Microcellular plastic foam is a plastic that has been specially fabricated so as to create micro-pores or cells in the polymer matrix. The common definition includes foams with pore size on the order of 10 micrometers in diameter (from 0.1 to 100 micrometers typically.) Since the size of cells is close to the wavelength of light, to the casual observer this foam retains the appearance of a solid light colored plastic. Microcellular foams have been made in the density range of 5 to 99% of the base material. Conventional commodity foam has cell diameters of 100 to 500 micrometers and density of very light foam to hard foam (0.3 to 50% of virgin material). Images of these plastics can also be found here: (source: http://microcel.me.washington.edu)
Microcellular plastic foams was invented at MIT under the direction of Professor Nam P Suh in the early 1980s. The basic theory for microcellular plastic foams and the actual processes were developed at MIT. The basic patents were licensed to Trexel, Inc., which has commercialized the process for injection molding, extrusion and blow molding. The advantages of the microcellular plastics are: reduction of materials consumption, accurate part, long-term stability due to low residual stresses in molded parts, higher productivity due to shorter cycle time, and unique appearances.
Some microcellular plastics are created by the Solid-State Foaming Process, which saturates a thermoplastic with an inert gas at very high pressures. The gas dissolves in plastic, which absorbs the gas like a sponge. Removing the plastic from the high pressure environment creates a thermodynamic instability. Heating the polymer above the effective glass transition temperature (of the polymer/gas mixture) then 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 mechanical properties compared to conventional foams. There are also environmental benefits to foaming using inert gases. Most conventional foams are produced using chemicals that could be bad for the environment.
Commercialization of the Solid-State process has begun by a company named Microgreen Polymers.
Microcellular plastics are also currently used in industry to produce injection molded and extruded thermoplastics under the tradename MuCell.
Recent developments at the University of Washington have produced Nanocellular foams. These foams are characterized by cell sizes in the 20-100 nanometer range.