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Microcellular plastic foam is plastic that has been specially foamed so as to created micro-pores or cells. The common definition includes foams with pore size on the order of 10 micrometres in diameter (from 0.1 to 100 micrometre typically.) Since the cells are so small, to the casual observer this foam retains the appearance of a solid plastic. Microcellular foams have been made in the density range of 8 to 99% of the base material. Ordinary foam has cell diameters of 100 to 500 micrometre and density of only 0.3 to 50% of virgin material. (source: http://swhite.me.washington.edu/~kumar/microcel/welcome.htm)
Most microcellular plastics are created by saturating a thermoplastic with an inert gas at very high pressures. The plastic absorbs the gas like a sponge. Removing the plastic from the high pressure environment and heating it causes the plastic to foam, creating a very uniform structure of small bubbles.
The very small structure, and uniform makeup of the plastic yield superior insulative and mechanical properties compared to conventional foams. There are also environmental benefits to foaming using inert gases. Most conventional foams are produced using hydrocarbons or CFC's that are bad for the environment.
Microcellular plastics are currently mainly an interest of academia, although limited commercial production is taking place.