(429c) Foaming Behavior of Polymers With Using Mixtures of CO2 and N2 As Foaming Agent

               Polymer foams are well known to have high shock absorbing ability, thermal insulating ability, good handling ability and many other good characteristics. Due to their properties, polymer foams are used as products for daily use like sponge, housing materials, car bumper and so on.

              Recently, due to the interest in environmental issues, non-CFCs such as carbon dioxide (CO₂) or nitrogen (N₂) are used for blowing agent for the production of the polymeric foams. It is well known that foams with CO₂ have large but non-uniform cells. On the other hand, foams with N₂have a lot of small and uniform cells.

              In this work, to control the final foaming structure, use of the mixtures of CO₂ and N₂ as blowing agent was examined through the foam morphology and visualization of the initial stage of foaming. Poly(methyl mathacrylate) (PMMA), Polystyrene (PS), CO₂ and N₂ were used for the foaming experiments. Molar fraction of CO₂/N₂ was varied in the range 0-100 mol% to observe the foam morphology of PMMA/CO₂/N₂ and PS/CO₂/N₂systems. The densities of the foamed polymer samples were determined by the Archimedean principle using a picnometer. For microstructural analysis, the foamed samples were observed with scanning electron microscope.

              Initial stage of foaming was observed with batch foaming visualization system. In this study, foaming behavior, or the leading time of foaming after pressure release and cell diameters growth rate as well as number density increase of cells, was examined with PMMA/CO₂/N₂systems.

              In the foam morphology, we found the difference in density, cell number density, and cell size, following the change in CO₂/N₂ molar fractions. The lowest density (0.047 g/cm³) was obtained with PMMA/CO₂/N₂ system at the CO₂/N₂ ratio of 40 mol%. It is 4 to 8 times lower than PMMA/CO₂ or PMMA/N₂ binary systems. Also PS/CO₂/N₂ system had the lowest density (0.038 g/cm³) at the CO₂/N₂ ratio of 83 mol%, which is different from PMMA/CO₂/N₂system. Polymer foams with the lowest density had larger cells than those obtained with the single gas foaming.

              The initial stage of foaming with mixtures of CO₂ and N₂ systems exhibited somewhat different behavior compared with the single gas foaming systems. At certain molar ratio of CO₂ and N₂, growth rate of the initially formed bubble changed drastically. This behavior might be caused by the plasticization effect from increasing the CO₂ molar fraction. Decrease in the interface tension of polymer/gas with the increase in the CO₂ molar fraction will encourage the cell diameters to grow. However, the plasticization also  decreases viscosity, which will cause unification and collapse of cells in the high CO₂ molar fraction range. The best CO₂/N₂molar fraction for low density of foam might be determined by the balance of the cell diameter growth with unification and collapse of cells.

              It could be concluded that use of the mixtures of CO₂ and N₂ as blowing agents is useful to control the foaming structure.