The Second Expansion Cycle in Ultramicrocellular Foams †

Ultramicrocellular PMMA foams made from the low temperature rubbery phase, obtained by saturating the polymer with CO₂ at 0°C and 34 atm, was subjected to a second expansion cycle by heating it rapidly to temperatures in the range 30 to 120°C. The blowing agent for the secondary expansion was either the residual CO₂ in the foam at about 19 wt% level or fresh CO₂ re-equilibrated at 0°C and 34 atm to a loading level of about 29 wt%. In the former case, the second expansion cycle led to lower foam densities whereas in the latter case it led to stretching and re-orientation of the cell walls with no significant gain in further foam density reduction or cell density increase.