Ultra-micro-cellular foaming using super critical CO_2

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The visual observation of physical foaming processes is conducted to understand the bubble nucleation and bubble growth behavior in polymers and to develop the technique of micro cellular plastic foams, whose bubble size less than 1 micron meter and bubble density is larger than 10^9 bubbles per cm^3.The visual observation elucidates that to make the bubble size smaller. the bubble nucleation should be enhanced by increasing the foaming agent concentration, making pressure release rate fester, and/or reducing the surface tension of the polymer and the bubble growth rate should be reduced by depressing the mass transfer of foaming agents from polymer matrix to bubbles. The viscosity is a less important factor during the bubble growth process but it becomes an important factor in the process of coalescence and bubble stabilization and break-up. Controlling the temperature of the foaming process and carefully selecting the polymer, the transparent foams of polyolefin foams were created, where the bubble size is less than 1-micron meter. However, because of the low foaming rate, any appropriate applications of the created foams were not found at this stage unless the foaming rote is increased. Nano-composites, which is a composite of polypropylene and clay, were foamed. The effect of clay on nucleation was investigated and orientation of clay around the bubble was also investigated in order to make the mechanical strength of the foam higher.