Abstract
A two-step foaming process was developed that uses different temperatures in each step and combines ultra-high saturation pressure (Ps > 50 MPa) and rapid depressurization (> 1.25 GPa/s) for production of nanocellular polymer foams. The effect of process conditions were investigated using polymethylmethacrylate (PMMA) and carbon dioxide. Foams with small cells (< 100 nm) and a large cell nucleation density (N0 > 1015 nuclei cm−3) were obtained, which were of a finer structure than those made by previous one-step foaming process. N0 tended to increase even at Ps =100 MPa. A fine cell structure (30.5 nm ± 10.4 nm) was observed with scanning transmission electron microscopy (STEM), and was confirmed when two-step process conditions were used. The process and advantages of ultra-high pressure are discussed with regard to porosity, cell size and N0.
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