Study On The Constrained Foaming Of Polymers In Supercritical CO₂

A one-step foaming process was applied to the constrained foaming of PS PMMA and PP polymers using supercritical CO2 as blowing agent. The effect of different dimensional foaming process on the final cellular structure has been investigated. The results showed that in one-dimensional foaming process, relatively high orientation of PS foam cell was obtained by increasing expansion space. And in two-dimensional foaming process, when blow-hole plate was used as barrier constraints, the saturation time changed little, however, the cell growth would be restrained dramatically under the relative high temperature, low pressure and low depressurization rate conditions, and uniform cell size distribution were achieved, this phenomenon didn't disappear with the increase of the sheet's thickness. The fractal dimension theory was applied to analyze the relationship between foaming dimensions and expansion ratio. The experimental data indicated that the expansion ratio increased with the foaming dimensions. What's more, when the foaming behavior was isotropic, the expansion ratio between two-dimensional and three-dimensional foaming process existed an inherent relationship, which was the same under different temperature and pressure. Based on the effect of constrained foaming on cell morphology, a novel method was proposed to produce the biomimetic material. By controlling the conditions of compression process and constrained foaming, a layered microcellular sheet was prepared, which was very close to the nature cork. Besides, a simulation device was applied to investigate the flatness of foamed sheet in the compression molded foaming process. The results showed that the key factor affecting foamed sheet's flatness was the stress caused by the constrained foaming process. Combined with the analysis of different dimensional foaming process, several relation formulas were deduced to set the original unfoamed sheet size for the flatness improvement.