This study analyzes the fundamental principles and characteristics of the microcellular foaming process (MCP) to minimize warpage in glass fiber reinforced polymer (GFRP), which is typically worse than that of a solid polymer. In order to confirm the tendency for warpage and the improvement of this phenomenon according to the glass fiber content (GFC), two factors associated with the reduction of the shrinkage difference and the non-directionalized fiber orientation were set as variables. The shrinkage was measured in the flow direction and transverse direction, and it was confirmed that the shrinkage difference between these two directions is the cause of warpage of GFRP specimens. In addition, by applying the MCP to injection molding, it was confirmed that warpage was improved by reducing the shrinkage difference. To further confirm these results, the effects of cell formation on shrinkage and fiber orientation were investigated using scanning electron microscopy, micro-CT observation, and cell morphology analysis. The micro-CT observations revealed that the fiber orientation was non-directional for the MCP. Moreover, it was determined that the mechanical and thermal properties were improved, based on measurements of the impact strength, tensile strength, flexural strength, and deflection temperature for the MCP.

In this work, a high-magnification extrusion-foaming technique for biomass-based biodegradable composite materials using water vaporization was examined. Starch was selected as the biomass and polylactic acid was selected as a biodegradable matrix resin. No additional plasticizer or additives were used in this extrusion-foaming process. The foaming ratio was deduced according to the conditions of the extrusion-foaming process to confirm the forming characteristics of the foaming materials. Scanning electron microscopy was performed to examine the morphology of the composite foam. To investigate the potential of the foam cushion as an ecofriendly packing material, we conducted experiments on its static compression and dynamic cushioning properties and examined whether its biodegradability could be controlled by varying the mixing ratio of the materials. Thus, we developed a water-foaming process that is ecofriendly and whose products can be recycled as compost after use.

This study investigates the properties of a polymer–gas mixture formed through diffusion, based on the changes in the partial pressure and observed changes in the impact and tensile strengths owing to the gas dissolution. The high-pressure gas dissolves into a solid-state polymer through diffusion based on the difference in the partial pressure. This dissolved gas is present in the amorphous region within the polymeric material temporarily, which results in the polymer exhibiting different mechanical properties, while the gas remains dissolved in the polymer. In this study, the mechanical properties of amorphous polyethylene terephthalate (APET) specimens prepared by dissolving CO₂ using a high-pressure vessel were investigated, and the resulting impact and tensile strengths were measured. These experiments showed that the increase in sorption rate of CO₂ caused an increase in the impact strength. At 2.9% CO₂ absorption, the impact strength of APET increased 956% compared to that of the reference specimen. Furthermore, the tensile strength decreased by up to 71.7% at 5.48% CO₂ sorption; the stress–strain curves varied with the gas sorption rate. This phenomenon can be associated with the change in the volume caused by CO₂ dissolution. When the APET absorbed more than 2.0% CO₂ gas, sample volume increased. A decrease in the network density can occur when the volume is increased while maintaining constant mass. The CO₂ gas in the polymer acted as a cushion in impact tests which have sorption rates above 2%. In addition to the reduction in the network density in the polymer chain, Van Der Waals forces are decreased causing a decrease in tensile strength only while CO₂ is present in the APET. These observations only occur prior to CO₂ desorption from the polymer.

This study investigated bio-based plastic cushioning materials foamed through water phase-change characteristics using the natural by-product wheat bran. Experiments were carried out while increasing the wheat bran content from 20% to 70% in a blended composite material of polypropylene (PP) and wheat bran (WB). From the experimental results, we were able to prepare a bio-based plastic cushion that contained a high amount of natural materials, with a bran content of over 50%. This indicates the possibility of meeting the criteria for biodegradable plastics, as well as bio-based plastics. In this study, by inducing a foaming ratio of 95% or more, a volume-expansion ratio from 16 times to over 62 times was achieved. In addition, the optimal mixing condition for inducing a high-expansion foam was when the mixing ratio of PP and WB was 50/50, and the water content of the foaming agent was 25 parts per hundred resin. Finally, dynamic cushioning characteristics of PP50/WB50 composite foam prepared in this study and Polystyrene (PS) bead based commercial products were compared. The composite foam of this study showed comparable values, confirming commercialization possibility.

In this study, the vibration and sound response characteristics of composites produced via injection molding applied with a microcellular foaming process (MCPs) were improved. The study was conducted using PA6 and glass fiber composites, which are representative thermoplastic engineering plastics. Two types of specimens were used: a plate specimen to confirm the basic sound and vibration characteristics, and a large roof-rack specimen from an actual vehicle with a complex shape. The frequency response function curve was calculated by conducting an impact test, and natural frequency and damping ratio were measured based on the curve. The results confirmed that, in the case of a specimen manufactured through the injection molding process to which MCPs were applied, the natural frequency was lowered, and the damping ratio decreased. The degree of change in the natural frequency and damping ratio was confirmed. To determine the cause of the change in the natural frequency and damping ratio, the mode shape at the natural frequency of each specimen was measured and the relationship was confirmed by measuring the density and the elastic modulus of the composite. In addition, the usability of the specimens to which MCPs were applied was verified by conducting impact strength and tensile strength tests.