart spacer inserted between the RTM molds, the part consolidation was effectively achieved after the complete mold filling, which consequently improved processing as well as performance characteristics of advanced composite materials. Visualizing the resin-front advancement, anisotropic permeabilities of several reinforcement fabrics were collectively determined by comparing the experimental results and Darcys law. A finite element/control volume method was used to predict the resin pressure and flow front advancement during the ARTM process. Finally, the processing variables of resin bleeding, consolidation pressure, and surface formers were investigated to identify the ARTM processing and structural characteristics in high-performance composite utilization.

Molecular weight distribution and controllability in living polymerization with chain-transfer agents

The same molecular weight distribution is always attained for both dead and living polymers when the living polymerization is carried out in a continuously stirred tank reactor, which cannot be achieved in a batch reactor. The average molecular weight and the yield are completely controllable to any desired values. However, the polydispersity index is determined by the average molecular weight only, its value increasing with increasing average molecular weight. The index ranges from unity to 2.

Effect of co-rotation and counter-rotation on suppression of melt convection in magnetic Czochralski growth

Abstract Melt flow phenomena of magnetic Czochralski growth are simulated numerically for a 5 inch diameter crystal grown from a 14 inch diameter crucible. It is found that, in conjunction with an axial magnetic field, co-rotation of the crystal with the crucible suppresses meridional circulation more thoroughly than counter-rotation.

COMPUTATIONAL SIMULATION OF MELT FLOW IN MAGNETIC CZOCHRALSKI GROWTH PROCESS

Melt flow phenomena of magnetic Czochralski processes are simulated numerically. A model is established for the system that can grow a 5 inch diameter single crystal from the melt in a 14 inch diameter cylindrical crucible. Effective conditions to suppress convection adequately are studied by varying rotation rates of the crucible and the crystal for given magnetic fields. When the axial magnetic force is used, the method of co-rotation of the crystal and the crucible is found more effective to suppress meridional circulation than the conventional counter-rotation operation.

Computational analysis of convective diffusion with chemical reaction in a cavity

Unsteady convective diffusion problems involving chemical reaction in a rectangular are numerically examined. The effect of various factors affecting the removal of the contaminant in the cavity has been analyzed systematically. The vorlicity and streamfunction are used for numerical computations. For low Reynolds number cases, the rate of removal of the contaminant increases as the Grashof number becomes larger. For high Reynolds number, the secondary flow hinders the rate of removal of the contaminant as the Grashof number becomes larger.